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The University of Manchester Research
Microphysical properties and radar polarimetric featureswithin a warm frontDOI:10.1175/MWR-D-18-0056.1
Document VersionAccepted author manuscript
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Citation for published version (APA):Keppas, S., Crosier, J., Choularton, T., & Bower, K. (2018). Microphysical properties and radar polarimetricfeatures within a warm front. Monthly Weather Review, 146. https://doi.org/10.1175/MWR-D-18-0056.1
Published in:Monthly Weather Review
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Download date:04. Aug. 2021
1
Microphysical properties and radar polarimetric features within a 1
warm front 2
S. CH. Keppas1, J. Crosier1,2, T.W. Choularton1, K.N. Bower1 3
1Centre for Atmospheric Sciences, SEES, University of Manchester, UK 4
2National Centre for Atmospheric Science, University of Manchester, Manchester, UK 5
6
Corresponding author e-mail: Jonathan Crosier, [email protected] 7
8
Abstract 9
On 21 January 2009, the warm front of an extensive low-pressure system affected the UK weather. 10
In this work, macroscopic and microphysical characteristics of this warm front are investigated 11
using in-situ (optical array probes, temperatures sensors and radiosondes) and S-band polarimetric 12
radar data from the Aerosol Properties, PRocesses And InfluenceS on the Earth’s climate-Clouds 13
project. The warm front was associated with a warm conveyor belt, a zone of wind speeds of up 14
to 26m s-1, which played a key role in the formation of extensive mixed-phase cloud mass, 15
ascending significant liquid water (LWC ~0.22g m-3) at a level ~3km and creating an ideal 16
environment at temperatures ~ -5°C for ice multiplication. Then, “generating cells”, which formed 17
in the unstable and sheared layer above the warm conveyor belt, influenced the structure of the 18
stratiform cloud layer dividing it into two types of elongated and slanted ice fall-streaks, one being 19
depicted by large ZDR values and another by large ZH values. The different polarimetric 20
characteristics of these ice fall-streaks reveal their different microphysical properties, such as the 21
ice habit, concentration and size. We investigate their evolution, which was affected by the warm 22
conveyor belt, and their impact on the surface precipitation. 23
2
1 Introduction 24
Extratropical cyclones have been extensively studied, as such systems often demonstrate 25
complicated cloud structures providing an ideal environment for microphysical studies [e.g. 26
Matejka, 1980; Chen and Cotton, 1988; Forbes and Clark, 2003; Stark et al., 2013; Crosier et al., 27
2014; Lloyd et al., 2014; Dearden et al., 2016]. The typical structure of cyclones consists of acold, 28
a warm and an occluded front, which usually affect the UK with strong winds and heavy rainfall 29
[e.g. Browning, 2004; Lavers et al., 2011]. The fronts are associated with warm and cold conveyor 30
belts which circulate air masses with different traits. Warm conveyor belts (hereafter WCB), which 31
usually originate in surface maritime areas, convey humid air making a substantial contribution to 32
the cloud structure and surface precipitation [Harrold, 1973; Browning, 1986; Eckhardt et al., 33
2004; Pfahl et al., 2014]. Although all types of fronts have particularities and mechanisms that 34
need further investigation, studying warm fronts is important as they account for the majority of 35
the precipitation associated with extratropical cyclones [Keyser, 1986; Wakimoto and Bosart, 36
2001]. 37
Dual-polarization radars have been a valuable tool for revealing cloud microphysical 38
properties, providing details about ice crystal habits, dimension, shape, orientation and phase of 39
hydrometeors. Various studies have been conducted on linking polarimetric radar variable 40
thresholds with specific hydrometeor types [e.g. Straka, 2000; Hogan et al.,2002; Kennedy and 41
Rutledge, 2011; Schrom and Kumjian, 2016], and determining polarimetric signatures, which can 42
provide insight into cloud microphysical processes [e.g. Hall et al., 1984; Illingworth et al., 1987; 43
Hubbert et al., 1998; Smith et al., 1999; Ryzhkov et al., 2005; Kumjian and Ryzhkov, 2008; 44
Kumjian, 2013]. Comparing and linking in-situ cloud measurements with radar data can improve 45
and validate the interpretation of the remote sensing data, and therefore the understanding of cloud 46
microphysics, including the role of primary [Koop et al., 2000; Vali et al., 2015], secondary ice 47
production [Mossop and Hallett, 1974; Pruppacher and Schlamp, 1975; Vardiman, 1978; Knight, 48
3
1979 and Choularton et al., 1980] and embedded convection mechanisms. Embedded convective 49
turrets, which appear as cells with horizontal and vertical extent of up to 6km and 2km 50
correspondingly, are referred to as “Generating Cells” (GCs), and have been documented several 51
times in the literature [Wexler and Atlas, 1959; Houze et al., 1976; Hobbs and Locatelli, 1978; 52
Herzegh and Hobbs, 1981; Kumjian, 2014; Plummer et al., 2014; Rosenow et al., 2014; Plummer 53
et al., 2015; Oue et al., 2015; Rauber et al., 2015; Keeler et al., 2016a; Keeler et al., 2016b; Keeler 54
et al., 2017 and many others]. Discussing some of the latest studies, Plummer et al. [2014, 2015] 55
used in-situ (2D-C and 2D-P) together with a doppler radar (reflectivity factor and doppler 56
velocity) in order to investigate the microphysics of GCs and the fall-streaks originated in them. 57
In addition, Rosenow et al. [2014] used an airborne W-band radar to quantify the magnitude of the 58
vertical velocities in GCs, while Keeler et al. [2016a, b, 2017] used models to simulate GCs and 59
examine their origin, forcing and their relationship to vertical wind shear, ambient thermal 60
instability and cloud top radiative forcing. A mechanism that commonly triggers GCs is Kelvin-61
Helmholtz instability, which happens when velocity shear occurs in a single continuous fluid 62
[Browning et al., 1973; Hogan et al., 2002]. GCs usually present updrafts of <3m s-1, and they are 63
often associated with descending trails of ice crystals and snow, which are commonly referred to 64
as “ice fall-streaks”. Ice fall-streaks, the shape of which is affected by the vertical wind shear, 65
originate in and form from generating cells, play an important role in the production of 66
precipitation [e.g. Marshall 1953; Langleben 1956; Douglas et al., 1957; Wexler and Atlas, 1959; 67
Bader et al., 1987; Kumjian, 2014; Rosenow et al., 2014; Oue et al., 2015]. 68
In this study, we use high-resolution co-located in-situ and active remote sensing observations 69
obtained from dual-polarisation data, to provide a) detailed picture of the microphysical structure 70
of a warm front and b) analysis of the structure, the origin and the effects of the ice fall-streaks, 71
which are frequently embedded within warm fronts. 72
73
4
2 Methodology 74
The data used in this study were obtained during the NERC-funded APPRAISE-Clouds project 75
(part of the NERC Aerosol Properties, PRocesses And InfluenceS on the Earth’s climate 76
programme), which took place in the UK during 2007-2010. This cloud project aimed to provide 77
a comprehensive investigation of mixed-phase clouds influencing UK weather systems and the 78
impact of aerosols on the microphysical properties and development of such clouds [Crosier et al., 79
2011; Crawford et al., 2012]. 80
a. The in-situ datasets 81
The UK-FAAM (Facility for Airborne Atmospheric Measurements) Bae-146 aircraft was used 82
to collect in-situ measurements of cloud properties during APPRAISE-Clouds. In particular, on 83
21 January 2009 during flight B424, the aircraft extensively sampled clouds associated with a 84
warm front over southern England, performing a series of straight and level runs, profiles, and 85
saw-tooth profiles over the period 1500-2000UTC. The sampling occurred along a radial of 255o 86
within ~100km range from the Chilbolton Facility for Atmospheric and Radio Research (CFARR) 87
located at 51.15oN, 1.44oW, to allow direct comparison with radar scans along the same azimuth. 88
Hourly radiosondes were also launched from the Chilbolton ground site between 0700-1800UTC. 89
The FAAM Bae-146 aircraft was equipped with microphysical probes, which provide 90
measurements of a wide range of hydrometeors as described in Crosier et al., 2014. Key 91
instrumentation used in this work included: A mie scattering Cloud Droplet Probe (CDP) 92
measuring number and size (~3 and 50μm) of cloud droplets [Lance et al., 2013]; 2D-S (Two-93
Dimensional Stereo) and CIP-100 (Cloud Imaging Probe-100) Optical Array Probes (OAP) 94
providing shadow imagery of particles with size 10-1280μm and 100-6200μm [Knollenberg et al., 95
1970; Lawson et al., 2006] respectively; Rosemount probe measured the ambient de-iced 96
temperature. It should be highlighted that the 2D-S probe provides better resolution (10μm) and 97
5
wider size range (10-1280μm) comparing to its previous version 2D-C (25μm; 25-800μm). Particle 98
phase was estimated based on a shape analysis of images obtained with the 2D-S and CIP-100, 99
while ice number concentration and diameter (from the diagonal of the bounding box around the 100
recorded particle image) was estimated according to Crosier et al. [2011, 2014]. An inter-arrival 101
time threshold was used to filter OAP images to remove artefacts associated with shattering of 102
large particles on the probes according to Field et al. [2006]. 103
In order to estimate bulk ice-phase parameters, including mean diameter and concentration, 104
the 2D-S and CIP-100 data sets were merged, and measurements in overlapping sizes were 105
averaged. Ice Water Content (IWC) was calculated from merged 2D-S and CIP-100 particle size 106
distributions using estimates of particle mass from Heymsfield et al. [2007]. Significant 107
uncertainties (~50% for IWC) can arise because of the diversity in OAP image processing options. 108
Liquid Water Content (LWC) was estimated from the CDP particle size distribution. 109
b. The remote sensing datasets 110
During the flight, the Chilbolton Advanced Meteorological Radar (CAMRa) performed Range 111
Height Indicator (RHI) scans along the 255o (WSW) radial every ~90 seconds to obtain near co-112
located radar and in-situ measurements. The CAMRa is a dual-polarization Doppler radar that 113
operates at 3GHz with a steerable 25m dish resulting in a narrow 0.28o beam [Goddard et al., 114
1994]. Additionally, the zenith-pointing dual polarisation 35GHz Copernicus radar, which is 115
located within 50 meters of the CAMRa, was operating throughout the day. In contrast to low-116
frequency radars, which are applied for precipitation observations [Kollias et al., 2007; Fukao et 117
al., 2014], high-frequency radars are significantly affected by attenuation due to precipitation 118
[Godard, 1970]. The cross section ratio between large and small particles at such high frequencies 119
is smaller compared to longer wavelengths, so are an ideal tool for monitoring clouds and fog. All 120
references to data from the 35GHz vertical pointing radar will be indicated by the subscript “35”; 121
otherwise the data being discussed are from the 3GHz radar. 122
6
In this study, we use radar variables, such as reflectivity factor (ZH), Doppler velocity (VRAD) 123
and differential reflectivity (ZDR) to examine cloud dynamical and microphysical structures. ΖΗ 124
can be affected by the size and concentration of the hydrometeors [Doviak et al., 1979]. Doppler 125
velocity provides an estimate of the motion of hydrometeors, which can indirectly be used to infer, 126
depending on the mode of operation, information on wind fields or the fall-speeds of hydrometeors 127
[Browning and Wexler, 1968]. Finally, ZDR is the ratio between the radar reflectivities at horizontal 128
and vertical polarisations expressed in logarithmic scale induced by hydrometeors within the radar 129
beam [Seliga and Bringi, 1976], and can be used to provide details about their shape, orientation 130
and phase. We highlight that ZDR data coming from elevations <0.2o have been excluded due to 131
vertical beam blockage [Ryzhkov et al., 2002; Giangrande and Ryzhkov, 2005]. 132
133
3 The Synoptic Condition 134
On 21 January 2009, a deep low-pressure system in the northwest Atlantic Ocean moved 135
eastwards and developed (figure 1a). This system generated multiple frontal boundaries, which 136
approached the UK from the west. The Bae-146 research flight sampled the warm front as it passed 137
across the south of England. This warm front was observed to be associated with a WCB, which 138
originated in the warm sector of the system (figure 1). Figure 1a, b show that the WCB transported 139
humid and warm air over the colder air found over the UK mainland. In figure 1b, the WCB 140
becomes perceptible through the zone of enhanced relative humidity and strong SSW-W winds 141
(up to 23m s-1), which reached a level of ~450mb (~6km height). 142
143
144
4. A Macroscopic Description of the cloud features 145
7
A brief description of the entire in-situ and radar dataset is provided in order to understand the 146
general cloud structures in the lower/middle troposphere during the passage of the warm front 147
shown in figure 1a. 148
The spatial structure of the frontal cloud obtained from CAMRa at 1712UTC is depicted in 149
figure 2. Several features, which can be seen in figure 2, were consistently observed throughout 150
the frontal passage and will now be discussed in turn. Firstly, we observed GCs presented as 151
protruded cloud tops, which demonstrated high-ZH/low-ZDR cores (>10dBZ / ~0dB) and low-152
ZH/high-ZDR (<10dBZ / >1dB) boundaries (figure 2a, d) [e.g. Bader et al., 1987; Kumjian, 2014; 153
Oue et al., 2015]. A ZH ice fall-streak [e.g. Bader et al., 1987; Kumjian, 2014; Oue et al., 2015] is 154
represented as a long (tens of km) and narrow (~1km) slant (angle of 3-7o) zone of high ZH 155
(enhanced by 10-15dBZ relative to the surroundings) (figure 2a) and ~0dB ZDR (figure 2d) 156
containing mostly rimed/aggregated crystals (figure 5, 2km<altitude<3.5km). Then, ZDR fall-157
streaks [e.g. Bader et al., 1987; Kumjian, 2014; Oue et al., 2015] typically located above a ZH fall-158
streak, exhibited a zone of high-ZDR (>1dB) (figure 2d) and low-ZH (<10dBZ) (figure 2a) values 159
consistent with the presence of pristine ice crystals (figure 5, 3km<altitude<5km). Although 160
figure 2 does not show an ideal example of GC, it highlights the connection between a GC and a 161
ZH/ZDR fall-streak (see details in section 5b). Finally, figure 2f presents two banded layers of 162
enhanced Doppler velocity associated with the so called Warm Conveyor Belt (hereafter WCB), 163
an air stream which appears in RHIs between 2-4km, which transports humid and warm air aloft. 164
An overview of the major temporal changes in the cloud structure can be seen in the vertical-165
pointing 35 GHz radar data (figure 3). It should be noted that clouds located at horizontal distances 166
between 30-90km range in the 3GHz radar scans are estimated to overpass the 35GHz radar 25-167
75 minutes later (based on a typical wind speed of ~20m s-1 along the radar profile). Although 168
comparisons between these two radar datasets will not be fully correlated as the system may 169
change during transit, it reveals useful general trends of the system over time. 170
8
In figure 3a, we can see a change in the height of the freezing level or “melting layer” (referred 171
to as ML). The ML appears as a bright band in the reflectivity field, the ZH signal of which 172
increases depending on the type and concentration of the hydrometeors that enter this region 173
[Fabry and Zawadski, 1995]. ZH increases within the ML due to the change of the phase and shape 174
of melting hydrometeors, which also affects ZDR [e.g. Stewart et al., 1984; Zrnic et al., 1993; 175
Szyrmer and Zawadski, 1999; Giangrande et al., 2008]. Before the passage of the warm front 176
(<2215UTC) the ML was observed at ~0.7-1km (ZH_35 reached 30dBZ at 0.75km; figure 3a). After 177
the passage of the warm front, the ML height raised to ~1.6km (2215 UTC) (figures 1a, 3a). The 178
ML is also depicted as a zone of dramatic increase of Doppler velocity (from ~1 to >4m s-1) 179
retrieved by the 35 GHz vertical-pointing radar (figure 3b), because hydrometeors may become 180
larger in size due to coalescence and denser due to melting [Mitchell, 1996; Heymsfield and 181
Iaquinta, 2000; Protat and Williams, 2011]. 182
Data from the 35 GHz vertical-pointing radar (figure 3a) show that the cloud top height 183
decreased (from 6.5km to 5km) at a mean rate of 0.24km/hour during the frontal passage, while 184
the cloud top temperature increased from ~-35°C to ~-25°C according to radiosondes (figure 4). 185
The lowering of cloud top height with time, which can be seen in figure 4c as a decrease of RH 186
with time by 50% at altitudes 6-7km, is also perceptible from figures 2b (3GHz radar) and 3b 187
(35GHz radar). The 35GHz radar data (figure 3c) show that >50% of the pixels were considered 188
as cloudy (where ZH>-40dBZ) at altitudes ≤5.5km for 1800-1900UTC. However, this height 189
gradually decreased to 4.5km (1900-2000UTC) and 4km (2100-2200UTC). The corresponding 190
heights for the 3GHz radar were always observed ~0.5km lower than the 35GHz radar (figure 2b), 191
as the 3GHz frequency is mainly used for the detection of precipitation and not of smaller cloud 192
particles. According to figure 3b, Doppler velocities between 0 and -1m s-1 were observed at cloud 193
tops (height >4km), which is consistent with weak vertical motions and slowly precipitating low 194
density ice crystals (like the small ice-plates and columns at altitudes >4km and temperatures <-195
15°C in figure 5) [Jayaweera and Cottis, 1969; Heymsfield, 1972; Kajikawa, 1972]. 196
9
In order to gain insight into the presence of the aforementioned features, a statistical analysis 197
of the ZH and ZDR datasets is shown in figures 2c, e and 3c. In relation to ZH fall-streaks, in figure 198
2c, ZH seems to increase through the time between 1646-1932UTC (95th percentiles of ZH ~20dBZ 199
at 2-4km height). At 1912-1932UTC, although average ZH decreased significantly to ~10dBZ, 200
high ZH 95th percentile values (>20dBZ) were observed at 2-4km, highlighting the presence of 201
discrete GCs. This is also demonstrated by the 35GHz radar dataset, as 95th percentiles of ZH-35 202
around the ML were enhanced at 2000UTC (from 15dBZ to ~30dBZ). This ZH-35 increase, but also 203
the enhanced 95th percentiles at 2-4km, implies the occurrence of more frequent and more intense, 204
but spatially restricted GCs, which occasionally affected the ML echo (figure 3d). This is also 205
demonstrated by the fact that ZH-35 increased in the ML and decreased at 2-4km with time, while 206
95th percentiles signals at 2-4km remained significantly high (~15dBZ). In addition, the vertical 207
pointing-radar at 1900-2030UTC observed larger Doppler velocities (-2 to -3m s-1) comparing to 208
the previous time period at 0.7-3km height, which indicate the existence of denser hydrometeors 209
(like the large ice particles at altitudes <3.5km in figure 5). 210
Switching focus to ZDR fall-streaks, these features can be observed in figure 2e as local ZDR 211
maxima at altitudes of ~4km (slightly higher than ZH fall-streaks). The ZDR fall-streaks manifest 212
mainly as perturbations to the 95th percentile (0.5-2dB) relative to the rest of the profile affecting 213
also the mean value (0.5-1dB). The ZDR fall-streaks are mainly observed during the second half of 214
the observing period, 1800-1932UTC. 215
The WCB was a clearly identifiable dynamical feature which likely “fed” the cloud system 216
with water vapor. It is presented, in the vertical-pointing radar dataset, as a zone of Doppler 217
velocity ~0m s-1, where strong westerlies (typically >20m s-1) of the WCB advected the 218
hydrometeors almost horizontally (figure 3b). Whilst the wind speed profiles suggest significant 219
stratification at all times (figure 4d), it seems that the highest wind speeds were observed near 220
cloud top, where relative humidity (figure 4c) and cloudiness (figure 3c) dramatically decrease. 221
10
The main region of high wind speeds (20-30m s-1) (figure 4d) and positive wind shear (figure 2h) 222
was firstly located at 5-6km (1500-1600) but gradually lowered to 3.5-4.5km (after 18:00UTC). 223
However, before 1800UTC, there were also other enhanced wind speed layers along the profile, 224
which likely represent slight differences in air mass characteristics/origin. As an illustration, two 225
well-distinguished layers centered at altitudes of ~2.5km and ~4km at ~1700UTC (figure 2f), 226
coincided with wind speed (17-20m s-1), relative humidity (>90%), clock-wise wind veering (~60o 227
km-1 comparing to the average of ~20o km-1 for altitudes 0-6km) and potential temperature (~ +9K 228
km-1 comparing to average rate of +4K km-1 for altitudes 0-6km) maxima (figures 4c, d, e, f). At 229
the same altitudes, temperature presented the highest increase (up to 1.3oC h-1) between 1500-230
1800UTC. All the above observations reveal that there was a main warm front at ~4km and a 231
possible sub-frontal zone at ~2.5km, the locations of which were associated with defined structures 232
in the wind speed and temperature data. As a general trend, it seems that as the warm front was 233
approaching, the various wind speed layers tended to merge (only one significant wind speed peak 234
can be observed at ~4km at 1800UTC – figure 4d) and were located at lower altitudes (figures 235
2g, h). 236
237
5 Microphysical properties of the warm front 238
Using in-situ and remote sensing data, the microphysical properties in specific features of the 239
warm frontal cloud, such as the WCB, ice fall-streaks, and regions of secondary ice production are 240
investigated. 241
a. The Warm Conveyor Belt (WCB) 242
The WCB originated in the warm sector of the deep low-pressure system shown in figure 1a 243
and, during the observing period, it intersected the surface in the Celtic sea. It was responsible for 244
11
the large-scale slantwise ascent of humid air, which led to widespread formation of mixed-phase 245
clouds. 246
At 1600UTC, CAMRa scans to the WSW identified three zones of enhanced Doppler Velocity 247
(up to ~20m s-1) at altitudes of 2.5km, 4km and 6km respectively (figure 6a). These three zones 248
caused significant vertical wind shear (up to -12m s-1 km-1), which could potentially release 249
instability and form convective GCs. Similar regions of wind shear were also identified in the wind 250
speed and direction data from radiosonde profiles (figure 6b). In regions where the radar scan 251
azimuth (255o) is closely aligned with the wind speed, (at altitudes 2.5-3km), the radiosonde wind 252
speed is in good agreement with Doppler velocity. 253
At 1800UTC, as the warm front approached the radar site, a broad slanted zone of high Doppler 254
velocities (up to 23m s-1) was observed spanning altitudes of 2-5km (figure 6c). The high Doppler 255
velocity zone above 3-5km highlighted the WCB location and presented enhanced wind speed 256
(15.6-26.0 m s-1) and wind direction shear (21o km-1). The peak wind speed was located near the 257
middle of the WCB, with two zones of wind speed and direction shear located below (+8.7m s-1 258
km-1, ~47o km-1) and above it (-3.8m s-1 km-1, ~+10o km-1). Again, there was a good level of 259
agreement between Doppler velocity and measured wind speed for wind directions close to the 260
radar beam azimuth (255o) (figure 6a-d). Comparing and linking the Doppler velocity with 261
radiosonde data, we estimate the main warm front location is as shown in figure 6c by the solid 262
red line. Smaller fluctuations in Doppler velocity fields and radiosonde wind profiles indicate the 263
presence of a small “sub-frontal” zone located at ~2km (dashed red line). 264
At 1935UTC, the frontal system had transited further to the east over the ground site. At this 265
time the WCB was located ~2km closer to the surface (between 1-3km), being represented by 266
distinct zone of Doppler velocities between 17-25m s-1 (figure 6e). Aircraft observations obtained 267
through the WCB at this time indicate the presence of significant liquid water (figure 6f) (up to 268
0.22g m-3) and cloud droplet concentrations (10-58cm-3), while IWC dramatically increases 269
12
outside the WCB (up to 0.044g m-3). These observations support the idea that slantwise ascent 270
from the WCB generates a large expanse of cloud containing super-cooled liquid water. The 271
lifetime and radiative properties of similar super-cooled layer clouds is sensitive to the presence 272
of Ice Nuclei [e.g. Pinto, 1998; Jiang et al., 2000; Morrison et al., 2005; Murray et al., 2012], 273
which can activate to form ice in the cloud leading to poor representation in weather models. 274
275
b. The Generating Cells (GCs) 276
As the warm front approached the UK, some GCs appeared in RHIs, especially after 1800UTC. 277
Despite the lack of in-situ measurements within these features, we try to investigate their 278
microphysics using the available radar data. In general, GCs were observed above the WCB and 279
above a braided structure [Chapman and Browning, 1998] associated with sheared wind (figure 280
7a). Sheared wind is associated with Kelvin-Helmholtz instability [Browning, 1971; Browning et 281
al., 1973], which is a mechanism that can trigger convection [Hogan et al., 2002]. 282
At altitudes where GCs appeared (4.5-6.5km), the rate of change in potential temperature with 283
altitude recorded by the radiosondes at 1700 and 1800UTC was 0-5K km-1 (figure 4f). It is 284
important that a small region of negative rate is observed at ~5km, which coincides with the typical 285
location of GCs, implying that GCs formed due to the release of instability through the Kelvin-286
Helmholtz mechanism. In the meantime, the wind speed exhibited an increasing trend with time 287
within the WCB (4.2km) (figure 4d). This increased the wind shear from ~4 to ~8m s-1 km-1 at 288
1700 and 1900UTC respectively at the top layer of the WCB, which also lowered from 4.5 to 289
3.5km (figure 2h). Although a radiosonde cannot provide representative data over a wide area, the 290
above measurements indicate the occurrence of instability and a triggering mechanism which can 291
explain the presence of GCs. As stated earlier (beginning of section 4), GCs demonstrated a core 292
of ZH >10dBZ, which increased to 20-33dBZ as the warm front approached the Chilbolton site. 293
13
GCs also demonstrated high ZDR at cloud tops (>2dB), which indicate regions of newly formed 294
ice, in which ZDR fall-streaks originated (e.g. figure 7e, range=85-95km). 295
In figures 7a-h, we present the evolution of a GC. At ~1928UTC (figures 7a, e), a GC was 296
triggered at approximately 90km distance from CFARR, above the WCB. The Doppler velocity 297
below the GC was ~ -23m s-1, but only ~ -14m s-1 in the newly formed GC, indicating wind shear 298
at the top of the WCB (figure 7a). The above observations suggest that the trigger which caused 299
the formation of GCs was the wind shear at the WCB top layer (Kelvin-Helmholtz instability). 300
Through this mechanism, significant amounts of liquid water from the WCB are lofted via 301
turbulence and weak updrafts. In the example presented in figures 7i, j, k, the aircraft measured 302
high cloud droplet concentration and LWC (up to 10cm-3 and 0.3g m-3 correspondingly) at the rear 303
region of a newly formed GC (located within the WCB at 55km<range<65km, 304
2km<altitude<3km). It is important to highlight that no positive values of Doppler velocity were 305
recorded by the vertical-pointing radar (figure 4d). According to the CAMRa, GCs were generally 306
forming at ranges >30km away, passing overhead the vertical-pointing radar as dissipated cells or 307
ZDR/ZH fall-streaks. In addition, due to short time of GC genesis (5-15minutes) and their narrow 308
updraft region (typically <5km), it is not highly likely that a GC was captured overhead the radar 309
during its genesis time. 310
The GC was further developed in height 10-20 minutes later (figures 7b, f and c, g), which 311
caused the intensification of the aggregation and riming processes and affected the ZH parameter 312
(ZH increased to ~32dBZ). At the later stage (figure 7g), a distinct region of very high ZDR and 313
low-medium ZH was observed (up to 4dB, <17dBZ) at the GC top. This suggests regions of newly 314
formed (as there was not such a region earlier) pristine ice-plates/dendrites at -15°C (figure 7f) 315
[e.g. Magono and Lee, 1966; Schrom et al., 2015; Bailey and Hallett, 2009; Schrom and Kumjian, 316
2016]. The fact that these ZDR structures, which indicate the presence of un-rimed ice particles, 317
typically form at the latter stages of GC formation, suggests that they result from ice nucleation in 318
regions where the initial convective feature has mostly decayed, but that significant regions of 319
14
supersaturation still remain. Finally, as the GC core sediments/precipitates into the WCB (figure 320
7d, h), the associated ZH structure changes from a largely vertical orientation, to being more 321
horizontally elongated due to strong westerlies within the WCB (figures 14d, i). The evolution of 322
this GC shows the connection between fall-streaks and GCs. Another example is also shown in 323
figures 12c, f (40km<range<60km, 2km<altitude<4km) 324
325
c. The ZDR and ZH ice fall-streaks 326
(i) General Characteristics 327
At 1800UTC, two zones with different radar polarimetric traits were detected within sheared 328
ice fall-streaks. ZDR fall-streaks appeared as slanted zones of significantly high ZDR (>1.5dB) and 329
low ZH (<15dBZ) values, containing pristine ice crystals (hexagonal plates and dendrites) [e.g. 330
Andric et al., 2013; Schrom et al., 2015; Schrom and Kumjian, 2016]. ZH fall-streaks, which 331
appeared as a zero ZDR (-0.5 to 1dB) and high ZH (>15dBZ) zone, were typically observed beneath 332
a related ZDR fall-streak containing mostly aggregated and rimed ice crystals. Essentially, these 333
fall-streaks were represented by a bipolar zone of high-low ZDR (or low-high ZH) zones (similar 334
fall-streaks have been investigated by Bader et al. [1987], Kumjian [2014], Oue et al. [2015]). 335
These zones originate in and descend from GCs (section 5b), as ice crystals fall into sheared flow 336
[Kumjian, 2014]. ZH fall-streaks can be also enhanced by pristine ice crystals precipitating into it 337
from the overlying ZDR fall-streak. 338
Examining the entire in-situ and polarimetric radar dataset, observations of ZDR >1.5dB were 339
mostly (>60% of the observations) linked with low ice concentrations (<2L-1) and small-sized ice 340
particles (<800μm) (figure 8a, b). Taking into account previous literature [e.g. Schrom et al., 341
2015; Schrom and Kumjian, 2016] and the fact that the data around the ML (<1km) were excluded, 342
it seems that large ZDR values are mainly linked with small pristine ice crystals in small 343
concentrations within the ZDR fall-streaks. Figure 8c also shows that regions of ZDR<1dB are 344
15
partially associated (by >45%) with observations of ZH >15dBz due to ZDR fall-streaks, while 345
regions of ZDR >1dB are associated (by 60-70%) with smaller ZH (ZH <15dBz) due to ZH fall-346
streaks. It should be highlighted that regions of lower ZDR values (in general <1.5dB such as ZH 347
fall-streaks) were observed to have a larger variety of ice particle number concentrations (from 348
<4L-1 to 10L-1) and mean size (from <500 to 3600μm). Smaller crystals in larger concentrations 349
can be explained by the effects of secondary ice production (see section 5d). To conclude, although 350
the ZH/ZDR fall-streak radar polarimetric boundary is not particularly clear, a rough threshold for 351
distinguishing them could be: ZH ~15dBZ, ZDR~1dB. 352
An example of the evolution of large scale, moderately intense ZDR/ΖΗ fall-streaks is illustrated 353
in figure 9. The first fall-streak started to form at ~1549UTC (figures 9a, e), being located at 354
range=100-110km and height=4-6km. In this region, a spatially more extensive but less intense, 355
in terms of ZH (ZH <17dBZ), GC occurred (compared to the GC investigated in section 5b). At 356
this stage, the ZDR fall-streak was relatively modest, occasionally approaching 1-2.5dB. The 357
corresponding ZH structure was also relatively modest, exhibiting values of ~15dBZ, which was 358
an enhancement of ~10dBZ over the ZDR fall-streak. As the wind speed in the WCB appears to 359
increase ~25 minutes later (16:13 UTC, figures 9b, f), both ZDR and ZH fall-streak signals become 360
more obviously enhanced (up to 2.7dB and 24.5dBZ respectively). The ZH fall-streak coincided 361
with a region of ZDR~0dB, and is first observed in a region of enhanced Doppler velocity (>20m 362
s-1). As this region was part of the WCB (which transported large amounts of liquid water), it is 363
highly likely that ice crystals became intensely rimed and less oblate within it. Ten minutes later 364
(~16:23, figures 9c, g), the ZH fall-streak was represented by -0.6dB<ZDR<1.5dB and ZH >15dBZ, 365
containing large (~984μm) aggregated and heavily rimed dendrites (figure 9i) in concentrations 366
of ~1.4L-1 (figure 9m). The ZH fall-streak was an almost glaciated zone typically exhibiting LWC 367
~0g m-3 and IWC >0.01g m-3. 368
16
After a further 30 minutes (~16:55), the fall-streak was fully developed demonstrating the 369
largest length of all the observed fall-streaks (~60km) during the day, and an expansion rate of 370
~17m s-1. Although the ZDR fall-streak was characterised by ZH<15dBZ and ZDR>1dB due to small 371
concentrations of ice-dendrites and plates (figures 9d, h), it was actually divided into two sub-372
fall-streaks: one (figure 9d, h; range=65-85km, ZDR >1.5dB, T~ -13°C) with small (~580μm) ice-373
plates (fig. 9j – frames 3-4) and another (figure 9d, h; range=55-65km, ZDR ~1.5dB, T~ -13.5°C) 374
with large (400-1400μm) ice-dendrites/dendrite aggregates (fig. 9j – first two frames; 9n), both 375
in small concentrations (<1 L-1 and <5L-1 respectively). We speculate that the sub-fall-streak which 376
contained ice-dendrites, formed when small and light ice-plates drifted away from the initial ZDR 377
fall-streak due to strong westerlies and remained within the WCB for a longer time. As a result, 378
these plates grew into ice-dendrites due to the high supersaturation regime at ~ -13oC (figure 9d) 379
[Bailey and Hallet, 2009]. In the transitional zone between the ZDR and ZH fall-streak; (figure 9d, 380
h; range=90km, height=3.5km), small graupel particles mixed with slightly larger pristine plates 381
were observed in small sizes (~460μm), but in larger concentrations (up to 8L-1) (fig. 9j – last two 382
frames). In total, although this enhanced ZDR fall-streak reached the ML retaining its 383
characteristics, there was a decrease of ZDR slightly above the ML, presumably due to aggregation. 384
An important conclusion that arises from both sections 5b and 5c(i) is that the slope of the fall-385
streaks increased as the warm front was approaching. In particular, the slope of the ZDR fall-streak 386
at 1927UTC (figures 7a, e) was ~5o comparing to ~3o and ~7o at earlier (16:56UTC) and later 387
(20:07, figure 7d) times. It seems that the wind speed intensity of the WCB (discussed in sections 388
4 and 5a) and, thus, the wind shear intensity at its upper boundary, played an important role in the 389
formation and shape of the fall-streaks. Thus, stronger wind shear might cause stronger updrafts 390
forming larger and heavier hydrometeors in a shorter period of time, which fell faster towards the 391
surface. 392
393
17
(ii) The impact of the ZH fall-streak on the surface precipitation 394
The identification of the ZH fall-streak is very important as it is directly related to precipitation 395
enhancement at the surface. In figure 10, we present the evolution of the ZH/ZDR fall-streak dipole, 396
which was described in section 5c(i), with the corresponding rain rate product from the UK Met 397
Office operational radar network (NIMROD). 398
The ZH fall-streak remained elongated (length ~60km) for ~30mins (1656-1717 UTC) before 399
dissipating, exhibiting up to ZH ~28dBZ. In the initial stages (1705UTC; (figures 10a, d) it 400
appeared to be moving slantwise downwards, being represented by ZH_3GHz >20dBZ and ZDR~0dB. 401
As the ZH fall-streak evolved over the next hour, ice crystals moved slantwise downwards, with 402
aggregation and riming leading to greater and greater enhancements of the ZH signal. At 1804UTC, 403
a region of significantly high ZH (30-47dBZ) was observed at the ML (~0.7km). Such enhanced 404
ZH values imply heavy aggregation due to higher temperatures (>-3.5oC; figure 2d) and, thus, 405
“stickier” ice crystals. Comparing the surface rain rate graphs (figures 10i-l) with the RHIs, it 406
seems that the surface precipitation is strongly affected by the ZH fall-streak. In particular, the rain 407
rate increased (at range=30km; fig. 10i-l) from <1mm h-1 to 5mm h-1. This suggests that fall-streaks 408
which are initiated near cloud top, and develop over time scales of approximately an hour, have a 409
significant impact on surface precipitation rates downstream. 410
Finally, although a co-polar correlation coefficient (ρHV) was not available in order to estimate 411
the ML height [e.g. Giangrande 2008; Boodoo et al., 2010], ZDR (2-4dB) peaked around the ML 412
at different heights in (range=32km) and out (range=25km) of the ZH fall-streak (figure 11). In 413
particular, at 1800UTC, the ML was located at 0.8km (figure 4a), which broadly agrees with the 414
ZDR/ZH peak [Brandes and Kyoko, 2004; Houze, 2014, pg.144] of the 25km vertical profiles 415
(figure 11, green lines). However, a slight lowering of the ZDR peak height was observed within 416
the enhanced ZH region (by 0.2-0.4km). The local depression of the melting layer was possibly 417
caused by the melting of large aggregates [Stewart,1984; Stewart et al., 1984; Oraltay and Hallett 418
18
2005; Griffin et al., 2013]. This is important as ~0oC isothermal layers can be produced close to 419
the surface allowing (partially melted) ice to precipitate [Findeisen, 1940; Szeto et al., 1988; 420
Ryzhkov et al., 2011; Griffin et al., 2013]. 421
d. Secondary ice in the ZH fall-streak 422
In this section, we try to analyse concurrent in-situ and radar data in order to detect, determine 423
and investigate the characteristics of regions, which appear to be greatly influenced by secondary 424
ice processes. 425
Between 19:12:54-19:17:42, the aircraft passed through a region (range=50-70km) of low-426
medium ZH (8-12dB) and low ZDR (0.5-1dB), which was located at the entrance of the WCB. 427
Although this region was located within a weak signal cloud top, we speculate that this region is a 428
mature/dissipated ZH fall-streak. At the rear of this mature ZH fall-streak (range=65-80km), which 429
was located within the WCB entrance (Doppler velocity >20m s-1), the aircraft measured enhanced 430
LWC and cloud droplet concentration (up to 0.17g m-3 and 16.7cm-3 respectively). In contrast, at 431
the bottom of the WCB (range=50-65km), both cloud droplet concentration and LWC decreased 432
by 10cm-3 and 0.10g m-3 respectively. This liquid water removal may imply intense riming, which 433
is a mechanism assisting in ice multiplication [Mossop and Hallett, 1974; Choularton et al., 1978; 434
Cloularton et al., 1980]. The occurrence of small (~300μm in average) ice-columns (figure 12g; 435
2D-S frames 1-4) in high ice number concentrations (up to 37.8L-1) and the enhanced IWC (0.11g 436
m-3) observed at temperatures ~ -4.8oC imply that Hallett-Mossop is the possible ice multiplication 437
mechanism here. Some aggregated ice-columns were observed at range=65-80km (figure 12g, 438
CIP-100 frames 1-2) formed due to the occurrence of ice-columns within a region of high LWC. 439
Further to the east (figure 12c, f, h, i), a region of quasi-spherical rimed ice particles (figure 440
12g; 2DS frames 4-5, CIP-100 frame 3) and supercooled water drops mixed with some ice-441
columns (range=43-55km; T ~ -4°C; ZH up to 28.6dB and ΖDR~ 0.5-1dB) was observed, followed 442
by another region of mostly ice-columns (range=31-43km; T ~ -5°C). As this region belonged to 443
19
a short and newly formed ZH fall-streak originating in the core of a GC (range ~53km, altitude 444
~3km), processes, such as riming and ice-multiplication could be at early stages. This may explain 445
the increased, but lower than the previous secondary ice region, ice concentrations (up to 14.1L-446
1). Decreased LWC (<0.005g m-3) and cloud droplet concentration (<0.4cm-3) could be measured 447
due to riming process and the aircraft position (outside the WCB). It should be noted that some 448
pristine ice crystals from the ZDR fall-streak above (range=40-50km) rimed (an example in figure 449
12; CIP-100 frame 4) and became heavier, when they moved into the WCB. As a result, they 450
might fall into the ZH fall-streak, which was finally characterized by small quasi-spherical (and 451
possibly mixed-phase) ice particles/supercooled water drops, but also of ice-columns and ice-452
lollies [Keppas et al., 2017]. 453
454
6 Conclusions 455
On 21 January 2009, multiple frontal zones affected the UK weather due to a maturing 456
depression, which originally formed over the North Atlantic Ocean. In the work presented, the 457
dynamics and microphysics of mixed-phase clouds, with embedded convective elements, 458
associated with the warm front of this system are investigated comparing high resolution (0.28° 459
beam) dual-polarisation S-band radar with in-situ aircraft data. The latter data collected by 2D-S 460
probe (providing better resolution than 2D-C), which were used together with CIP-100 probe data 461
in order to provide a wider size range of measured particles. It should be noted that this is the first 462
time that a warm front is comprehensively investigated with high resolution data. 463
A few studies previously investigated warm fronts and GCs comparing both in-situ and radar 464
datasets. Here we discuss some of their results, which generally come to an agreement with the 465
results of the present study. Herzegh and Peter [1980] found that large ice particles within GCs 466
grew by riming, deposition and aggregation as occurred in ZH fall streaks. They also suggested 467
that GCs work as feeders of ice to stratiform clouds below. According to Matejka et al. [1980], as 468
20
GCs mature, they tend to be glaciated. They supported that surface precipitation is mainly 469
associated with embedded convection in warm fronts. Hogan et al. [2002] demonstrated some 470
evidence that embedded convection within a warm front is triggered by Kelvin-Helmholtz 471
instability. However, they found that embedded convection was linked with narrow vertical high 472
ZDR zones of ice columns, which coincided with updrafts and was a feature that was not observed 473
in the present study. Murakami et al. [1992] observed only a shallow layer of supercooled droplets 474
above the warm frontal zone. Additionally, they found that GCs provided a favorable environment 475
for ice crystals to grow rapidly, which agrees with Plummer et al. [2014]. Finally, Plummer et al. 476
[2015] noticed that ice mainly grew in regions below GCs, where enhanced moisture was 477
observed. 478
As a conclusion, the general structure of the warm front is schematically summarised in figure 479
13, according to the airborne in-situ and ground-based polarimetric radar measurements. The main 480
conclusions of the study are outlined as follows: 481
Regarding the macroscopic characteristics of the investigated warm front: 482
The height of cloud tops gradually decreased from 6km to 4.5km as the warm front was 483
approaching. Cloud top temperatures were, generally, >-25oC, which implies the ice in 484
the cloud system was formed via primary heterogeneous processes, and potentially 485
enhanced by secondary multiplication processes. 486
The height of the 0oC level, or the melting layer (ML), was firstly located at 0.7km 487
rising by almost 1km after the passage of the warm front. 488
The horizontal dimension of the cloud mass was larger than 120km, the maximum 489
range of the RHIs presented in this work. 490
The cloud mass consisted of distinctive features which demonstrated definite radar 491
polarimetric and/or microphysical characteristics. These features are: 492
21
the Warm Conveyor Belt (WCB), was initially depicted by a group of multiple zones 493
of enhanced Doppler velocity. These multiple zones merged into a single one, which 494
decreased in altitude, as the warm front moved further inland away from the coast. High 495
radar Doppler velocities (20-30m s-1) and radiosonde wind speeds (up to 26m s-1) were 496
recorded within the WCB presenting, essentially, a fair agreement for wind directions 497
along the radar azimuth. The WCB was vitally important for transporting large amounts 498
of liquid water (up to 0.37g m-3) into the system, offering at the same time an ideal 499
regime for secondary ice production at temperatures ~ -6oC. 500
the Generating Cells (GCs), formed at the unstable, based on potential temperature 501
profiles, layer above the WCB, where vertical wind shear triggered Kelvin-Helmholtz 502
instability. The GCs were represented by a core of high-ZH (10-33dBZ)/~0dB ZDR and 503
a shell of low-ZH (<10dBZ)/high-ZDR (up to 4dB). 504
the ice fall-streaks, formed as GCs were moving into a sheared flow caused by the 505
WCB. The strong westerlies “converted” the GCs’ high-ZDR shells and high-ZH cores 506
into an ice fall-streak, which consisted of two individual fall-streaks with different 507
polarimetric characteristics: a ZDR and a ZH fall-streak respectively. Ice fall-streaks 508
exhibited lengths of up to 60km and slope of 3o, which increased with the time (up to 509
7o) due to the intensification of the GCs. The ZDR fall-streaks were divided into two 510
low ZH (<15dBZ) subzones: a.) a zone of ice-plates with ZDR>1.5dB and b.) a zone of 511
dendrites with ZDR~1.5dB, which might form from small ice-plates that were moving 512
from GC shells to the high supersaturated regions of the WCB. As icing can be harmful 513
for aviation in dendritic ice regions, this can help to improve existing icing detection 514
algorithms [Serke et al., 2008; Pitersev and Yanovsky, 2011; Ellis et al., 2012]. Beneath 515
the ZDR fall-streaks, ZH fall-streaks were represented by high-ZH (>15dBZ) and 516
ZDR~0dB being consisted of mostly rimed and/or aggregated ice crystals in variable 517
sizes (from <500μm to 3600μm) and concentrations (from <4L-1 to >10L-1). ZH fall-518
22
streaks that form higher up in the clouds can affect the surface precipitation. The time 519
and spatial evolution depends on the profile of wind direction and speed in the clouds. 520
As an illustration, a case where a ZH fall-streak enhanced the surface precipitation by 521
4mm h-1 an hour after its genesis was presented. 522
the secondary ice regions, which presented large ice concentrations (up to 37.8L-1) 523
and mostly consisted of ice-columns. Such regions were located within the intersection 524
region of the ZH fall-streaks with the WCB, where riming was intense at temperatures 525
~ -5oC and liquid water was dramatically depleted potentially affecting the cloud 526
lifetime. 527
In the present paper, both heterogeneous ice nucleation and secondary ice formation were 528
observed in mixed-phase clouds, which are highly uncertain processes. Heterogeneous ice 529
nucleation plays an important role in the formation of GCs, as new ice forms from freezing liquid 530
water transported aloft along the rear updraft region of GCs. As in-situ measurements within GCs 531
and ice-fall-streaks are limited, more observations should be performed aimed at investigation of 532
the cloud processes (aggregation, riming, ice multiplication) in order to obtain a deeper 533
understanding of cloud microphysics. Except for observing real GCs, they could also be created 534
and observed in labs. Comprehemsive datasets, which could describe the structure and evolution 535
of such clouds could be used for the validation of complex microphysics models [Stoelinga et al., 536
2003; Keeler et al., 2016a]. 537
538
539
540
Acknowledgements 541
23
The APPRAISE-Clouds programme was funded by NERC (grant NE/E01125X/1). Radar and 542
aircraft datasets were made available by the British Atmospheric Data Centre (BADC). We would 543
like to acknowledge the support from FAAM and Direct Flight in obtaining the aircraft dataset. 544
ECMWF Interim Re-Analysis archive data were used for the brief presentation of the synoptic 545
condition of the present case (http://data.ecmwf.int/data/). Nimrod rain rate data were obtained 546
from UK CEDA (http://catalogue.ceda.ac.uk/). We would like to thank anonymous reviewers and 547
Prof David Schultz for the valuable feedback they provided, which improved the quality of the 548
paper. Finally, Mr Keppas was funded by the University of Manchester. 549
550
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892
40
Figure Captions 893
Figure 1. ECMWF Re-Analysis (ERA Interim) (0.125o resolution) for 21/01/2009 at 18UTC. (a) 894
Temperature (oC) on the 850mb pressure level (colour shading and dashed contours) and mean 895
sea level pressure (mb) (white contours). Surface fronts have been plotted in accordance with 896
UK MetOffice analysis charts. (b) Vertical section of relative humidity (colour shading) and 897
wind speed/direction (standard wind barbs) across (purple line in figure 1a) the investigated 898
warm front at 18z. Warm front position is highlighted by the red line according to wind and 899
potential temperature fields. 900
Figure 2. 3GHz radar RHI scans of (a) ZH (dBZ), (d) ZDR (dB) and (f) unfolded Doppler 901
velocity (m s-1) at 17:11:00-17:12:30UTC (negative values indicate flow towards the radar). (b) 902
Percentage of cloudy pixels (ZH dataset was used) with height. Graphs (c), (e), (g) and (h) show 903
average (solid lines) and 95th percentiles (dots) of ZH, ZDR, Doppler velocity and vertical wind 904
shear produced from Doppler velocity, which were calculated from the 3GHz radar dataset for 905
30≤Range≤90km and 0≤Altitude≤8km for the most representative scans during each time period 906
showed in the legend. Different colours indicate different time periods for (b), (c), (e), (g) and 907
(h). 908
Figure 3. (a) ZH (dBZ) and (b) Doppler velocity (m s-1) evolution with time as captured by the 909
35GHz radar. In (a) the melting layer height is highlighted by the blue (time <2100UTC), the 910
green (2100UTC< time <2215UTC) and the red (time <2215UTC) dashed lines. The black 911
dashed line shows the linear fitting of the cloud tops height between 1500-2100UTC. The 912
coloured lines on the left corner indicate the time that radiosondes of figure 4 were launched 913
comparing to the time colourscale of the figure 3a. In (b) negative Doppler velocity indicates 914
flow towards the radar. (c) Similar to figure 2b. (d) shows average (solid lines) and 95th 915
percentiles (dots) of ZH as calculated from 35GHZ radar data. Both (c) and (d) were calculated 916
from the first 10 minutes of data of each hour. 917
Figure 4. Graphs demonstrating (a) temperature (oC), (b) potential temperature (K), (c) relative 918
humidity (%), (d) wind speed (m s-1), (e) wind direction, (f) change of potential temperature 919
with height obtained from radiosonde data. Different colours indicate different times and were 920
selected to be comparable with the colour scheme of figures 2b, c, e, g and 3b, c, e. 921
Figure 5. Example of ice particle images captured by the 2D-S probe. Ice particles are grouped 922
by the temperature (left axis) and altitude (right axis) at which they were observed. 923
Figure 6. Unfolded Doppler velocity (m s-1) in RHI scans for three scanning periods: (a) 924
16:00:50 – 16:02:20, (c) 18:00:06 – 18:01:35, (e) 19:35:13 – 19:36:43 (the grayscale line shows 925
the aircraft track between 19:33:34 and 19:40:05 -black for the initial and white for the final 926
position). Negative values indicate flow towards the radar. The orange dashed line is the -20m s-927
1 contour. The solid red line in figure (c) determines the warm front boundary location and the 928
dashed red line the possible sub-front. (b) and (d) show radiosonde wind speed and direction 929
(colour scale) over the Chilbolton region (shaded curve) and Doppler velocity (blue dashed line) 930
along the vertical black line on graphs (a) and (c) correspondingly. (f) shows IWC, LWC and 931
cloud droplet concentrations for the corresponding aircraft track in (e) at T~ -1.9oC. 932
41
Figure 7. RHIs of ZH (dBZ): (a)-(d) and (i); and of ZDR (dB): (e)-(h) and (j). The horizontal 933
grayscale line shows the aircraft track between 19:03:00-19:07:48 in (i) and (j) (white for the 934
final, and black for the starting position). The mean temperature recorded along the track was ~ -935
7°C. The black contour line in (a)-(d) is for Doppler velocity equal to -20m s-1 and in (e)-(h) for 936
ZH=15dBZ. In figure (a) the red contour presents wind shear dV/dz=7m s-1 km-1. Figure (k) 937
shows LWC and cloud droplet concentration along the aircraft track in (i) and (j). 938
Figure 8. Probability distribution functions of different ZDR thresholds for in-situ (a) ice number 939
concentration, (b) mean ice particle diameter and (c) ZH. The data used for (a) and (b) come 940
from regions where the aircraft flew through ZDR/ZH fall-streaks. The data used for (c) come 941
from the 3GHz-radar for ranges of 30-90km and elevations >1km (to avoid the ML and focus on 942
fall-streaks) between 16:61:21-19:28:28 943
Figure 9. RHIs of (a)-(d) ZH (dBZ), (e)-(h) ZDR (dB). The grayscale line shows the aircraft 944
track between 16:21:13-16:24:22 in (c) and (g) and 16:52:39-16:59:06 in (d) and (h) (white for 945
the final, and black for the starting position). The black isoline in (a)-(d) is for Doppler velocity 946
equal to -20m s-1 and in (e)-(h) for ZH=15dBZ. Figures (i) and (j) show 2D-S images captured 947
during the flight track in (c), (g) and (d), (h) at temperatures -7.8°C and -12.9°C respectively. 948
Figures (k) and (l) show IWC/LWC and ice number concentration/ice mean diameter for the 949
aircraft track in (c), (g). Similarly, (l) and (n) correspond to the aircraft track in (d) and (h). 950
Figure 10. As in figure 9. Figures (i), (j), (k) and (l) show the surface rain rate (Nimrod) along 951
the radar range of the RHIs in (a,e), (b,f), (c,g) and (d,h) respectively. 952
Figure 11. (a) ZDR and (b) ZH profiles (0-1km height) from data taken from the radar scans 10d 953
and 10h. Green curves depict the profiles out (range=25km) and red curves within the ZH fall-954
streak (range=32km). 955
Figure 12. As in figure 9. In (g)) both 2D-S and CIP-100 images are shown. The time and range 956
from the Chilbolton radar are indicated at the bottom of the figure in order to be comparable with 957
the radar scans. In (h), the green curve shows the cloud droplet concentration. The mean 958
temperature along the aircraft track was ~ -4.8°C. 959
Figure 13. A schematic representation summarizing the investigated processes and polarimetric 960
signatures of the frontal clouds observed. 961
42
962
963
964
Figure 1. ECMWF Re-Analysis (ERA Interim) (0.125o resolution) for 21/01/2009 at 18UTC. (a) Temperature (oC) 965
on the 850mb pressure level (colour shading and dashed contours) and mean sea level pressure (mb) (white contours). 966
43
Surface fronts have been plotted in accordance with UK MetOffice analysis charts. (b) Vertical section of relative 967
humidity (colour shading) and wind speed/direction (standard wind barbs) across (purple line in figure 1a) the 968
investigated warm front at 18z. Warm front position is highlighted by the red line according to wind and potential 969
temperature fields. 970
971
972
Figure 2. 3GHz radar RHI scans of (a) ZH (dBZ), (d) ZDR (dB) and (f) unfolded Doppler velocity (m s-1) at 17:11:00-973
17:12:30UTC (negative values indicate flow towards the radar). (b) Percentage of cloudy pixels (ZH dataset was used) 974
with height. Graphs (c), (e), (g) and (h) show average (solid lines) and 95th percentiles (dots) of ZH, ZDR, Doppler 975
velocity and vertical wind shear produced from Doppler velocity, which were calculated from the 3GHz radar dataset 976
for 30≤Range≤90km and 0≤Altitude≤8km for the most representative scans during each time period showed in the 977
legend. Different colours indicate different time periods for (b), (c), (e), (g) and (h). 978
979
44
980
Figure 3. (a) ZH (dBZ) and (b) Doppler velocity (m s-1) evolution with time as captured by the 35GHz radar. In (a) 981
the melting layer height is highlighted by the blue (time <2100UTC), the green (2100UTC< time <2215UTC) and the 982
red (time <2215UTC) dashed lines. The black dashed line shows the linear fitting of the cloud tops height between 983
1500-2100UTC. The coloured lines on the left corner indicate the time that radiosondes of figure 4 were launched 984
comparing to the time colourscale of the figure 3a. In (b) negative Doppler velocity indicates flow towards the radar. 985
45
(c) Similar to figure 2b. (d) shows average (solid lines) and 95th percentiles (dots) of ZH as calculated from 35GHZ 986
radar data. Both (c) and (d) were calculated from the first 10 minutes of data of each hour. 987
988
Figure 4. Graphs demonstrating (a) temperature (oC), (b) potential temperature (K), (c) relative humidity (%), (d) 989
wind speed (m s-1), (e) wind direction, (f) change of potential temperature with height obtained from radiosonde data. 990
Different colours indicate different times and were selected to be comparable with the colour scheme of figures 2b, c, 991
e, g and 3b, c, e. 992
993
46
994
Figure 5. Example of ice particle images captured by the 2D-S probe. Ice particles are grouped by the temperature 995
(left axis) and altitude (right axis) at which they were observed. 996
997
47
998
Figure 6. Unfolded Doppler velocity (m s-1) in RHI scans for three scanning periods: (a) 16:00:50 – 16:02:20, (c) 999
18:00:06 – 18:01:35, (e) 19:35:13 – 19:36:43 (the grayscale line shows the aircraft track between 19:33:34 and 1000
19:40:05 -black for the initial and white for the final position). Negative values indicate flow towards the radar. The 1001
orange dashed line is the -20m s-1 contour. The solid red line in figure (c) determines the warm front boundary location 1002
and the dashed red line the possible sub-front. (b) and (d) show radiosonde wind speed and direction (colour scale) 1003
over the Chilbolton region (shaded curve) and Doppler velocity (blue dashed line) along the vertical black line on 1004
graphs (a) and (c) correspondingly. (f) shows IWC, LWC and cloud droplet concentrations for the corresponding 1005
aircraft track in (e) at T~ -1.9oC. 1006
1007
1008
1009
48
1010
Figure 7. RHIs of ZH (dBZ): (a)-(d) and (i); and of ZDR (dB): (e)-(h) and (j). The horizontal grayscale line shows the aircraft track between 19:03:00-19:07:48 in (i) and (j) (white 1011
for the final, and black for the starting position). The mean temperature recorded along the track was ~ -7°C. The black contour line in (a)-(d) is for Doppler velocity equal to -20m 1012
s-1 and in (e)-(h) for ZH=15dBZ. In figure (a) the red contour presents wind shear dV/dz=7m s-1 km-1. Figure (k) shows LWC and cloud droplet concentration along the aircraft track 1013
in (i) and (j).1014
49
1015
Figure 8. Probability distribution functions of different ZDR thresholds for in-situ (a) ice number concentration, (b) 1016
mean ice particle diameter and (c) ZH. The data used for (a) and (b) come from regions where the aircraft flew through 1017
ZDR/ZH fall-streaks. The data used for (c) come from the 3GHz-radar for ranges of 30-90km and elevations >1km (to 1018
avoid the ML and focus on fall-streaks) between 16:61:21-19:28:281019
50
1020
Figure 9. RHIs of (a)-(d) ZH (dBZ), (e)-(h) ZDR (dB). The grayscale line shows the aircraft track between 16:21:13-16:24:22 in (c) and (g) and 16:52:39-16:59:06 in (d) and (h) (white 1021
for the final, and black for the starting position). The black isoline in (a)-(d) is for Doppler velocity equal to -20m s-1 and in (e)-(h) for ZH=15dBZ. Figures (i) and (j) show 2D-S 1022
images captured during the flight track in (c), (g) and (d), (h) at temperatures -7.8°C and -12.9°C respectively. Figures (k) and (l) show IWC/LWC and ice number concentration/ice 1023
mean diameter for the aircraft track in (c), (g). Similarly, (l) and (n) correspond to the aircraft track in (d) and (h).1024
51
1025
Figure 10. As in figure 9. Figures (i), (j), (k) and (l) show the surface rain rate (Nimrod) along the radar range of the RHIs in (a,e), (b,f), (c,g) and (d,h) respectively. 1026
1027
52
1028
1029
Figure 11. (a) ZDR and (b) ZH profiles (0-1km height) from data taken from the radar scans 10d and 10h. Green curves 1030
depict the profiles out (range=25km) and red curves within the ZH fall-streak (range=32km). 1031
53
1032
Figure 12. As in figure 9. In (g)) both 2D-S and CIP-100 images are shown. The time and range from the Chilbolton radar are indicated at the bottom of the figure in order to be 1033
comparable with the radar scans. In (h), the green curve shows the cloud droplet concentration. The mean temperature along the aircraft track was ~ -4.8°C.1034
54
1035
Figure 13. A schematic representation summarizing the investigated processes and polarimetric signatures of the 1036
frontal clouds observed. 1037
1038