ATMOSPHERE PHYSICSM. Balmez, S. Ştefan 5 796 0T ln R p VkT fp = ×∇ G G (1) where R (J⋅molK-1 -1) is the specific gas constant for air, f (s-1) is the Coriolis parameter, kG is

ATMOSPHERE PHYSICS

ON THE FORMATION MECHANISM OF LOW-LEVEL JET OVER BUCHAREST’S AIRPORTS

M. BALMEZ1,2, S. ŞTEFAN2*

1 Romanian Air Traffic Services Administration, 10, Ion Ionescu de la Brad Bv., 013813, Bucharest, Romania

2 University of Bucharest, Faculty of Physics, P.O.Box MG-1, 077125, Măgurele, Romania E-mail: [email protected]; E-mail: [email protected]

Received April 10, 2014

The presence in the lower atmosphere of the Low-Level Jets (LLJs) affects the safety of aircrafts at landing or take-off. In this work, the physical mechanisms of LLJ formation are studied. For this purpose, three cases of observed LLJs associated to three main types previous determined for Bucharest area, were selected. A synoptic and mesoscale analysis for all cases was made and a thermal wind computation for 6 hours before and 6 hours after the appearance of the low-level jet was performed, using the hourly ALARO model data. The analysis has shown that the mechanisms found act either alone or in combination. Thus, for type I, the baroclinicity induced by the development of a Mediterranean cyclone is the mechanism that leads to the low-level jet formation; for type II- the Blackadar mechanism is responsible and for type III- the combination of Blackadar mechanism and the orography determine the formation of LLJs.

Key words: low-level jet, airport, mechanism, thermal wind, wind shear.

1. INTRODUCTION

The definition of the low-level jet is depending on the purpose of the study. The most commonly used definition is that of any maximum in lower troposphere in the vertical profile of the horizontal component of the wind. Specific requirements for the maximum wind speed and the wind shear that accompanies the LLJ are used in different papers [1, 2, 3, 4]. For aviation, the low-level jet is important in the approach phase and on landing/take-off path, where an increase or decrease of headwind leads to an increase or decrease of the aircraft height (Figure 1). This is why the definition used for the LLJ in this study is that of a region of maximum wind speed of at least 12m/s at some altitude in the boundary layer, with the wind speed at surface and below the 700hPa pressure level (about 3000 m) decreasing to at least half of the maximum wind speed.

Rom. Journ. Phys., Vol. 59, Nos. 7–8, P. 792–807, Bucharest, 2014

2 The mechanism of low-level jet over Bucharest’s airports 793

With this definition and a statistic made for Bucharest’s airports area using the regional climate model RegCM3 which was run for Romania, for winter season between 1959 and 1982, it was shown that LLJs can be organized in three main types, depending on the synoptic situation in which they occurred [5]. In addition, the statistical results emphasized that the majority of the cases appears between 975 and 925hPa for 3–12 hours, with a depth for the maximum wind speed of 10–50hPa [5].

The three main types found are: • Type I: a Mediterranean cyclone at surface and a cut-off or a trough in

altitude which, if it is sharp, it is accompanied by a jet stream and if it is relaxed, no upper jet stream is present

• Type II: a trough of a northern or north-eastern European low, both at surface and in altitude

• Type III: an anticyclone of different origins at surface and a weak circulation in altitude.

Fig.1 – The effect of the LLJ on an aircraft [photo: C. Balmez].

The main goal of this study is to find the mechanism for these three types of LLJ that affects the Bucharest’s airports area. The theoretical considerations that are trying to explain the dynamical origin of the jets are presented in Section 2. Methodology used is presented in Section 3 and Section 4 is dedicated to the three case studies corresponding to the three types of LLJ found. The paper ends with conclusions.

M. Balmez, S. Ştefan 3 794

2. SHORT THEORETICAL CONSIDERATION ABOUT MECHANISMS FOR LLJ FORMATION

Blackadar (1957) describes the mechanism of formation of a maximum wind below 1000m height, which was observed in the cloud-free nights, through the inertial oscillation theory [6]. This means that, after sunset, when the turbulent mixing ceases and the radiative cooling leads to a temperature inversion, the layer above the inversion becomes frictionless and starts to accelerate due to the imbalance between the pressure gradient and Coriolis forces. The wind begins an oscillation around the equilibrium vector gV (the geostrophic wind), the wind vector rotating clockwise in the northern hemisphere (Figure 2).

Fig. 2 – At a given height z, the deviation vector D rotates clockwise (in the northern hemisphere) around the equilibrium vector (here gV ). The initial vector at the height z is denoted by 0U [7].

Another theory is based on shallow baroclinicity [2]. In the regions with significant changes in the surface characteristics (coasts, the edge of large ice zones), the horizontal differences in sensible and latent heat fluxes produce low-level baroclinicity which produces LLJ oriented parallel to the low-level horizontal temperature gradients. The effect of a baroclinicity produced by sloping terrain is a mechanism analysed by Holton [8]. From the diurnal cycle in the horizontal temperature gradients over sloping terrain, it results a cycle in the geostrophic wind and, in consequence, an oscillation of thermal wind. This mechanism explains why LLJ has the tendency of setting over the Great Plains (which has a gentle slope). The development and evolution of an extratropical cyclone produces vast regions of low-level baroclinicity [9], [2]. Temporal variations in synoptic-scale pressure field can increase wind speed in LLJ during the intensification and movement of the cyclone. Additionally, diabatic effects have an important role in LLJ development, as it is shown in numerical simulations [10].


The isallobaric forcing intensifies the LLJ which appear beneath the exit region of an upper-level jet as part of an indirect circulation [11] (Figure 3). Reiter (1969) has demonstrated that LLJ can be formed as a response to an orographic cyclogenesis [12]. The decoupling of the boundary layer during the night allows wind to accelerate along the pressure gradient. The orographic cyclogenesis amplifies the pressure gradient force and it has a substantial contribution to the high frequency of LLJ in the region of the Great Plains [13], [1].

Fig. 3 – LLJ coupled with the upper-jet stream [13].

In addition, the terrain effects can be the mechanism for LLJ formation. Diurnal heating in complex terrain regions produces both slope and valley winds that can form LLJs. The air flow is accelerated through a channel (valley) due to the horizontal pressure gradient oriented along the flow. Also, the presence of a mountain barrier can block the low-level flow of a cold, stable air mass and channel this flow along the barrier [2]. For Romania, the Carpathian Mountains represent a barrier in the north-easterly flow which, in winter season, can lead to extreme weather conditions [14].

These theoretical considerations were analysed in conjunction with the theories which express the behaviour of the wind in the planetary boundary layer (thermal wind) since the wind speed and direction are the representative parameters for the low-level jet.

In the planetary boundary layer, the pressure gradient force and the Coriolis force are not usually in balance, so the geostrophic wind cannot be used to express the behaviour of the wind. That is why the thermal wind is used for this.

The thermal wind is the vector difference between geostrophic wind at pressure level p and geostrophic wind at pressure level p0, p < p0 and it can be written as follows [15]:


0lnTpRV k T

f p

= ×∇

(1)

where R ( -1 -1mol KJ ⋅ ) is the specific gas constant for air, f (s-1) is the Coriolis parameter, k is the unit vector for Oz axis and T is the mean temperature of the layer between level pressures p and p0.

From equation (1) it follows that thermal wind is orthogonal to the mean temperature gradient so it can be used to estimate the horizontal mean temperature advection in a layer (for a cyclonic rotation of the thermal wind there is a cold advection in the layer and for an anticyclonic rotation a warm advection). The zonal component of the thermal wind vector can be written using the geopotential height:

( ) ( )0T p pg gT Tu Z Z Z

y yfT fT∂ ∂

= − − = − ∆∂ ∂

(2)

where Zp and Zp0 are the geopotential heights of the two pressure levels p and p0. Thus, the greater the thickness of the layer, the higher the thermal wind speed.

3. METHODOLOGY

For each type of low-level jet found in [5], a case from the winter period of December 2011–November 2013 was analysed for the area around Bucharest. The synoptic and mesoscale pressure patterns and thermal wind theory were used for analysis. All the synoptic maps used are ECMWF operational analysis maps and all the mesoscale maps are from ALARO model run at National Meteorological Administration in Bucharest. For the thermal wind, the hourly geostrophic wind horizontal components were computed from ALARO model using geopotential height values for two pressure surfaces (e.g. 950 and 925hPa):

2 1( )pg

Z Zguf y∆ −

= −∆

(3)

3 1( )pg

Z Zgvf x∆ −

=∆

(4)

The model was chosen such that it best represented the geopotential height for Bucharest after which, three locations: Bucharest, Ploieşti and Slobozia were used in the above mentioned equations as points 1, 2 and 3, in order to compute the thermic lapse rates.


The Skew-T diagram (thermodynamic diagram) for the lowest part of the atmospheric vertical radio-sounding at Bucharest was also used in order to know vertical profiles of wind, temperature and humidity.

The METEOSAT satellite images were used for analysis of the air mass characteristics for the three studied cases.

The algorithm of the analysis follows the next steps: – check the type of the LLJ [5]; – analyse the vertical profiles of the wind and temperature using a

thermodynamic diagram; – note the presence of the temperature/pressure gradients; – note the layer where LLJ acts and the time interval; – compute the hourly geostrophic wind from ALARO model; – represent the hodograph of the thermal wind in the layer.

4. RESULTS AND DISCUSSIONS

4.1. THE LLJ CASE IN 07.01.2012

During the night of 07.01.2012, a low-level jet developed in the area of Bucharest. At 0000UTC, a maximum wind speed of 30m/s (60kt) was detected in the vertical profile of the horizontal wind at 928hPa (515m above ground level –AGL), blowing from north-east (Figure 4), with wind speed over 25m/s (50kt) between 955hPa and 903hPa (279 and 736m AGL, respectively). No temperature inversion was present.

Fig. 4 – The Skew-T diagram for the lowest part of the atmospheric sounding at Bucharest, in

07.01.2012/0000UTC; temperature profile in black solid line, dew point temperature profile in blue solid line, isotherms in black dashed line (-100 and 00 in thick black dashed line); isobars in horizontal

light blue dashed line; wind denoted with barbs (direction and speed in kt).


The 300hPa wind analysis is showing a jet stream core of 60-70m/s at 0000UTC in the south of Europe and north of Africa, with the jet exit over Turkey, after 6 hours being moved south-easterly, with a branch of 40m/s reaching the coastal region of Romania.

The 500hPa pressure analysis shows a large trough with a cut-off low at 532gpdam (rising from 528gpdam over the last 6 hours) over Balkan Peninsula while a large upper ridge was situated over Western Europe (Figure 5a). The cut-off low continued to fill during the next 6 hours. The mean sea level pressure analysis shows a low of Mediterranean origin in Macedonia region, Greece (of 990hPa at 0000UTC, rising from 982hPa and continuing to fill) and an Azores High ridge over Western Europe reaching the western part of Romania and the northern part of Moldova (Figure 5b). This led to a high pressure gradient over eastern Romania.

Fig. 5 – a) The geopotential at 500hPa pressure level (black solid lines at 4gpdam intervals) and temperature (pink and blue dashed lines at 20C intervals); b) Mean sea level pressure (green and

orange solid lines at 2.5hPa intervals) from ECMWF analysis in 07.01.2012/0000UTC. Surface low is indicated by “L”.

This analysis shows that the conditions in which the LLJ occurred during the night of 07.01.2013 are those of type I.

In addition, from the satellite imagery of air mass (Figure 6) it can be observed a dry intrusion of stratospheric air [16], intrusion associated with the tropospheric jet.


Fig. 6 – METEOSAT air mass RGB satellite image from 07.01.2012/0000UTC.

The 925hPa pressure and temperature map (Figure 7a) from ALARO model in 07.01.2012/0300UTC shows a southerly warm air flow in the south-eastern part of Romania with strong temperature gradients and a cold air in Moldova region, while the wind map (Figure 7b) shows an area with high winds (more than 25m/s), blowing from north-east, in the eastern part of Muntenia region.

Fig. 7– a) The geopotential at 925hPa pressure level (black solid lines at 2gpdam intervals) and

temperature (in coloured contours at 20C intervals, colour scale attached); b) 925hPa wind speed (coloured contours) and direction (arrows) on 07.01.2012/0300UTC; Moldova and Muntenia regions

are marked with black dashed lines.

The vertical cross sections along line A-B (Figure 8a), with Bucharest in the middle, from ALARO model shows that, below 1000m, a wind of 9m/s had blown from 06/1800UTC (confirmed by a wind shear report from a pilot, received at 1945UTC) until 07/0300UTC, reaching 26m/s at 0000UTC and a peak of 28m/s at 0200UTC (not shown here), after that starting to decrease (Figure 8b, valid for 0300UTC).


Fig. 8 – a) Constant longitude line A-B, with Bucharest in the middle; b)Vertical cross section along A-B line; winds are shown as isotachs (black solid lines every 2m/s), arrows indicate velocity and direction of the rotated horizontal wind, temperature fields are represented on colour scale at 40C

intervals – from ALARO model in 07.01.2012/ 0300 UTC.

The hourly geostrophic wind was computed from ALARO model for 950 and 925hPa pressure surfaces, from 1800UTC in 06.01.2012 to 0600UTC in 07.01.2012. The best model was the one run in 06.01.2012 at 1800UTC, which had the wind speed at 925hPa of 27m/s in 07.01.2012, 0000UTC and wind speeds of 25m/s at 950hPa and 26m/s at 900hPa, a peek in the wind speed of 28m/s at 925hPa at 0200UTC/07.01.2012 (with wind speeds greater than 25m/s at 925hPa between 2300 and 0600UTC), while the atmospheric sounding showed a wind speed peek of 30m/s at 0000UTC.

Fig. 9 – The hodograph of the thermal wind in 950-925hPa layer between 06.01.2012/1800UTC

and 07.01.2012/0600UTC; the units for u and v components are m/s.


A cyclonic turning of the thermal wind (with one exception, between 2100 and 2200UTC) from southerly to easterly wind, thus cold advection in the 950-925hPa layer, can be observed during a 12-hour period (Figure 9).

The cold advection inferred from the thermal wind behaviour shows the increase of pressure gradient and so, the increase of wind speed. In fact, this high pressure gradient in the eastern part of Romania was caused by the interaction between the Mediterranean cyclone and the Azores High ridge, which can lead to the low-level jet formation [9].

Hence it follows that the mechanism which leads to the formation of a low-level jet of type I is the baroclinicity associated with the development of the Mediterranean cyclone.

4.2. THE LLJ CASE IN 13.01.2012

During the night of 13.01.2012, a low-level jet developed in the area of Bucharest. At 0000UTC, a maximum wind speed of 13m/s (25kt) was detected in the vertical profile of the horizontal wind between 958hPa (429m AGL) and 835hPa (1533m AGL), blowing from west (Figure 10). Two temperature inversion were present, one of 4.20 C from 990 to 972hPa (162-311m AGL), the other one (a subsidence inversion) of 3.10C from 841 to 818hPa (1476-1698m AGL), the LLJ lying between these two inversions.

Fig. 10 – The Skew-T diagram for the atmospheric sounding in Bucharest, 13.01.2012/0000UTC.

The atmospheric sounding and the 300hPa wind from ECMWF analysis map (not shown here) show no tropospheric jet stream over Romania.


The 500hPa analysis map and MSLP analysis map show a trough of a north-eastern low over Eastern Europe both in altitude and at surface (not shown).

Thus, the type of LLJ corresponds to type II. At 925hPa, a warm air was present in the western and extra-carpathian

regions of Romania in 13.01.2012/0000UTC (Figure 11a), with a westerly flow of 10-15m/s in the southern region (Figure 11b), increasing to 15-20m/s by 0600UTC.

Fig. 11 – a) The geopotential at 925hPa pressure level (black solid lines at 2gpdam intervals) and

temperature (in coloured contours at 20C intervals, colour scale attached) and b) 925hPa wind speed (coloured contours) and direction (arrows) on 13.01.2012/0000UTC.

The hourly geostrophic wind was computed from ALARO model for 950 and 850hPa pressure surfaces between 1800UTC in 12.01.2012 and 0600UTC in 13.01.2012. The best model was the one run in 12.01.2012 at 1800UTC, which had the wind speed at 950, 925 and 900hPa of 12m/s in 13.01.2012/0000UTC, 11m/s at 875hPa, intensifying to 16m/s by 0600UTC at 925hPa, while the atmospheric sounding showed a wind speed peek of 13m/s at 0000UTC between 950 and 850hPa.

In the layer 950-850hPa, the thermal wind has a cyclonic rotation in the first 2 hours, then an anticyclonic turning (with one exception, between 2200 and 2300UTC) in the next 4 hours, again a cyclonic rotation in the next 4 hours and finally, anticyclonic in the last 2 hours (Figure 12). This behaviour shows that in this type of cases, the thermal wind is not a good tool for analysis of the LLJ formation mechanism.

The variation of thermal wind rotation is a consequence of the presence of the two temperature inversions. In this case, the vertical profile of the temperature induced oscillation of the wind due to the variation of the temperature gradients. The effect of these vertical gradients was the decoupling of the layer above the lower inversion and the enhancing of the wind, leading to the formation of a low- level jet. This means that the mechanism for this type of LLJ is the Blackadar mechanism.


Fig. 12 – The hodograph of the thermal wind in 950–850hPa layer between 12.01.2012/1800UTC and 13.01.2012/0600UTC; the units for u and v components are m/s.

4.3. THE LLJ CASE IN 10.02.2012

During the night of 10.02.2012, at 0000UTC, a maximum wind speed of 13m/s at 971hPa (395m AGL) was detected in the vertical profile of the horizontal wind in Bucharest, and 12m/s at 981hPa (317m AGL), blowing from north-east (Figure 13). A temperature inversion of 3.40C was present from ground to 981hPa (0-317m AGL), the LLJ lying above this inversion. The surface pressure was 1022hPa and the wind speed at surface was 1m/s.

Fig. 13 – The Skew-T diagram for the atmospheric sounding in Bucharest, 10.02.2012/0000UTC.

The synoptic situation over Romania is one of quiescent conditions: no jet stream at 300hPa, weak circulation at 500hPa and, at surface, an East European High ridge with 1035hPa in northern Moldova, after a blizzard episode.


The above conditions denote a type III low-level jet. The 950hPa pressure and temperature maps from ALARO model show a cold

air mass slowly moving southerly between 0000 and 0600UTC, while the 950hPa wind maps show a north-easterly intensification of the flow (10-15m/s) near the curvature of the Carpathian Mountains, the area with this intensification expanding between 0000 and 0600UTC.

The vertical cross-section along line A–B (Figure 8a) shows an area of winds with more than 10m/s above Bucharest at approximately 500m AGL (Figure 14).

Fig. 14 – Vertical cross section at constant longitude along A-B line (Figure 9a), with Bucharest in the middle; winds are shown as isotachs (black solid lines every 2m/s), arrows indicate velocity and

direction of the rotated horizontal wind, temperature fields are represented on colour scale at 40C intervals- from ALARO model in 10.02.2012/ 0000 UTC.

The hourly geostrophic wind was computed from ALARO model for 975 and 950hPa pressure surfaces between 1800UTC in 09.02.2012 and 0600UTC in 10.02.2012. The best model was the one run in 09.02.2012 at 1800UTC, which had a wind speed of 10m/s at 975hPa and 950hPa in 10.02.2012/0000UTC and wind speeds of 10-11m/s at 950hPa from 09.02.2012/2200UTC to 10.02.2012/ 0600UTC, while the atmospheric sounding showed a wind speed of 13m/s at 971hPa in 10.02.2012/0000UTC.

An anticyclonic turning of the thermal wind (Figure 15), thus warm advection, can be observed during the first 2 hours, followed by a cyclonic turning (cold advection) in the next 8 hours and then again an anticyclonic rotation in the last 2 hours. The overall cold advection is due to the cold air mass which enters in eastern Romania from the northern regions.

The low-level inversion caused by the nocturnal radiation led to the decoupling of the layer above it, while the acceleration of the frictionless air led to a low-level jet profile. The area in which this LLJ had developed and the direction of it is linked also to the Carpathian Mountains, the northerly flow in Moldova being turned north-easterly in Muntenia by the curvature of the mountains. Hence it follows that the mechanism for the formation of type III LLJ in winter season is a combination of the Blackadar mechanism and the terrain effects (the curved mountain barrier).


Fig. 15 – The hodograph of the thermal wind in 975-950hPa layer between 09.02.2012/1800UTC

and 10.02.2012/0600UTC; the units for u and v components are in m/s.

5. CONCLUSIONS

For the three types of low-level jet found for winter season in Bucharest’s airports area [5], three cases of LLJ were selected and a synoptic and mesoscale analysis was made for each type, along with a thermal wind computation for 12 hours around the time of LLJ appearance.

Type I LLJ is the type that develops in conditions of development and evolution of a Mediterranean cyclone affecting the southern part of Romania. The low level baroclinicity is the mechanism that drives the formation of this type of LLJ.

Type II LLJ develops in conditions of westerly flow in southern Romania, with low level inversion during the night, the Blackadar mechanism being the driving mechanism.

Type III LLJ is a north-easterly flow which develops in quiescent conditions, after a blizzard episode in eastern and southern Romania or a cold front passage from west to east and the cold air mass behind it entering the eastern part of Romania. The Carpathian Mountains curve the northerly flow from Moldova to north-easterly in Bucharest area. The mechanism behind this type of LLJ is the Blackadar mechanism in conjunction with the terrain effect (the curved mountain barrier).


From these findings, three criteria for LLJ appearance can be extracted: 1. If a Mediterranean cyclone is evolving in the Balkan Peninsula and a ridge

of Azores or East European High is affecting north-eastern Romania, then a north-easterly low-level jet in Bucharest area is likely to appear

2. If a westerly flow is affecting southern Romania and a radiative inversion appear during the night, then a westerly low-level jet in Bucharest area is likely to appear

3. If quiescent conditions are present over Romania, with a cold air mass advection in eastern Romania, then a north-easterly low-level jet in Bucharest area is likely to appear.

For more fine analysis of the mechanisms involved in the low-level jet manifestation at Bucharest’s airports, a data base of wind speed and direction measurements are needed below 500m which, with a SODAR set on the landing/ take-off path at “Henri Coanda” airport since 01.03.2014, will be possible in the future.

Acknowledgements. The authors of this paper would like to thank the members of the

ALADIN group, from NWP Division, National Meteorological Administration in Bucharest (D. Banciu, S. Tascu, M. Pietrisi and A. Craciun) for providing data used in the case studies.

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Documents

ATMOSPHERE PHYSICSM. Balmez, S. Ştefan 5 796 0T ln R p VkT fp = ×∇ G G (1) where R (J⋅molK-1 -1) is the specific gas constant for air, f (s-1) is the Coriolis parameter, kG is