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Radar Detection of Shallow Weather and Orographic
PhenomenaPaul Joe
EUMETCAL Weather Radar
20130605
Who am I?•a research scientist specializing in developing applications (severe weather, aviation, quantitative precipitation). • I work for Environment Canada and live in Toronto.•the chair of the WMO/Working Group on Nowcasting Research and have had a lot fun demonstrating the use of radar for nowcasting at the Sydney (2000), Beijing (2008), Vancouver (2010) and Sochi (2014) Olympics.•contribute to several Expert Teams in the WMO/Commission of Instruments, Methods and Observations (Upper Air and Remote Sensing and Surface Observations).
Beijing Vancouver Sochi Photography Skiing
This module briefly explores “radar meteorological” issues of low level weather detection in a generic way.
Module Objective
Outline
• Some “radar calculations”:– Typical reflectivities of rain, drizzle, fog, snow
(detection issue)– Beam height (detection issue)– Beam width (quantitative and detection issues)– Sensitivity (detection issue)
• Impact on Meteorology– Drizzle– Lake Effect Snow– Clear Air Echoes– Orographic Precipitation
Low Level Phenomena Detectable by Radar
• Meteorological Targets– Precipitation (Drizzle, Rain, Snow, Hail)– Lake Breezes, Convergence Lines, Gust
fronts, cold pools• Biological Targets (example at the end)
– Insects, birds, bats• Electro-magnetic Targets (examples)
– Other radars, RLANs, Sun, second trip echoes
• Other (not discussed)– Building, Mountains, Forests– For calibration of humidity retrieval– Turbulence (Bragg scattering)
• Hard Targets (not discussed here)– Wind turbines, Cars, ships, space debris– Forest fires– Sea Clutter
Romanian Gust Front
General Comments – Low Scanning
• Wide variety of phenomena and intensity of targets– Turbulence (too weak) to Mountains (very intense)– From very weak to very strong (-30 dBZ to 95 dBZ)
• Different Doppler signatures– Some have 0 velocity– Some have aliased velocity (> Nyquist)
• Advanced uses of weather radar– VDRAS – variational doppler radar assimilation system– Refractivity retrieval – use of ground clutter echoes
• Commonality – Limited range! – Low echo strength (generally), – Low height of weather, – Radar sensitivity and scanning are issues
Drizzle
Some Radar Examples
Drizzle reported in surface observations but no radar echoes.
Drizzle in surface observationsBUT NO/Little RADAR DATA
Germany Example 1
Lang, DWD
Drizzle (mm/h) but very fewechoes
Germany Example 2
Lang, DWD
Drizzle in Finland!
Saltikoff, FMI
1. Why was drizzle observed in Finland but not Germany?
2. Why is the drizzle observed only around the radar?
3. Why is the reflectivity pattern stronger near the radar and decreases away from the radar?
4. Why is there a range limit to see drizzle?
Minimum Detectable Signal
Concept
Minimum Detectable SignalThe detection threshold (as a function of range).
Range [km]
Ref
lect
ivit
y [d
BZ
]
Color is Probability Distribution of Reflectivity with Range (not important for this discussion). Function of Wx.
Minimum Detectable Signal (constant power)
P = C Z r2
The Radar Equation
MDS can expressed as a noise temperature or a power measurement but for meteorologists it more useful to express as reflectivity at a particular range. Typically, -1 dBZ at 50 km.
Some Radar Considerations
P = C Z r2
P = power, C = radar constant, r = range
Z = N D6
[Z] = mm6/m-3
dBZ = 10 log Z
Reflectivity Factor - Linear
Radar Equation and MDS Pmin = C Zmin(r)
r2
• The Radar measures “P” – power received• The Radar Equation converts P to Z for a given
range (r)– Radar Equation accounts for expanding beam
with range (1 /r2)• Sensitivity (or MDS) is a certain power level
– Just above the noise (hsssssss) level – In terms of P (power), it is a constant – In terms of Z (reflectivity), it is a function of
range (1 /r2)• A limitation for long range detection of weak
echoes is the radar sensitivity! – If the reflectivity of the target is below MDS
then the radar does not detect it!– Beware of artificial MDSartificial MDS! The display of the
radar data may be thresholded! Some data may not be displayed!
Range
Ref
lect
ivy
Range
Pow
er
Homework Question 1
• If the MDS is -10 dBZ at 10 km
• What is the MDS at 100 km?
Homework Answer 1
Homework Answer 2
Homework Answer 3
Rain Rate
Note that if N is number concentration and D is particle size and uniform
Z = N D6
Terminal Velocity of Hydrometeors
Gunn-Kinzer1949
Beard and Pruppacher, 1969Radius [microns]
Vel
oci
ty [
cm/s
]
Homework Answer 3
Homework Answer 3
So, can your radar see drizzle of a reflectivity of say -5 dBZ?
So, how far can you see drizzle (-5dBZ)?Or anything else?
P = C Z r2
Minimum Detectable Signal (power)
~ 25km
-5dBZ
Can you see drizzle – part 2?The Artificial MDS Situation
7dBZ
Data in this shaded area is thresholded (not displayed)!
~ 25km
Typical Drizzle reflectivity
Reflectivity vs Range for Constant Power (1/r2)
Where does your radar fit on this diagram? Go ask your radar engineers.
Typical Radars
Beamheight Considerations
Do you know what your minimum elevation angle is?
Minimum Elevation Angle and Beamwidth Impact
0.5oBeam totally overshoots the weather beyond this range! No detection at all!
Shallow Weather
The weather is detected but the beam is not filled beyond this range, so reflectivities are quantitatively underestimated from this range and beyond
Note: the lower the beam the longer the range for detection ability!
1o beamwidth
Drizzle
Drizzle is due to warm rain process. Slow growth which results in small drops (0.1 mm, 1 mm/h)
Note: Colour scales are different!
dBZ
dBZ
ZDR
Saltikoff, FMI
Drizzle is round!
1 km
Summary: Drizzle in Finland!
Saltikoff, FMI
1. Why was drizzle observed in Finland but not Germany? Thresholded!
2. Why is the drizzle observed only around the radar? Sensitivity
3. Why is the reflectivity pattern stronger near the radar and decreases away from the radar? Beamfilling
4. Why is there a range limit to see drizzle? ~80-100km, function of sensitivity, beamfilling, depth of the drizzle!
5-6°C
Drizzle ,,
Unusual widespread drizzle from cloud echoes aloft. At surface only few echoes above 1dBZ. Note: change in threshold for DWD, see more drizzle!
Hamburg
Germany Example 3
Lang, DWD
Homework Answer 4
Homework Answer 5
Homework Answer 6
Major Factors for Detection
• Radar Sensitivity – Target Reflectivity/Radar MDS combination
• Overshoot– Lowest Angle of Radar/Height of weather / Earth
Curvature combination
• Beam filling (quantitative) – Weather is too shallow or too low– Beam is very broad
• Thresholding– Artificial MDS = Minimum Displayed Signal*
Use laser to select the most significant for your radar system/display!
* Saltikoff
FOG
Can the radar see fog?
FogSpecial Cloud/Fog Radar (35 GHz or Ka Band)
Fog has drop sizes from 10 to 30 microns, so very low reflectivities.
An operational radar has a sensitivity as -8 dBZ at 50 km.
What is the controlling factor of detecting fog for this radar?
Drop Size Distributions
dBZ
10 km
Non-operational
Beamheight Again
Quantitative measurements
Partial Beam Filling
Range bins that are partially beamfilled, decreasing reflectivity with range!
0.5 degree
Vertical Profile of Snow Function of Range
1. Snow originates aloft but grows as it falls.
2. The same vertical profile as observed by radar at increasing range due to beam filling, beam broadening (smoothing) and Earth curvature (can’t see lowest levels)!
Quantitative Impact of Beamfilling
Michelson, SMHI
Note the fall off of values with range.
This is NOT attenuation to which this is commonly attributed.
It is a beam filling effect!
Effective Range of a Radar for QPE before Vertical Profile Adjustments
Impact of Beamwidth / Beamfilling30 day Accumulation
Example of the impact of beamwidth or beamfilling on quantitative precipitation estimation. One radar is 0.65o and the rest are 1.1o beamwidth radars. Smaller beamwidth means less beamfilling problems with range and farther quantitative reflectivity information.
0.65o
(no blue)
Patrick, EC
1.0o
(blue)
Applying the Correctionaka Vertical Profile Correction
aka Range Correction
Koistinen, FMI
Other Shallow Features
Atmosphere is very layeredYou want to see low levels!
3 flow regimes evident but really 5?
5 layers? Virga
1
2345
Vertical profile of reflectivity
Radars in the MountainsOften on top for surveillance reasons
Germann, MCH
Can’t see low levels
Ke = 5/4
Precipitation is on the flow when the flow is blocked.
Radial velocity Reflectivity
You want to see low levels and non-precipitating echoes
Lake Breeze TStorm Outflow
Lake Breeze
ExplosiveGrowth
Reflectivity [dBZ] 10 Hour Reflectivity Accumulation [dBZ]
Airplanes as points
Airplanes fly along prescribed tracks
Different Color Tables
Summary• Focused on drizzle to illustrate the
impact of the radar sensitivity, lowest elevation angle on detection
• Understand why you can detect or not certain phenomena
• Do not “threshold data out”, it is all useful
• You should find out how low your radar scans and the impact on your forecasting– Sensitivity (actual and threshold)– Lowest elevation scan– Beamwidth– How far are your radars apart
