radar detection of shallow weather and orographic phenomena paul joe eumetcal weather radar 20130605

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

Thank You

Questions?

paul.joe@ec.gc.ca