Radar Meteorology LaboratoryRadar Meteorology LaboratoryRain from Doppler Radar dataRain from Doppler Radar data

Prof. LEE, Dong-In

23 November 2000

Object: To get rainfall from Object: To get rainfall from radar dataradar data

Theory:Theory:

Add appropriate theory from text or other sources.

Radar reflectivity factorRadar reflectivity factor

where z is radar reflectivity factor (mm6/m3), Ni is the number of drops per unit volume (m-3) and Di is diameter (mm) for drops in the ith size category.

6ii DNz

Rain RateRain Rate

Rain rate is the rate at which water volume accumulates over a unit area and is given by

where A is the area involved and t is time. This equation actually calculates the

volume of water per unit area per unit time.

RN D

Ati i

6

3

Liquid Water ContentLiquid Water Content

Liquid water content is the mass of liquid water in a volume in space, given by

where vi is the terminal velocity of the ith diameter raindrop, and w is the density of water.

i i

iiw

v

DN

AtM

3

6

Z-R RelationshipZ-R Relationship

z (mm6/m3) R (mm/h) z (mm6/m3) R (mm/h)

562.3 1.254 Z=200R 1.6 MP478.6 1.720 10 0.153765501.2 1.200 100000 48.62462

1023.3 1.610

398.1 1.460 Z=300R 1.5 J-W851.1 2.480 10 0.103574

1122.0 4.300 100000 48.074992238.7 4.570

138.0 1.138 Z=429R 1.59 RADAR7

Procedure:Procedure:

Get Radar data during rain situation Trace contours of reflectivity from each

time onto a single image. “Connect the dots” and draw swaths of the

path of the storm at each reflectivity level. Convert Z to R Get rain at each time by integrating amount

over each location at each time.

(Ex.) NEXRAD images of rain(Ex.) NEXRAD images of rain

Get data at hourly intervals. One possible way is to get it from:

http://wxp.atms.purdue.edu/radar/previous/rad_comp-X.gif where X can be 1, 2, 3, 4, 5, or 6.

Alternatively, get more detailed data from a specific site.

We can combine these into a We can combine these into a single image.single image.

You may want to select a single You may want to select a single region for your analysis.region for your analysis.

Transfer Z data from each image Transfer Z data from each image to a common image.to a common image. Draw outlines of each reflectivity level onto

a single piece of acetate or a separate image.

Then combine these.

Now combine individual images Now combine individual images into a single image.into a single image.

““Connect the dots” to produce Connect the dots” to produce rain swaths.rain swaths.

Fill the contours if you like.Fill the contours if you like.

Convert Z to RConvert Z to R

Use Z-R relationship. z = 300 R1.5 where R is rainrate in mm/h and z

is radar reflectivity factor in mm6/m3

Draw new contours of R (parallel to Draw new contours of R (parallel to the Z contours already on image)the Z contours already on image)

Razzelfratz!!!Razzelfratz!!!

After spending an hour or so generating all the (ugly) images before this, it finally dawned on me that this is not necessarily going to answer the question: “How much did it rain at point X?”

How can we answer that question?

Questions:Questions:

Define “rainfall” How do we go from radar data to rainfall

accumulations? What points (on the ground) are good ones

to use? How much time resolution in the radar data

is needed? Does it depend on the kind of rain falling?

Rainfall totalRainfall total

where R is the rainrate (mm/h) at a given place and time, t is the time interval between consecutive radar data sets.

Rtotal = the total depth of rain at the location over the entire time interval.

end

start

end

start

t

t

t

t

Total tRRdtR

How do we get How do we get RR??

Radar give radar reflectivity factor Z (or z). From a z-R relationship, we can convert z

into R. z = ARb where A and b are empirical

constants, R is rainrate in mm/h, and z is the linear radar reflectivity factor in mm6/m3.

Use: z = 200R1.6, the Marshall-Palmer z-R relationship.