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Radar Meteorology Laboratory Rain from Doppler Radar data

Radar Meteorology Laboratory Rain from Doppler Radar data. Prof. LEE, Dong-In 23 November 2000. Object: To get rainfall from radar data. Theory:. Add appropriate theory from text or other sources. Radar reflectivity factor.

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Radar Meteorology Laboratory Rain from Doppler Radar data

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  1. Radar Meteorology LaboratoryRain from Doppler Radar data Prof. LEE, Dong-In 23 November 2000

  2. Object: To get rainfall from radar data

  3. Theory: • Add appropriate theory from text or other sources.

  4. Radar 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.

  5. Rain 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.

  6. Liquid Water Content • Liquid water content is the mass of liquid water in a volume in space, given by where viis the terminal velocity of the ith diameter raindrop, and rw is the density of water.

  7. Z-R Relationship

  8. 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.

  9. (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.

  10. We can combine these into a single image.

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

  12. Transfer Z data from each image to a common image. • Draw outlines of each reflectivity level onto a single piece of acetate or a separate image. • Then combine these.

  13. Now combine individual images into a single image.

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

  15. Fill the contours if you like.

  16. Convert 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

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

  18. 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?

  19. 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?

  20. Rainfall total where R is the rainrate (mm/h) at a given place and time, Dt is the time interval between consecutive radar data sets. Rtotal = the total depth of rain at the location over the entire time interval.

  21. How do we get R? • Radar give radar reflectivity factor Z (or z). • From a z-R relationship, we can convert z into R. • z = ARbwhere 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.

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