ECE Satellite Radar TRMM Precipitation Radar Cloud mm Radar - Cloudsat. Tropical Rainfall Measuring Mission

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1 Tropical Rainfall Measuring Mission ECE Satellite Radar TRMM Precipitation Radar Cloud mm Radar - Cloudsat -TRMM includes 1st spaceborne weather radar - performs cross-track scan to get 3-D view of rain HURRICANE FLOYD ON Tropical Rainfall Measuring Mission Dual Wavelength Precipitation Radar (PR) TRMM Microwave Imager Visible and IR Scanner 35 Degree Obit Inclination at 4 Km Altitude Summary of Features of the PR sensor: 1) uses radar frequencies of and 13.8 GHz, with horizontal polarization ) horizontal resolution = 4.3 km at nadir 3) obtains data in km swaths 4) can perceive rain through clouds 5) makes quantitative measurements of rain (mm/h) over land and ocean with a sensitivity better than.5 mm/h 6) measures rain from the ground to an altitude of 15 km, with a vertical ("range") resolution of 5 m. 7) provides 3-dimensional rainfall distribution

2 A NASA EARTH SYSTEM SCIENCE PATHFINDER MISSION -, - f ' A ) Z-Rainfall There is not a one-to-one relation between Rainfall and radar reflectivity (Stephens, p49) Example of horizontal and vertical cross sections of rainfall rate measured by TRMM PR for Typhoon on August. Marshall Palmer n( D) N e Z N D e Z N (6!) 1 R m( D) n( D) v( D) dd R N 6 D 6 D dd 3 D e D (4 b) R Na 4b 6 7 b ad dd insert fig. 8.9 Z B AR TRMM PR algorithm involves dual wavelengths and ground validation TRMM Monthly Precipitation from TRMM Cloud Radar TRMM Merged Precip Jan Radar reflectivity Involves Rayleigh scattering (cloud particle diameter smaller than approximately one fourth of the radar wavelength) where the cross section increases as the inverse fourth power of wavelength. Cm wavelength rain radar fails to have sensitivity to see almost all cloud particles. Mm wavelength cloud radars (94GHz) were developed in the eighties specifically to profile clouds ''Ll4 E1 I i --' \ The reflectivity equation shows the dramatic increase in signal with wavelength (dbz = 1 log Z units) r TRMM Merged Precip Aug (mmfd) 4 8

3 Radar Attenuation Attenuation by Gases Pr ( R) G [ 3 P (4 ) R t h n( r) C ( r, ) dr]exp( b R ext ( R') dr') At microwave wavelengths, attenuation by gases is solely due to absorption by water vapor and molecular oxygen: D D D gas HO O P K r C P R t Z R exp( ext ( R') dr') Attenuation Radar signals are attenuated by atmospheric gases, cloud particles, and precipitation. D ext D gas k w k c p R (usually expressed in db/km) Water Vapor ( GHz): Oxygen (6 GHz): D T H O f 3 644/ 1 6 v T 1 T e 1.x1 db / km f 4 f P GHz 113 T P f 113 T f 6 P 3.85 db / km 113 T 1 db km D O / f (From Ulaby et al (1981).) Cloud Radar Frequency Selection is Limited by Gas Absorption Cloud Particle And Rain Attenuation Are Significant Factors

4 Liquid 1 cm 14 GHz 94 GHz -8 C 1 C C -8 C 1 C C -8 C 1 C C n k Im(-K) kc* *In db/km/g/m^3. attenuation by ice particles is negligible since they have extremely small imaginary refractive indices liquid particles are negligible for weather radar but become important at high frequency Ice (- C) 1 cm 14 GHz 94 GHz n k..7.3 Im(-K) 9.E kc* 7.E *In db/km/g/m^3. Data from University of Wyoming 95 GHz cloud radar in marine stratus off the coast of Oregon. (Vali and Haimov (1998)). T storm over Kansas ER- High Altitude Aircraft Cloud Radar airborne Reconstructed unattenuated reflectivity Precip attenuation Observed Precip attenuation at 94 GHz Li et al., surface

5 Example of 14, 94 GHZ radar Precipitation retrievals and effects of attenuation mm-wave Z and cloud water/ ice water content Z- Ice water content relationships At mm wavelengths (& for non - Rayleigh scatterers, e.g. ice crystals, precipitation) 6 Z K n(d)d f(d)/. 93 f(d) is a Rayleigh - to - Mie correction factor.93 value of ( K K o.686 at C at 94 GHz) for water at cm wavelengths LWC =.6 Z 1/ For the same reasons, there is not a one-to-one relation between cloud water and ice contents and mm wave radar reflectivity

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8 CLOUDSAT A NASA EARTH SYSTEM SCIENCE PATHFINDER NI/SSION

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11 CloudSat Profiles Tornadic Outbreak The intense thunderstorms responsible for this week's deadly outbreak of tornadoes in Tennessee, Kentucky, Mississippi, Alabama and Arkansas were imaged by the Cloud Profiling Radar on NASA's CloudSat satellite on February 5.