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Remote Estimation of Cloud Liquid and Droplet Size Using Radar and Satellite Measurements

Remote Estimation of Cloud Liquid and Droplet Size Using Radar and Satellite Measurements. J. Vivekanandan National Center for Atmospheric Research Boulder, Colorado Email: vivek@ucar.edu. Collaborators: Marcia Politovich, Guifu Zhang, Merritt Deeter, John Tuttle, Brooks Martner,

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Remote Estimation of Cloud Liquid and Droplet Size Using Radar and Satellite Measurements

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  1. Remote Estimation of Cloud Liquid and Droplet Size Using Radar and Satellite Measurements J. Vivekanandan National Center for Atmospheric Research Boulder, Colorado Email: vivek@ucar.edu Collaborators: Marcia Politovich, Guifu Zhang, Merritt Deeter, John Tuttle, Brooks Martner, Ed Brandes, Scott Ellis, John Williams,David Serke, and Tim Schneider

  2. Liquid droplets are classified into three categories based on size Background • Cloud droplet size < 50 mm • Drizzle droplet size is between 50 and 500 mm • Raindrop size > 500 mm • Scientific applications • Cloud radiation studies • Numerical weather prediction • In-flight aircraft icing • Measurement Techniques • Scattering: Radar - Active technique • Emission: Satellite - Passive technique

  3. Aircraft icing Scientific Motivation • Inflight icing is caused by impingement of supercooled droplets onto an aircraft • Median volume diameter > 30 mm and LWC > 0.2 gm-3 • cause hazardous icing condition (Politovich, 1989) • Percentage of ice accretion ( or catch ratio) onto an airplane depends on liquid particle size • Detection of droplets in a mixed phase cloud is much more challenging

  4. Initiation of warm rain ( no ice phase) Scientific Motivation (cont.) • Precipitation growth in a cumulus cloud is not well understood • There is a major difference in time scales of formation of • large drops between model and observation • (Knight et al., 2002) • e.g. Large drops appear in cloud base well before main burst of precipitation • Both observations and modeling of cloud droplet formation • need to be improved

  5. Cloud radiation studies Scientific Motivation (cont.) • Influence of clouds on earth’s radiation budget is significant (Heymsfield, 1993, Stephens 1999) • Radiation impact is quantified by droplet size, liquid water • content, and cloud thickness (Stephens et al, 1998) • LWP= 2/3 * optical depth * effective radius • e.g. For a cloud with an optical depth of 30 and • effective radius 10 mm, LWP = 200 g m-2 • Even a small change in droplet size may cause large change • in cloud albedo (Slingo, 1990)

  6. Limitation of single wavelength radar method Dual-wavelength radar technique Single wavelength radar and radiometer method Microwave satellite-based technique Summary and conclusions Outline

  7. * * * * * * * * * * * * * * * * t e g r t a o t e g n a r = r Pulsed Doppler Radar BASIC MULTIPARAMETER RADAR THEORY r = range to target Differential Reflectivity, ZDR Reflectivity, dBZ C = Radar Constant <P> = Average Received Power Ratio of co-polar returns In Rayleigh, Measure of the mean axis ratio (g) and bulk density reflectivity weighted axis ratio ZDR Reflectivity Shape Hydrometeor type Cloud and drizzle drops Raindrop 0 dB < 0 dBZ 2 dB > 20 dBZ

  8. drizzle mixture of cloud and drizzle Reflectivity ~ (droplet size)6 A million droplets of 10 mm give the same radar reflectivity as one droplet of 100 mm! A million droplets of 10 mm contain a thousand times as much water as one droplet of 100 mm. And so: one drizzle droplet changes the reflectivity significantly without changing the liquid water content. cloud droplets Ref: Herman Russchenberg, and Oleg Krasnov, 2004

  9. Problem of using radar to infer liquid water content: Very different moments of a drop size distribution: Reflectivity often dominated by drizzle drops ~100 mm LWC dominated by ~10 m cloud droplets An alternative is to use dual-wavelength radar Radar attenuation proportional to LWC, increases as transmit wavelength of radar decreases Radar reflectivity difference (i.e. attenuation) between two wavelengths ( e.g. 10 cm and 8 mm) is proportional to LWC with no size assumptions necessary Dual-wavelength radar method can be difficult to implement in practice Need very precise radar measurements Limitation of Radar Reflectivity

  10. Dual-wavelength radar

  11. S-band antenna Ka-band antenna S-Polka radar system at the Marshall field site near Boulder during WISP04 project.

  12. 1 ì ü 3 = RES í ý î þ ò = = - 6 6 6 3 D N ( D ) db D mm m For -10 C, l = 8mm, + j = (7.54 + j 15.1) 1 - = ´ 4 RES ( 7 . 12 10 ) mm Z 3 A RADAR ESTIMATED SIZE (RES) Reflectivity (Z) 2 ò ¢ km-1 ¢ - Attenuation (A) = 3 3 p 4 . 34310 k D N ( D ) dB 3 ( ) + r 6 2 r Liquid Water Content (LWC) 3 = C <D > dB km-1 where C = 7.12 x 10-4, km-1 LWC o

  13. a b % Liquid water content accretion Liquid water content, g m-3 10 100 100 10 Size, mm Size, mm Figure: (a) Liquid water content vs size, and (b) Percentage of liquid accretion on aircraft vs size

  14. 20 0 X-band, dBZ -20 -40 0 2 4 6 8 10 12 14 16 18 20 20 0 Ka-band, dBZ -20 -40 0 2 4 6 8 10 12 14 16 18 20 10 Reflectivity difference, dB 5 0 16 0 2 4 6 8 10 12 14 16 18 20 10 2 14 2 12 18 0 8 4 6 0 Range, km X-and Ka-band reflectivity and reflectivity difference along a radial for a stratified cloud with light drizzle. The data were collected in northeastern Colorado on 18 March 1991.

  15. 0.5 0.5 0.4 0.3 Liquid water content, g m-3 0.2 0.1 0.0 0 2 4 6 8 10 12 14 16 18 20 0.5 0.4 0.3 Droplet size, mm 0.2 0.1 0.0 0 2 4 6 8 10 12 14 16 18 20 Range, km Retrieved Liquid Water Content (LWC) and Radar Estimated Size (RES) along a radial for a stratified cloud with light drizzle. The data were collected in northeastern Colorado on 18 March 1991.

  16. Ground-based radiometer

  17. Radiometers measure brightness temperatures Tb, that are converted into optical depths, . Optical depths are linearly related LWP and VWP kl and kv are path averaged coefficients. d is the ‘dry’ optical depth Two wavelengths, two equations, two unknowns – retrieve LWP and VWP. Radiometer Technique

  18. Radiometer Dual wavelength radar Comparison of dual-wavelength radar and radiometer-derived liquid water path for 22 March 1991 from 0900 to 0912 GMT. National Center for Atmospheric Research

  19. Dual-wavelength radar Summary • Estimates spatial distribution of liquid water content • Large droplets introduces artifacts in LWC estimation • Sample volume mismatch at two radar wavelengths • introduce error in LWC estimation • The technique is still in research phase • Additional radar measurements are necessary in the case • of a mixed phase cloud ( ice and liquid)

  20. Mm-wavelength radar and Radiometer

  21. Less sensitive to ground clutter More sensitive to cloud droplets (Rayleigh gain ~ l-4) Minimal Bragg scattering i.e. (l11/3) Absorption at millimeter wavelength is proportional to LWC Suitable for airborne platform i.e. smaller size radar Merits of Millimeter Wave Radar

  22. Ka-band (l=8mm) reflectivity S-band (l=10cm) reflectivity S-band (l=10cm) velocity Ka-band (l=8mm) velocity PPI scan from WISP04, 2004 showing reflectivity and radial velocity observations of S-band and Ka-band. Note at close range, Ka-band cloud reflectivity is unaffected (Ka-band Doppler is non-zero) by ground clutter.

  23. Marshall Test Site S-Pol radar Radiometer

  24. Retrieval of LWC and droplet size using radar and radiometer y-axis, km y-axis, km Altitude, m y-axis, km x-axis, km Droplet size, mm

  25. + In-situ Radar Altitude, m Liquid water content, g m-3 Comparison of liquid water content between radar/radiometer retrievals and in-situ measurements during WISP04. In-situ measurements are from a liquid water probe on board the UND citation research aircraft.

  26. Mm-wavelength radar and radiometer Summary • Radiometer provides total liquid water path or absorption • Particles should be small compared to radar wavelength • i.e. Rayleigh scattering • Data acquisition time for radiometer is much longer than radar • Usually radar and radiometer observations are mis-matched in time and space • Radiometer observations are corrupted if the radome is wet

  27. Dual-polarization and dual-wavelength radar measurements S-band Reflectivity, dBZ S-band ZDR, dB Ka-band Reflectivity, dBZ

  28. Liquid water content comparison Dual-wavelength technique Dual-polarization technique Liquid water content, g/m3 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 North of S-Pol, km 1.6 1.8 2.0 West – East of S-Pol, km

  29. Microwave Satellite Technique

  30. Advanced Microwave Scanning radiometer for EOS (AMSR-E) Background • Satellite-based retrieval of cloud liquid over the oceans has • matured (Weng and Grody, 1994) • Satellite-based retrievals of cloud liquid over land are less • mature (Greenwald et al., 1997) • Technique relies on • (a) difference in surface emissivity at V and H polarization • (b) absorption by cloud liquid reduces difference between brightness temperatures at V and H polarizations

  31. Parameterized approach Dual-wavelength satellite method • Polarization difference: DTB= De (Ts-TD)T • De: Emissivity difference between V and H polarizations • Ts : Surface temperature • TD : Down-welling brightness temperature • T : Atmospheric transmittance • TD, T depend on temperature, vapor and liquid profiles • Using a ‘training dataset’ of atmospheric profiles that describe natural variability in vapor, liquid and temperature a parameterized relation is derived • DTB = De exp(b0 + b1Ts + b2 LWP + b3 PWV) • LWP: liquid water path • PWV: Precipitable water vapor path

  32. Relation between normalized DTb and atmospheric parameters (LWP, PWV, Ts)

  33. Scatter plot of LWP retrieved by AMSR-E (microwave) versus LWP retrieved by MODIS (Vis/IR) for single overpass of Southern Great Plains in Oklahoma on 12/3/03 AMSR-E LWP Retrievals

  34. Comparison of LWP retrieved by AMSR-E with LWP retrieved by MODIS for single overpass of Southern Great Plains region on 12/3/03 AMSR-E and MODIS LWP Retrievals

  35. Microwave satellite-based method Summary • It is a ‘stand-alone’ method -- previous method depend on Vis/IR radiances and radiosonde measurements • Microwave satellite-based method is applicable during day and night • Insensitive to overlaying cirrus or multi-layer clouds • Underestimates LWP when compared to ground-based radiometer • Microwave satellite provides only a limited temporal and spatial coverage • Further validation of this technique is in progress

  36. Summary and Conclusions • Various remote sensing methods for estimating cloud liquid and cloud droplet size were discussed • Effect of drizzle droplet on retrieval of cloud liquid water content is almost eliminated by dual-wavelength radar technique and radar/radiometer methods • A novel technique for retrieving cloud liquid over the land using microwave satellite measurements was presented

  37. Summary and Conclusions (cont.) • A combination of remote observations are required for verification of cloud microphysical retrievals • S-Pol radar and the proposed HIAPER cloud radar with dual-polarization and dual-wavelength capabilities are attractive for both cloud radiation and precipitation microphysics research • Joint analysis of radar and satellite observation will improve spatial and temporal resolution of cloud microphysical products • Remote sensing of mixed-phase clouds is much more challenging

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