Response of active and passive microwave sensors to precipitation at mid- and high altitudes

Ralf Bennartz

University of Wisconsin

Atmospheric and Oceanic Sciences Department

Outline

• Introduction

• Comparison of ground-based radar with satellite data

• Combined active and passive mw modeling approach for future sensors

• AMSU precipitation classification: 89 versus 150 GHz

• Conclusions and Outlook

Passive microwave precipitation signal

• Emission signal from liquid precipitation: Most directly linked to surface precip, ocean only.

• Scattering signal from frozen precip-sized ice particles: Only indirectly linked to surface precipitation,

• Most directly linked to surface precipitation

• Over cold (water) surfaces only

• All types of surfaces• More indirect

Observation geometry

Altitude of radar beam (elevation 0.5°):

@100km distance: 2.2 km

@200km distance: 5.2 km

273 K isothermal typically at 2-3 km

ThunderstormGraupel

(Cold air outbreak)Frontal

precipitation

Radar reflectivity [dBz]

Different precipitation events

Radar versus passive microwave precipitation estimate

ThunderstormGraupel

(Cold air outbreak)Frontal

precipitation

Bennartz and Michelson (in press, Int. J. Rem. Sens.)

Simulating passive mw-response for current and future sensors and missions (AMSR, SSMIS,(E) GPM)

waterofconstant Dielectric

efficiency Extinction)/(

velocityfall Terminal)(

ondistributi size DropN(D)

DiameterDrop

)/()(extinction Volume

)(ZtyreflectiviRadar

)()(6

Rrate Rain

ext

K

DQ

DU

D

dDDQDN

dDDNK

dDDUDN

e

e

2

6

3

D

D

D

Bennartz and Petty, JAM, 2001

Bennartz and Bauer Radio Sci. In press

Volume extinction at 85 GHz and 150 GHz

To SSM/Iresolution

To TMIresolution

Every 2ndscanline

85 GHz 150 GHz

150 GHz @ TMI-resolution85 GHz @ TMI-resolution

Graupel shower 85 & 150 GHz

183-7 GHz 183-3 GHz

183-3 GHz @ TMI-resolution183-7 GHz @ TMI-resolution

Graupel shower 183-7 & 183-3 GHz

Frontal Graupel shower

Intensive convection

85 GHz 16 25 45

150 GHz 28 44 42

183-7 GHz 8 22 18

183-3 GHz 4 7 6

Maximum brightness temperature depressions

Sensitivity to surface emissivity

(very dry atmosphere wvp = 6 kg/m2)

Sensitivity to surface emissivity

(moist atmosphere, wvp= 35 kg/m2)

Sensitivity to surface emissivity

(high cloud liquid water path, wvp=20 kg/m2)

High-latitude precipitation classification based on AMSU-data for nowcasting purposes

(see presentation A. Thoss)

Rain rate

Class 1: Precipitation-free 0.0 - 0.1 mm/h

Class 2: Risk for precipitation 0.1- 0.5 mm/h

Class 3: Light/moderate precipitation 0.5 - 5.0 mm/h

Class 4: Intensive precipitation 5.0 - ... mm/h

RGB AVHRR ch3,4,5 PC product RGB:red: very lightgreen:light/moderateblue:intense

Radar compositedifferent projection!

NOAA15 overpass 13 September 2000, 05:48 UTC

150 GHz versus 89 GHz scattering index (land)

Bennartz et al. (Met. Apps., 2002, 9, 177-189)

• 150 GHz enhances dynamic range of SI by a factor of 2

• About 15% of the precip free areas are falsely identified as raining at 89 GHz. This is reduced to 2.6% at 150 GHz.

AQUA AMSR-E

• Data coverage: August 2002-....

• AQUA AMSR-E/AMSU/HSB

• Latitude range 50 N -70 N

• Network of 25 radars

• Radar reflectivities every 15 minutes

• Gauge-adjusted rain rates every 15 minutes

• volume scans of Gotland radar

Combined active/passive dataset for high latitudes

(UW-Madison/SMHI)

AMSR-ESSM/I

Conclusions

Scattering signal shows good correlation to rain rate,

Active+passive mw simulation tools in place and show good agreement with observations

However, sensitivity varies strongly with type of precipitation event

High frequencies (e.g. AMSU 150 GHz) show much better response than lower (AMSU 89 GHz). Sensitivity is about a factor of 1.5 to 2 better.

Sensitivity of scattering signal to variations in surface emissivity is only critical for very dry atmospheres

Collecting AQUA+radar data