response of active and passive microwave sensors to precipitation at mid- and high altitudes ralf...
TRANSCRIPT
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