water rain effects - byrd polar and climate research...

Effects of Rainfall on WatER

Ernesto RodriguezJet Propulsion Laboratory

California Institute of Technology

Effects of Rainfall on WatER

• Water is a Ka-band instrument• Ka-band radiation is strongly attenuated in the

presence of rain• Unlike nadir altimeters, the main effect of rain on the

radiometer will be a loss of signal-to-noise ratio, withan increase in height noise.

• Currently, the WatER budget allows for meeting thescience requirements with a drop in power of 6dB

• There is also a concern about rainfall data loss beingcorrelated with time of day if a sun-synchronous orbitis used

Attenuation Effects at Ka-Band

6 dB311.5 dB5

215 dB10073 dB3050 dB2024 dB10

35 GHz Attenuation(5-km path)

Rain rate mm/hr

WatER will only be able to collect valid data at rainrates smaller than 3-5 mm/hr (depending onsurface water brightness)

Walsh, et al., Rain and Cloud Effects on aSatellite Dual-Frequency Radar AltimeterSystem Operating at 13.5 and 35 GHz,IEEE Trans. GRS, 22, 1984

Rain Probability

Petersen WA, Nesbitt SW, Blakeslee RJ, et al.TRMM observations of intraseasonal variability in convective regimes over the Amazon JOURNAL OF CLIMATE 15 (11): 1278-1294 JUN 1 2002

The total WatER data loss, if data were uncorrelated with time of day,will be less than 10% in the tropics. Similar numbers also hold at otherlatitudes.

Data Loss vs Time of Day

• In non-tropical regions, the correlation between time of day andrain events is weak– Rainfall dominated by fronts– Data loss expected to be < 10%

• Tropical rainfall is governed by convective instabilities whicharise due to daytime heating– Thunderstorms tend to happen in the late afternoon or

evening– Rainfall pattern is patchy– Correlation distance between rain events < 50 km to 100 km– 6 am is near the rainfall probability minimum, while 6 pm is

close to the rainfall probability maximum– See results below for more details

Spatial Distribution of Rain Cells-1

Spatial Distribution of Rain Cells-2

Monthly Weather Review, 134, 2004

Note localized rain cells with patchydistribution

Seasonal Variation of Precipitation


Local Time of Maximum Precipitation


Evolution of Rain Rate vs Local Time-NW South America-1


Evolution of Rain Rate vs Local Time-NW South America-Rain Gauges


Rainfall vs Time of Day

Journal of Climate, 16, 2003

Amazon/LBASoutheast US


Conclusions

• There will likely be a global data loss <10% due rainfall• The data loss away from the tropics is not correlated with time

of day in a significant way• In the tropics, the likelihood of rain for the 6pm passes is

significantly higher than for the 6am passes. However,averaging between the two, a 10% data loss is still expected.

• Temporal sampling in the tropical regions may be moreimpacted than away from the tropics due to greater loss of datain the 6pm passes

• However, the temporal sampling will still meet the samplingrequirements for the mission

• A 10% data loss is acceptable for spaceborne remote sensingmissions: e.g., Topex or Jason have 81% data recoveryrequirements

• Due to power considerations, a 6am/6pm sun-synchronousorbit is still the most attractive possibility.

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