Precipitation and altimeter Precipitation and altimeter missionsmissions

Jean Tournadre

Laboratoire d’Océanographie Spatiale

IFREMER Plouzane France

1. Basic principles of satellite altimetry

Interaction of the pulse of duration τ with a smooth sea surface.

For an altimeter in a 1000-km orbit a pulse duration of about 3 ns would lead to a footprint of diameter 2.8 km.

In this way, the altimeter is able to average out the effect of the ocean waves. This parameter is called a Significant Wave Height (SWH), max power : sigma0 (wind), epoch (sea surface height)

In practice sea surface is rough rather than flat. As a result, the first reflection of energy commences when the leading edge of the pulse reaches the topmost crests of the waves, earlier than for the flat surface, but the reflected energy does not achieve its maximum until the trailing edge reaches the lowest wave trough.

• Problem of rain for altimeter: perturbations of the signal: attenuation and change of propagation speed.

– Attenuation: modification of the waveforms

– Speed change : conversion time/distance• Since Topex: Radar altimeters operate at two frequencies:

Ku and a C or S bands, mainly for correction of ionospheric effects

• Rain detection from dual-frequency backscatter coefficient data well established (based of Ku vs C band attenuation)

• Used operationally for Jason and Envisat rain flagging

• Several studies showed that rain rate estimates are possible (McMillan et al 2002, Tournadre, 1998)

Context

• Two main problems for precipitations studies

– Low time and space sampling of the ocean by one altimeter (nadir view only)

– No estimation of the height of rain necessary (assimilated to the freezing level FL) to infer the surface rain rate from the attenuation over the atmospheric path

• New method to infer FL from radiometers (that are part of the altimetric missions)

• Merging of different altimeters data to improve the time and space sampling

Rationale

• Rain attenuation is frequency dependant :

• Attenuation by rain one/two order of magnitude larger at Ku band than that a C/S band

Detection of occurrence where Kuattenuated vs C/S

• Use of rain free Ku/C band relation

Overview of the Method

Mean « rain free » KU/C(S) relation (f)

Δσ 0 =f (σ0C / S)−σ0

Ku>1 . 8 rms(σ0C /S )

Lz>200 μm

RMS of the f relation

Lz:liquid water content from radiometer

Rain rate and Freezing level determination

• Rain rate from attenuation using Marshall-Palmer relation

• a and b frequency dependent coeff.

• H height of rain ~ freezing level• Problem : determination of FL?• Model of TB’s as a function of rain

rate and FL• Wilheit et al 1977 atmosphere

model• Rosencrantz 2002 Radiative

transfer model• FL estimated by inversion of TB

for rain flagged samples

b

HaR

/1

20

Topex TMR microwave TB

modelling

Radiometer Brightness Temperatures

• TB near 18 and 22 GHz selected for inversion for Topex and Jason (36.5 and 22 for Envisat). More sensitivity.

• Distribution of the TB for rain flagged samples.

FL and Rrad by inversion of TB18 and TB22

~35% of TBs outside model because of difference of sensor resolution

Validation of Freezing level estimates

• About 50 % of FL for low rain rate (<2mm/hr) and more than 80% for R>5 mm/hr

• Higher proportion for high latitude than in the Tropics.

Proportion of valid FL estimates

• Comparison with NCEP and ECMWF FL for 2003

• Underestimation of high FL (>4 km)

• Overestimation for FL<3 km• Overestimation at mid and

high latitude• Underestimation in the Tropics• Similar results with SSM/I and

TRMM Fl estimates using PR bright band

• Better than SSM/I and good overall agreement

Validation of Freezing level estimates

Topex

Jason

Envisat

Mean FL for winter 2003

JASONNCEPSSM/I F14

• Log-normal distribution method (Berg and Chase (1992) • The lognormal density : non linear function of two variables,

μ and .

• where p is the probability of non zero rainfall

value. and expressed by• Ri : ensemble of instantaneous rainfall estimates

Mean rain rate

P (R, μ,σ )= prσ √(2 π

exp [ 1

2σ 2

( ln r - μ )2

] R> 0

μ̂=1 /n∑i= 1

n

ln R i

σ̂=1 /n∑i=1

n

[ ln R i− μ̂ ]2

Mean annual rain rate fields for 2003

TopexTopex

JasonJason

EnvisatEnvisat

MergedMerged

GPCPGPCP

SSM/IF13SSM/IF13

•Very good agreement between Jason , Envisat and Topex expected for high northern latitude

•Good agreement with Global Precipitation Climatology Project

•More smaller scale details (coherent with TRMM climatology)

•Merged data set using histogram equalization for Envisat.

Mean annual rain rate fields for 2003

Mean latitudinal distribution

OTHER PARAMETER RAIN CELL LENGTH

•Very good along-track resolution by altimeter (~5-8 km)•Length= number of consecutive rain samples•Rain Cell Diameter Distribution (RCDD): exponential pdf

•Altimeter : Rain Cell Chord exponential pdf with l=2/•Mean rain cell diameter 1/

N ( D,τ )=N 0 ( τ ) exp [− λ ( τ ) D ]

Topex param

Can allow the study of rain event nature (stratiform, convective,..)

Online web server at IFREMER CERSAT

• All the data for the Topex,Jason and Envisat missions have been processed and are available at Ifrermer both on ftp and in a browser

MESCAL-ALTIKAAltika : altimeter in Ka band . Better performances but one major problem at Ka band (35.75 GHz) rain and cloud can strongly attenuate the signal and distort the echo waveform.

At Ka band the attenuation by cloud droplet is about 1.1 dB/km par g/m3 (10 time larger than Ku band), not negligible. Cloud at Ka = rain at Ku .

Cloud more frequent than rain. Necessity to analyze in detail the effect of cloud on the signal

Estimation of attenuation, off-nadir angle, leading edge slope at 1Hz and 20 Hz as a function of cloud parameters (IWC, height, diameter,..)

Estimation of the impact of cloud on the geophysical parameters (ssh,s0, SWH) retrieval: waveform modeling and MLE4 (Stenou et al). Determination of data availibility

Previous studies (Tournadre 1999). In presence of rain larger than 1-2 mm/hr the distortion of the waveform will inhibit the geophysical parameter retrieval.

Waveform distortion by rain and cloud

Ka band : strong attenuation by rain and cloud

But more important:

strong distortions of the waveform shape : modification of leading edge and plateau slope

10

1

0.1

Altika WF over a 10 km 2.5 mm/hr rain cell

Return power Attenuation

Modeled waveforms using (10 km resolution 10 km) AIRS liquid water data from AIRS on AQUA

attenuationOff-nadir angle variations

Ex of Altika pass over cloud or low rain

Waveforms in log scale

Off-nadir

sigma0

Variation of zeta Local attenuation ? Need of a 2nd frequency

Raw waveforms in log scale

Change of slope

LWC

WVC

TB23 & 37 GHz

• Altimeter can be use to estimate oceanic precipitations.• A 13 years database already exist (pb of time to

complete for a 20 year one• In the perspective of MESCAL the use of Ka band could

allo to estimate the integrated precipitation for low rain rate.

Conclusion