Precipitation and altimeter missions Jean Tournadre Laboratoire d’Océanographie Spatiale IFREMER...
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Transcript of Precipitation and altimeter missions Jean Tournadre Laboratoire d’Océanographie Spatiale IFREMER...
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