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Calibration Method of Microwave Tb for Retrieving Accurate SST
Akira ShibataJAXA / EORC
Advances of Satellite Oceanography at Vladivostok , Oct. 3-6, 2007
Microwave SST• Microwave radiometers with lower frequencies (6-10GHz) can provide SST through clouds, which is very valuable both for operational and research works • Two radiometers, Advanced Microwave Scanning Radiometer–E (AMSR-E) and TRMM Microwave Imager (TMI), have been operated normally for five years and ten years, respectively• But, a calibration issue of brightness temperature (Tb) for both sensors still remains
Comparison of Two RadiometersTMI AMSR-E
Coverage 40S-40N GlobalOrbit non-sun sun-sym (1PM)Duration Dec. 1997 ~ May 2002 ~Freq. (GHz) 10,19,21,37,85 6,10,18,23,36,89
Monitoring of El Nino and La Ninaby AMSR-E
Monitoring of Positive SST Anomaly in Arctic Ocean by AMSR-E
Tb Calibration Issue
• Latest version (V3) of JAXA for AMSR-E still contains a calibration error of Tb with 1K order, and NASA V6 TMI also contains an error with similar order
• Sensitivity of microwave Tb of 6V and 10V to SST is 0.5K/ºC for SST above 10 ºC, and its value of 6V around SST 0 ºC is 0.3K/ ºC
• Therefore, Tb must be calibrated with an error of less than 0.1K to obtain accurate SST of 0.1-0.2 ºC error
Microwave SST Algorithm Outlinein case of AMSR-E
1. Atmospheric correction on 6V using 23V and 36V2. Wind correction on 6V using 6H
• dependable on relative wind direction, of which information can be estimated by a combination of 36GHz and 6H • also dependable on air-sea temperature difference, in which air temperature is obtained from a weather forecast model
3. Salinity correction on 6V4. Convert 6V to SST5. Remove following areas
• sea ice, sun glitter, and coast
6H*(K)
up
down
z0
Wind Correction on 6/10V (1/2)
slope=sp
Δ 6/10V
= 0 for 6/10H* less than z0
= (6/10H* – z0) × sp for 6/10H* greater than z0
6V*(K)
cross
6V(H)* = amsr_6V(H) – atmos_effect_6V(H) –
calm_ocean_6V(H)
SST dependent
10V(H)* defined similarly for TMI 10GHz
Wind Correction on 6/10V (2/2)
Tuning of Tb (1/3)
Calibration of 6/10V ;To minimize a difference of two SSTs in a range of 6/10H* between (z0-1) and z0
Calibration of 6/10H ;To minimize a jump of SST difference around z0
Tuning of Tb (2/3) – 6/10V -
Sensitivity of 6/10V to SST ~ 0.5K/ °C
+0.2K
-0.2K
-0.2K
+0.2K
Tuning of Tb (3/3) – 6/10H -
Impact of 6/10H on SST ~ 1K/ °C
Day
in Apr.-May
10H error ~ 10V × 0.7
Result of Error Estimation for TMI 10Vin case of Satellite Forward Direction
Day
Result of Error Estimation for TMI 10Vin case of Satellite Backward Direction
in Apr.-May
After Tuning of Tb Local time
TMIJan.1, 2006
Result of Error Estimation for AMSR-E 6Vin Ascending Orbit
Equ.
60S 60N
Jan.
Dec.
1K
AMSR-E SST Accuracy -RMS-
comparison with buoys during 5 years in Northern Hemisphere (N.H.)
July January
rms = 0.49ºC rms = 0.60ºC
AMSR-E SST Accuracy –Bias- (unit ºC)
SST Range Jan. Apr. Jul. Oct.N.H. 0- 5ºC 0.37 0.19 — —
5- 10 0.07 0.19 0.03 0.0010-15 0.00 0.14 0.02 0.0815-20 -0.03 0.03 -0.05 0.0320-25 -0.08 0.05 -0.03 -0.0325-30 -0.05 0.02 0.03 0.00
S.H. 25-30 -0.06 -0.09 -0.03 0.0220-25 -0.09 -0.14 -0.12 -0.1015-20 -0.01 -0.03 -0.08 -0.0310-15 0.00 0.03 -0.07 -0.01
5-10 -0.08 -0.01 -0.05 0.11 0- 5 -0.25 -0.28 -0.05 0.06
Long Term Trend of AMSR-E Tb
Conclusions• Method of calibrating microwave Tb at 6 and 10GHz was presented, which was based on a wind correction on 6/10V• After tuning of TMI Tbs, a diurnal cycle of SST was clearly observed • After tuning of AMSR-E Tbs, a difference of AMSR-E and buoy SSTs was mostly laid within 0.1ºC, though 0.2 or 0.3 ºC difference was found in low SST in both hemispheres• Trend of AMSR-E Tbs was not observed, which might indicate an application for monitoring the global SST warming in a near future, by combining AMSR-E and its following