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