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Cooperative Institute for Meteorological Satellite Studies / University of Wisconsin-Madison
Tropical Cyclones
AMSU

Additional information on the CIMSS AMSU TC Intensity Method can be found at the CIMSS AMSU Homepage
 
The Advanced Microwave Sounding Unit (AMSU): The AMSU consists of two instruments, AMSU-A and AMSU-B. These instruments are capable of measuring the brightness temperatures over a number of layers of the atmosphere. The CIMSS AMSU TC intensity method uses 5 channels from the AMSU-A instrument and 1 channel from AMSU-B. AMSU was launched in 1998. Currently there are AMSU's aboard NOAA-15, NOAA-16, NOAA-17 (AMSU-A is no longer functioning), NOAA-18, Aqua and METOP polar orbiting satellites.
 
Using AMSU to Measure TC Intensity: A Tropical Cyclone (TC) is warm core low pressure system. The strength of the warm core is directly related to the storms intensity. As the magnitude or height of the warm anomaly of the TC changes, so does the TC intensity. While the AMSU instrument can not measure the true atmospheric temperature (these temperatures can be obtained through a variety of retrieval routines) the antenna temperatures measured by the intrument can be converted to brightness tempperatures that are direct corellaries to the true temperature. Using the brightness temperature anomaly the TC intensity can be obtained. There is a direct relationship between the the temperature anomaly measured by AMSU and TC intensity. However this relationship can be strengthened if some of the AMSU error sources are acounted for. There are 4 primary sources of error:
 
1) The presence of mixed-phase and frozen hydrometeors can act to reduce the Tb sensed by the instrument. This effect is most severe at lower channels but can reach as high as channel 8. Because the CIMSS AMSU algorithm uses channels 6,7 and 8 to determine the TC Tb anomaly this scattering effect must be accounted for.
 
2) AMSU is a cross-track scanning radiometer. This scanning strategy leads to a decrease in resolution as the scene being viewed moves away from nadir. The resolution at nadir is ~ 50 km decreasing to ~ 100 km at the limb. Thus storms viewed near the scan limb will not be as well-resolved as those near nadir.
 
3) Even at nadir the highest resolution of the instrument is 50 km. Because the eye of a TC constrains much of the warm core eyes with a diameter less than 50 km will result in the warm core being sub-sampled. Ancillary eye size information from the IR-based ADT is used to correct the AMSU estimates when a clear eye is present. In cases when a clear eye is not present the information comes from TC structure messages issued by the TC warning centers. These messages contain the Radius of Maximum Winds (RMW) parameter. The RMW is used as a proxy for eye size. In cases when the eye size is less than the resolution of the AMSU scan view being used by the algorithm the eye size information is used to lower AMSU MSLP estimates.
 
4) The storm may fall in-between scan views (an effect known as bracketing). Because the storm core may only be 20 km in diameter and AMSU scan views may be 100 km apart this effect can result in sub-sampling of the warm core. Occasionally a strong TC with a very large eye may be very well resolved by the instrument. In these cases there can be a tendency for the estimates to be too deep.
 
AMSU Algorithm Performance: The AMSU algorithm was developed using reconnaisance-based verification of MSLP and MSW for 470 cases from 1998-2004 then independently tested using 264 cases from 2005-2006. Results are listed in Table 1 below.
 
Bias AAE RMSE
MSLP 0.3 mb 5.4 mb 7.8 mb
MSW -1.9 kts 7.8 kts 10.0 kts

 
Table 1. AMSU TC intensity algorithm statistics for 264 independent cases from 2005-2006. Negative bias indicates method was too weak.

References:
Herndon, D. and C. Velden, 2004: Upgrades to the UW-CIMSS AMSU-based TC intensity algorithm, 26th Conf. on Hurr. And Trop. Meteor., Miami, FL, Amer. Meteor. Soc., 118-119.
 
Brueske, K. F., and C. Velden, 2003: Satellite-Based Tropical Cyclone Intensity Estimation Using the NOAA-KLM Series Advanced Microwave Sounding Unit (AMSU). Mon. Wea. Rev., 131, 687-697.

 

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