POES MSU Temperatures

The primary goal of the Microwave Sounding Unit (MSU) on the NOAA POES satellites is to make temperature soundings through clouds.  This is important since clouds cover approximately 50% of the Earth.  MSU resolution is coarse relative to other infrared sounding channels we have discussed, this is a result of the microwave properties (with wavelengths on the order of 1cm). It has a nadir FOV of 111km diameter, while the HIRS/2 has a nadir FOV of 18.5km.   As noted, microwave channels are typically referred to by their frequencies.  The MSU channels 1-4 cover microwave frequencies of 50.3 GHz, 53.74 Ghz, 54.96GHz, 57.95 GHz, these are O2 absorption regions.  On the new NOAA 15 satellite, the MSU and SSU instruments have been replaced with two Advanced Microwave Sounding Units (AMSU-A and B).   The MSU channels measure radiance emitted by molecular oxygen in the atmosphere, as we know, the radiance emitted varies with the temperature of the gas.  Using this approach, the temperature of different layers of the atmosphere can be remotely sensed.  Like the other sounding retrievals we discussed (e.g., for the GOES Sounder), there is a complex procedure for obtaining a vertical temperature profile.

•  First there are corrections for the satellite viewing angle, so that all observations can be treated as if they were nadir viewing.
• solar zenith angles, terrain elevation and initial guess values of the surface (or skin) temperature and surface albedo must be specified
• the HIRS window channels are corrected for water vapor absorption
• an atmospheric radiance module is used to calculate spatially averaged clear-column radiances
• the HIRS brightness temperatures are used in regression equations to estimate the MSU brightness temperatures, if clouds are present, the estimated MSU brightness temperatures will be less than the observed brightness temperature.  For entirely cloudy FOV's, only the MSU values are used.

How accurate are these soundings of temperature retrieved from satellite-measured randiances?  This is an important question to ask.  A lot of effort has gone in to addressing this question, although its difficult to answer.  Typically, soundings are compared to radiosonde observations, which are not error free, and furthermore the two approaches are fundamentally different.  Satellite soundings have difficulty when there are sudden changes in the temperature lapse rate with height.  Especially in the case of moisture soundings. There is some evidence that for cloudy soundings, which rely on microwave radiances for retrieving tropospheric temperatures, that there is a negative bias comparing satellites and radiosondes (the satellite temperatures are too cool in the middle troposphere).  There have been significant efforts to deal with and account for these kinds of differences.

 The application of these remotely sensed soundings to address changing tropospheric temperatures is an exciting area of study, and is worthy of further investigation.  One site that has done considerable research into the use of tropospheric temperatures to evaluate climate is the University of Alabama at Huntsville (John Christy), in conjunction with the Marshall Space Flight Center.  We can go to their site to look at images of current tropospheric temperature data for the US or the GLOBE, and we can evaluate the changes in tropospheric and stratospheric temperatures through time with an interactive applet which shows global and regional temperature anomolies (deviations from the long term average).  

Global Hydrology and Climate Center

Link to GHCC microwave temperature sounding material.  A link the the GHCC home page is also provided here, and from the EVSC494 Links page.