Uses of satellite microwave measurements in weather and climate studies: status, challenges and requirement for future Instrumentation

In the past two decades, much progress has been made in the utilization of satellite microwave data in numerical weather prediction (NWP) models and is attributed to improvement in the satellite instrumentation, continued increase in the computational power and related improvements in the numerical models and data assimilation techniques. Remotely sensed microwave data for the atmosphere, land, and oceans provide critically important information to better understand and predict the effects of both weather and climate, and are now a major component of the global environmental observing system. The advanced microwave sounding unit (AMSU) on board a series of NOAA satellites has alone significantly increased the accuracy of global medium-range forecasts. The microwave sounding unit (MSU) on board earlier NOAA satellites has also provided temperature observations of lower troposphere to stratosphere and a 20 year length of the MSU tropospheric temperature record has been constructed and utilized for diagnosing and projecting the trend of global tropospheric temperature. Many other applications of microwave data include the global precipitation climatology produced from combined microwave, infrared and other in-situ data.

Recently, new satellite microwave data are available from Windsat, SSMIS, AMSR-E and MHS. These instruments provide unprecedented observations of the earth's environments and offer some good opportunities to further improve our understanding on weather and climate and benefit significantly numerical weather predictions. The Windsat provides four Stokes components at 10, 18 and 37 GHz and allow for simultaneous retrievals of wind speed and direction over oceans. The SSMIS the first time observes the atmospheric temperature from a conical scanning mode. Both SSMIS and MHS have some channels in millimeter wavelength between 150 and 191 GHz, which is vital for improvement in precipitation analysis and estimation. The observations from AMSR-E and Windsat at 6 and 10 GHz over land are uniquely linked to soil moisture content and other land surface parameters.

However, our current knowledge is not adequate for understanding all observed phenomena from these advanced microwave instruments. For example, Windsat data signify cloud and precipitation boundary well from its third and fourth Stokes components whose intensities could be the same as or stronger than those produced from ocean rough surfaces. In Greenland and Antarctic regions, theWindsat displays pronounced surface features. In the instrument calibration area, both SSMIS and Windsat are not well characterized for their biases and noise. For instance, systematic biases occur at the SSMIS temperature sounding channels and within several latitudinal zones and presumably result from its main reflector emission or scattering, and solar contamination on the warm calibration targets. It has been found that several Windsat polarimetric channels display systematic biases up to a few tenth degrees in Kelvin.

Current satellite microwave measurements from polar-orbiting platforms are constellated well by offering a good coverage for synoptic scale weather systems. A major observational gap remains for sub-synoptic, mesoscale and convective systems whose temporal evolution is so rapid and for which the performances of the current sounding algorithms and the data assimilation techniques are degraded due to clouds and precipitation. Uses of satellite microwave data for these events remain challenging although a major development effort is undergoing in the US Joint Center for Satellite Data Assimilation (JCSDA).

This presentation will be an overview of uses of satellite microwave measurements for weather and climate studies, challenges in understanding and utilization of new sensor data and some critical issues related to new microwave instrument calibration and product validation, and perspectives on future satellite microwave programs.