Highlights of Analysis

Satellite microwave (MW) radiometry has a 35-year heritage of providing highly accurate geophysical retrievals for Earth science. By viewing Earth over a broad spectral band ranging from 6 to 90 GHz, a large set of environmental parameters can be simultaneously estimated. The lower frequency channels penetrate the layers of cloud, giving an uninterrupted view of Earth’s surface. MW surface measurements include sea-surface temperature and wind, sea ice extent, snow cover, and soil wetness. The higher frequencies provide information on atmospheric moisture in all of its various forms: vapor, cloud, rain, and ice.4

The alteration of the planet’s hydrologic cycle due to global warming is one of the most (if not the most) critical issues associated with climate change, and satellite MW radiometry products are important inputs for the calculation of the planet’s changing water and energy budget. MW radiometry provides direct measurement of precipitation over both land and ocean. Over the ocean, sea-surface temperature, wind speed, and water vapor are retrieved, all of which are needed for the computation of evaporation; there is also the potential to determine water vapor advection and storage, drivers for air-sea fluxes, and global ocean circulation and upwelling. Additional hydrologic parameters are sea ice extent, snow cover, and soil wetness, although the latter two are more qualitative measurements.

The importance of sea-surface temperature (SST) to climate research/science is hard to overstate. For example, SST is a key parameter in determining how the water and energy fluxes at the air-sea interface affect the hydrologic cycle and the surface radiation balance (e.g., Curry et al., 2004). The intensity, frequency, and location of hurricanes are in part determined by the availability of oceanic heat to sustain, encourage, or dissipate these storms. Climate oscillations such as the El Niño Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and Pacific Decadal Oscillation (PDO) all have characteristic signatures that are visible in patterns of SST, precipitation, water vapor, cloud cover, and surface winds (scalar and vector). Because 6 GHz MW observations penetrate the clouds, are not affected by aerosols, and are only slightly affected by water vapor (an effect that is easily removed using higher frequency channels), the microwave radiometer has a distinct advantage over infrared sensors. The endemic cloud cover at high latitudes prevents monitoring of ocean temperatures by infrared radiometers, and microwave radiometers provide the only way to continually measure SST in these vital Arctic regions, which are now experiencing rapid climate change. Tropical convergence zones are also prime examples of persistently cloudy regions where SST detection by infrared sensors is problematic. However, the MW radiometer cannot provide the high spatial resolution or the near-coastal retrievals offered by the IR techniques. The two techniques are thus highly synergistic, and the combination of the MW radiometer with IR measurements provides the best means to accurately measure SST.

Microwave radiometers, specifically the Scanning Multi-channel Microwave Radiometer (SMMR) and Special Sensor Microwave Imager (SSM/I), provided the first convincing evidence that the Arctic polar ice was depleting. The Arctic ice plays a critical role in global climate change by regulating ocean-atmosphere transfers of energy and water and helping control ocean surface salinity. Sea ice albedo feedbacks amplify climate impacts in the polar regions. Variables such as ice extent, concentration, and type are important for navigation as well as for marine habitat assessment. Satellite observations of September minimum sea ice show that sea ice extent has declined 8.6 percent +/− 2.9 percent per decade over the period 1979-2006 (Serreze et al., 2007). Snow cover in the Northern Hemisphere has also declined by 1.28 × 106 km2 over the period 1972-2006 (Déry and Brown, 2007).5 This decline in snow cover is significant because, compared with other land cover types, snow has a very high albedo and climate feedbacks are felt on local, regional, and even hemispheric scales.


Monitoring water vapor via its emission at 22.235 GHz, the MW radiometer provides the most accurate means to measure total columnar water vapor. In the absence of rain, the MW radiometer also provides the most accurate means to measure total columnar cloud water. By design (i.e., by including the 89 GHz channels), the MW radiometer can detect scattering from ice particles, the third phase of water. However, unlike the measurements of water vapor and liquid water, this detection is not a quantitative measurement.


In the IPCC 4th Assessment it is stated that “continuous satellite measurements capture most of the Earth’s seasonal snow cover on land, and reveal that Northern Hemisphere spring snow cover has declined by about 2 percent per decade since 1966, although there is little change in autumn or early winter. In many places, the spring decrease has occurred despite increases in precipitation” (IPCC, 2007, p. 18). Recent unpublished work concludes that snow cover in the Northern Hemisphere has been declining at a rate of about 3 to 5 percent per decade during spring and summer (Brodzik et al., 2006).

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