(SST) measurements can be used to determine seawater density, which regulates ocean circulation and the formation of water masses.

SEA SURFACE TEMPERATURE

Global all-weather sea surface temperature data are critical for NWP and climate research. SST measurements are important for understanding heat exchange and coupling between the ocean and atmosphere, and SST data are required by operational ocean analyses in order to properly constrain upper-ocean circulation and thermal structure.2 SST measurements in clear air can be obtained using electrooptical (traditional) instruments; however, clouds prevent these measurements, so passive microwave measurements within the 5 to 6 GHz region are critical for obtaining coverage in areas that are seasonally cloud-covered. For example, areas in the U.S. Exclusive Economic Zone off the coast of Washington and Oregon are not imaged with traditional satellite SST sensors for weeks at a time owing to persistent stratus cloud cover, necessitating an all-weather solution. The standard SST measurement uncertainty for space-based SST measurements is 0.5 K at 50 km (passive microwave, all-weather capability). To achieve this standard for microwave measurements, interfering signal power within a (typical) receiving bandwidth of 350 MHz (e.g., AMSR-E) must be below approximately –126 dBm3 using a factor of 10 power margin. For reference, this power level is effectively 3 dB higher than recommended levels from International Telecommunication Union-Radio (ITU-R) RS.1029, but still far below the level of interference that would be considered acceptable for nearly all other communication and signal systems. Space-based SST measurements near 6 GHz near land are impacted primarily by land-based emitters operating in the fixed service within full compliance of their regulations. Less pervasive RFI impacts are encountered from shipboard radar. For SST measurements using 10.7 GHz such as TRMM’s TMI and AMSR-E, substantive RFI is incurred from geostationary transmitters operating immediately adjacent to the upper edge of the 10.7 GHz EESS band segment as depicted in Figure 2.16 in Chapter 2 in this report.

SOIL MOISTURE

Global, high-quality soil moisture measurements are expected to advance weather forecasting and Earth hydrology studies significantly. A proposed NASA

2

C.J. Donlon, P.J. Minnett, C. Gentmann, T.J. Nightingale, I.J. Barton, B. Ward, and M.J. Murray, “Toward Improved Validation of Satellite Sea Surface Skin Temperature Measurements for Climate Research” Journal of Climate, 15: 353-369 (February 2002).

3

With a factor of 0.5 sensitivity, this value is (1.38 × 10–23 J · K–1)(0.05 K)(350 × 106 Hz) = 2.42 × 10–16 W.



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