Using the special sensor microwave imager to monitor surface wetness

The frequencies flown on the Special Sensor Microwave Imager (SSM/I) are se nsitive to liquid water near the earth's surface. These frequencies are pri marily atmospheric window channels, which receive the majority of their rad iation from the surface. Liquid water near the surface depresses the emissi vity as a function of wavelength. The relationship between brightness tempe ratures at different frequencies is used to dynamically derive the amount o f liquid water in each SSM/I observation at 1/3 degrees resolution. These d ata are averaged at 1 degrees resolution throughout the globe for each mont h during the period of 1992-97, and the 6-yr monthly means and the monthly anomalies of the wetness index are computed from this base period. To quant ify the relationship between precipitation and surface wetness, these anoma lies are compared with precipitation anomalies derived from the Global Prec ipitation Climate Program. The analysis was performed for six agricultural regions across six continents. There is generally a good correspondence bet ween the two variables. The correlation generally increases when the wetnes s index is compared with precipitation anomalies accumulated over a 2-month period. These results indicate that the wetness index has a strong corresp ondence to the upper layer of the soil moisture in many cultivated areas of the world. The region in southeastern Australia had the best relationship, with a correlation coefficient of 0.76. The Sahel, France, and Argentina s howed that the wetness index had memory of precipitation anomalies from the previous months. The memory is shorter for southeastern Australia and cent ral China. The weakest correlations occurred over the southeastern United S tates, where the surface is covered by dense vegetation. The unique signal, strengths, and weaknesses of the wetness index in each of the six study re gions are discussed.