Measurements of Microwave Emission from a Foam-Covered, Wind-Driven Sea

Measurements were made from aircraft of the 1.55-cm microwave emission from the North Sea and North Atlantic at surface wind speeds ranging from less than 5 to 25 m sec−1. Brightness temperatures in the nadir direction increased almost linearly with wind speed from 7 to 25 m sec−1 at a rate of about 1.2C (m sec−1)−1. At 70° from nadir the rate was 1.8C (m sec−1)−1. This increase was directly proportional to the occurrence of white water on the sea surface. At wind speeds <7 m sec−1, essentially no white water was observed and brightness temperatures in the nadir direction were ∼120K; at wind speeds of 25 m sec−1 white water cover was on the order of 30% and average brightness temperatures at nadir were −142K. Maximum brightness temperatures for foam patches large enough to fill the entire radiometer beam were 220K.