Characteristics of the Variation of Raindrop Size Distribution in Baiu Season

Abstract

Observational data of RSD (Raindrop Size Distribution) in the Baiu season were analyzed by using Principal Component Analysis in order to detect objectively the characteristics of variation of RSD. The results indicated that 88.4% of the variation of RSD was represented by the first and second principal components. The primary component of variation was to increase the number concentration for all sizes of drops; in other words, a parallel movement in a semi-logarithmic plot (linear diameter-logarithmic number concentration). It was found that this variation has a good correlation with that in the rainfall rate. This is different from the Marshall-Palmer-type RSD, which assumes only the slope parameter λ depending on the rainfall rate. The secondary component of variation was to increase (decrease) number concentration for larger (smaller) drops, in other words, the rotational movement in a semi-logarithmic plot. This component was considered to be connected with physical factors other than rainfall rate. With respect to small rainfall rate, comparison between RSDs in convective, and stratiform precipitations, showed that the slope of RSD was smaller in stratiform than in convective precipitation for equal rainfall rate. This means that the difference between convective and stratiform precipitation is one of the possible factors relating to the rotational variation.