Time variation of cloud distribution near surface wind convergence zone in the Nobi Plain during daytime on summer sunny days

Abstract

The present study investigates the relationship between surface wind convergence and clouds by performing statistical analyses of time variations in surface wind field, its horizontal divergence, cloud distribution, and appearance frequency of radar echoes in the Nobi Plain in the daytime on summer sunny days using various observation data from 1990 to 1999. The 99 summer sunny days when general wind was weak were selected objectively in advance of the analyses. The results of analyses show that a westerly wind from the Biwa Lake direction and a sea breeze from Ise Bay converge in the north of Nagoya after 1300 JST on summer sunny days. In addition, clouds (cumulus) tend to be formed all over the Nobi Plain from late in the morning to about noon by thermal convections, and the formation of clouds continues near strong surface wind convergence zones until around 1500 JST on summer sunny days, although clouds are hardly formed in the afternoon in almost all parts of the plain due to the decline of the thermal convections. The continuation of the cloud formation is especially remarkable at the northeast of Nagoya. Thus, in the Nobi Plain on summer sunny days, the conditions where clouds are easily formed continue at the northeast of Nagoya until around 1500 JST, because of a strong surface wind convergence between a westerly wind from the Biwa Lake direction and a sea breeze from Ise Bay. The analysis of radar echoes observed around 1500 JST shows that the echoes occasionally appear near the northeast of Nagoya on summer sunny days. These results indicate that the strong surface wind convergence plays an important role in the formation of clouds, after thermal convection has weakened, on summer sunny days. © 2004, Meteorological Society of Japan.