INTERACTION OF SECONDARY CIRCULATIONS WITH THE SUMMER MONSOON AND DIURNAL RAINFALL OVER HONG-KONG

A complete yearly record (1988) of surface measurements is used to exa mine the atmospheric diurnal secondary circulations over the entire ar ea of Hong Kong in conjunction with spatial and temporal variations of surface temperature, wind speed and rainfall. Evidence of atmospheric diurnal secondary circulations is found at 10 sites. The occurrence o f a summer morning rainfall maximum over the coast results from the in teraction of the large-scale summer monsoon and local mesoscale second ary circulations. The afternoon onshore secondary circulation accelera tes the advection of warm, humid unstable air and, coupled with the up ward orographic lifting, produces enhanced rainfall along windward mou ntain ridges. Dynamical and scaling considerations suggest that the bl ocking effect is negligible and the primary forcing mechanism is land- sea temperature difference, but terrain effects are also important. Al though the secondary circulation system's strength and timing vary, th e circulation behaves like a classic sea-land breeze circulation, comp licated by superimposed mountain-valley breezes.