Diurnal Variations of Snow Precipitation in Wakasa Bay during Winter

Abstract

The diurnal variation of snow precipitation in the west coastal area near Wakasa Bay has been investigated for two winter seasons of 2001 and 2003 using surface precipitation radar, rain gauge, and satellite infrared data. Radar reflectivity-derived precipitation intensity shows a clear diurnal cycle within Wakasa Bay for both years, although the cycle is clearer in 2001 than in 2003. The precipitation maximum occurs in the early morning, and the minimum occurs in the evening. Using radar data collected during January 2003, the precipitation diurnal cycle within the Bay is compared with three nearby regions: offshore—open water region to the north, inland—land region to the northeast, and inshore—coastal region to the northeast. It appears that all the other three regions have the precipitation maximum during the day and the minimum during night, although the diurnal variation inland (over land) is very small. Additionally, in the offshore region, there exist two precipitation maxima and minima during a 24-hour day. Analysis of precipitation data from AMeDAS (Automatic Meteorological Data Acquisition System) rain gauge stations basically agrees with the radar observations. It shows that a similar diurnal cycle, found in the radar data within Wakasa Bay, can also be found for coastal stations, although the different patterns are shown depending on the location and time period in 2003. When averaging data from both January and February during 2001 and 2003, the diurnal cycle tends to be smoothed out. Cloud top temperature and cloud fraction derived from satellite infrared data do not show a clear diurnal cycle within Wakasa Bay, nearby inshore and inland regions, although the decreases of brightness temperatures are seen around noon in the offshore region for both cases of 2001 and 2003. Analysis using collocated satellite and radar data indicates that cloud top temperature has little skill in reflecting surface precipitation for the winter convective clouds associated with cold air outbreaks. Finally, possible causes of the diurnal variation are discussed, including local land-sea breeze and mountain wind effects, as well as the radiative cooling effect.