Common microfronts and other solitary events in the nocturnal boundary layer

Microfronts with abrupt changes of temperature and/or wind vector are found to be common for weak winds and thin stable boundary layers. In this study, microfronts and their parent structures on time-scales of minutes or tens of minutes are sampled from fast-response tower measurements of temperature and the three velocity components during FLOSSII. Cold microfronts are generally characterized by rising motion, followed by stronger stratification and weaker turbulence. Sinking warm air prior to the cold microfront, followed by rising cold air after the microfront passage, corresponds to conversion of kinetic energy to potential energy. This conversion requires a source of external energy for maintenance of the circulation. The shallow cold microfronts appear to be often related to deeper fast-moving disturbances in the horizontal velocity field. Warm microfronts generally lead to stronger wind and turbulence after the microfront passage. Gust microfronts induce rising motion in advance of the microfront. Solitary waves cause little net change of temperature and are systematically embedded within larger-scale deeper disturbances at this site. The sensitivity of the results to the sampling window width and sampling criteria is examined. Copyright (C) 2010 Royal Meteorological Society