Winter Diurnal Trends of Sierra Nevada Supercooled Liquid Water and Precipitation

Abstract

Ten-year climatologies from the Sierra Cooperative Pilot Project (SCPP) show diurnal variations of clouds, precipitation, supercooled liquid water, stability and temperature. Diurnal variations were generally more pronounced in March than in January and February. March, in particular, developed a pronounced maximum of afternoon convection due to strong modulation of spring air masses by surface heating. Supercooled liquid water was most abundant between midnight and dawn and least abundant at midday; this trend grew stronger from January through March. Two effects help to explain the diurnal trend of supercooled liquid water at Squaw Peak, a mountain top location. First, when the freezing level stayed above Squaw Peak during the day but descended below it at night, water cloud was only supercooled at the site at night. Second, low-level water cloud often dissipated as surface heating raised the temperature of overlying air above its dewpoint; water cloud formed at night through a reverse process.