A midlatitude cirrus cloud climatology from the facility for atmospheric remote sensing. Part I: Macrophysical and synoptic properties

A uniquely extensive high cloud dataset has been collected from the Univers ity of Utah Facility for Atmospheric Remote Sensing in support of the First (ISCCP) International Satellite Cloud Climatology Project Regional Experim ent extended time observations satellite validation effort. Here in Part I of a series of papers examining the climatological properties of the cirrus clouds studied over Salt Lake City, Utah, similar to 2200 h of data collec ted from 1986-96 is used to create a subset of 1389 hourly polarization rub y (0.694 mum) lidar measurements of cloud layer heights. These data were ob tained within +/-3 h of the local 0000 UTC National Weather Service radioso nde launches to provide reliable cloud temperature, pressure, and wind data . Future parts of this series will consider the inferred cirrus cloud micro physical and radiative properties. In addition to describing the cirrus macrophysical properties in terms of t heir yearly, seasonal, and monthly means and variabilities, the synoptic we ather patterns responsible for the cirrus are characterized. The strong lin kage between cirrus and weather is controlled by upper-air circulations mai nly related to seasonally persistent intermountain region ridge/trough syst ems. The cloud-top heights of cirrus usually associated with jet streams te nd to approach the local tropopause, except during the summer season due to relatively weak monsoonal convective activity. Although a considerable deg ree of variability exists, 10-yr average values for cirrus cloudbase/top pr operties are 8.79/11.2 km, 336.3/240.2 mb, -34.4 degrees/-53.9 degreesC, 16 .4/20.2 m s(-1), and 276.3 degrees /275.7 degrees wind direction. The avera ge cirrus layer physical thickness for single and multiple layers is 1.81 k m. Estimates of cloud optical thickness tau based on a "thin'' (i.e., bluis h) visual appearance suggest that tau less than or similar to 0.3 occur sim ilar to 50% of the time for detected cirrus, implying that the cirrus in th e region of study may be too tenuous to be effectively sampled using curren t satellite methods. The global representativeness of this extended cirrus cloud study is discussed.