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 University of Utah Facility for Atmospheric Remote Sensing in support of the First (ISCCP) International Satellite Cloud Climatology Project Regional Experiment 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 collected from 1986-96 is used to create a subset of 1389 hourly polarization ruby (0.694 mu m) lidar measurements of cloud layer heights. These data were obtained within plus or minus 3 h of the local 0000 UTC National Weather Service radiosonde launches to provide reliable cloud temperature, pressure, and wind data. Future parts of this series will consider the inferred cirrus cloud microphysical and radiative properties. In addition to describing the cirrus macrophysical properties in terms of their yearly, seasonal, and monthly means and variabilities, the synoptic weather patterns responsible for the cirrus are characterized. The strong linkage between cirrus and weather is controlled by upper-air circulations mainly related to seasonally persistent intermountain region ridge/trough systems. The cloud-top heights of cirrus usually associated with jet streams tend to approach the local tropopause, except during the summer season due to relatively weak monsoonal convective activity. Although a considerable degree of variability exists, 10-yr average values for cirrus cloud-base/top properties are 8.79/11.2 km, 336.3 /240.2 mb, -34.4 degrees /-53.9 degrees C, 16.4/20.2 m s super(-) super(1) , and 276.3 degrees /275.7 degrees wind direction. The average cirrus layer physical thickness for single and multiple layers is 1.81 km. Estimates of cloud optical thickness tau based on a ``thin'' (i.e., bluish) visual appearance suggest that tau < approximately 0.3 occur similar to 50% of the time for detected cirrus, implying that the cirrus in the region of study may be too tenuous to be effectively sampled using current satellite methods. The global representativeness of this extended cirrus cloud study is discussed.