Eight years of high cloud statistics using HIRS

Over the last 8 yr frequency and location of cloud observations have been c ompiled using multispectral High Resolution Infrared Radiation Sounder (HIR S) data from the National Oceanic and Atmospheric Administration polar-orbi ting satellites: this work is an extension of the I-yr dataset reported by D. Wylie ct al. The CO, slicing algorithm applied to the HIRS data exhibits a higher sensitivity to semitransparent cirrus clouds than the cloud algor ithm used by the International Satellite Cloud Climatology project; the thr eshold for cloud detection appears to require visible optical depths (tau(v is)) greater than 0.1. The geographical distributions of clouds in the 8-yr dataset are nearly the same as,those reported from 4 yr of data. The detection of upper-troposphe ric clouds occurs most often in the intertropical convergence zone and midl atitude storm belts with lower concentrations in subtropical deserts and oc eanic subtropical highs. The areas of concentrated cloud cover exhibit lati tudinal movement with the seasons as in other cloud datasets. HIRS finds cl ear sky in 25%, opaque cloud in 32%, and semitransparent cloud in 43% of al l its observations. The effective emissivity of the all semitransparent clo uds (T-vis < 6) ranges from 0.2 to 0.6 With an average value of about 0.5. Time trends are reexamined in detail. A possible cirrus increase in 1991 re ported by Wylie and coauthors in 1991 is found to be diminished upon reinsp ection. The revised 8-yr record has indications of an increase in high clou ds in the northern midlatitudes (0.5% yr(-1)) but little change elsewhere. The seasonal cycle of cloud cover in the Southern Hemisphere becomes very n oticeable in 1993.