Clouds and shortwave fluxes at Nauru. Part I: retrieved cloud properties

The datasets currently being collected at the Atmospheric Radiation Measurement (ARM) program's sites on the islands of Nauru and Marius represent the longest time series of ground-based cloud measurements available in the tropical western Pacific region. In this and a companion paper, a shortwave flux closure study is presented using observations collected at the Nauru site between June 1999 and May 2000. This paper presents frequency of occurrence of nonprecipitating liquid and ice clouds detected by the millimeter wavelength cloud radar (MMCR) and statistics of retrieved microphysical properties. The companion paper presents results from a closure study in which the retrieved cloud properties are input to a radiative transfer model, and the modeled surface fluxes are compared to observations. The liquid cloud properties are retrieved from MMCR and microwave radiometer (MWR) measurements using a Bayesian retrieval technique. Properties of ice phase clouds are retrieved from MMCR measurements using regression equations based on in situ observations taken during the Central Equatorial Pacific Experiment (CEPEX). Nonprecipitating liquid clouds were observed at Nauru in 35% of the radar observations. These clouds were primarily shallow cumulus with bases less than 1 km. Of the retrieved liquid clouds, 90% had liquid water path less than 100 g [m.sup.-2]. The average retrieved effective radius was 7.5 [micro]m. The frequency of liquid cloud detection and height of the liquid cloud base showed a clear diurnal cycle, which is most likely related to the island effect and the existence of the Nauru cloud plume. Ice clouds with no underlying liquid clouds were detected in 16.5% of the radar observations and ice clouds above liquid clouds in 7.7% of the observations. The mean retrieved IWP of the radar-detected ice clouds was 22.1 g [m.sup.-2] and the mean effective diameter retrieved was 72 [micro]m. Large monthly variability was seen in both the amount of cirrus detected and the retrieved ice water path. Ice clouds were observed by the radar more frequently at night than during the day at Nauru, but there was no clear diurnal trend in the retrieved microphysical properties.