A high-resolution analysis of cloud amount and type over complex topography

This paper reports on the first application of a multispectral textural Bay esian cloud classification algorithm ("SRTex'') to the general problem of t he determination of high-spatial resolution cloud-amount and cloud-type cli matological distributions. One year of NOAA-14 daylight passes over a regio n of complex topography (the South Island of New Zealand and adjacent ocean areas) is analyzed, and exploratory cloud-amount and -type climatological distributions are developed. When validated against a set of surface observ ations, the cloud-amount distributions have no significant bias at seasonal and yearly timescales, and explain between 70% (seasonal) and 90% (annual) of the spatial variance in the surface observations. The cloud-amount distributions show strong land/sea contrasts. Lowest cloud frequencies are found in the lee of the major alpine feature in the analys is domain (the Southern Alps) and over mountain-sheltered valleys and adjac ent sea areas. Over the oceans, cloud frequencies are highest over sub-Anta rctic water masses, and range from 90% to 95%. However, over the sea adjace nt to the coast on the western side of the Southern Alps, there is a distin ct minimum in cloud amount that appears to be related to the orography. The cloud-type climatological distributions are analyzed in terms of both s imple frequency of occurrence and conditional frequency of occurrence, whic h is the frequency of occurrence as a fraction of the total number of times that the cloud type could have been observed. These distributions reveal t he presence of preferred locations for some cloud types. There is strong ev idence that uplift over major mountain ranges is a source of transmissive c irrus (enhancing occurrence by a factor of 2) and that the resulting cirrus coverage is most extensive and frequent in spring. Over the ocean areas, S ST-related effects may determine the spatial distributions of stratocumulus , with higher frequencies observed over sub-Antarctic waters than over subt ropical waters. Also, there is a positive correlation between mean cloud-to p height and SST, but no similar relationship is found for other cloud type s.