Synoptic and satellite aspects of the southwestern u.s. summer ‘monsoon’

Abstract

Satellite infrared imagery for three years (1980, 1981, 1982) are analysed using a colour enhancement technique to derive a cloud climatology of the southwest United States summer 'monsoon'. Diurnally-stratified and weekly statistics are obtained for different cloud levels and used as an index of the monsoon intensity in each year. Strong diurnal variations in cloudiness, related to surface heating, are identified for July and August, as are marked interannual differences. These regional-scale cloud variations are found to correlate more closely with the 700 mb wind direction than with the height of the 700 mb pressure surface when considered for a representative station (Winslow, Arizona). A satellite-based definition of 'monsoon' best considers variations of the total cloud rather than of any individual cloud level(s).Several surface and 700 mb synoptic circulation indices are identified and related statistically to the intraseasonal cloudiness changes. The most significant association is found for the variations in latitude of the Bermuda high pressure ridge that presumably relates to changes in subsidence. However, the sign of this relationship may reverse in years when other circulation systems, notably the North Pacific anticyclone, also influence the desert Southwest. Consideration of the zonal westerly index between 45 and 65"N tends to improve the correlation between the Bermuda ridge and summer monsoon cloudiness over the region. Such cloud-circulation studies have implications not only for climate dynamics but also for ongoing solar energy research in the American Southwest.

The climate of the US Southwest

“…A secondary maximum in winter (typically from November through March) provides an average of 30% of the annual rainfall (Barry & Chorley 1998). Areas with a summer precipitation peak usually experience an arid fore-summer prior to the onset of summer rains and a relatively dry autumn, which is especially notable in Arizona (Bryson & Lowry 1955, Reitan 1957, Carleton 1985, Adams & Comrie 1997. Although the winter precipitation peak is generally smaller than that of summer, winter precipitation is still considered to be hydrologically important, because much of the summer rainfall evaporates before it infiltrates into the ground and because winter rain falls primarily as snow at higher elevations (Bryson & Hare 1974).…”

“…Physiography of the western US and northwestern Mexico is characterized by large upland areas bordered by lowlands, and the monsoon system is aided by the seasonally warm land surfaces in both the uplands and the lowlands in combination with atmospheric moisture supplied by the nearby maritime sources (V.L. , Trewartha 1981, Carleton 1985, Adams & Comrie 1997.…”

“…8) (Higgins et al 1997(Higgins et al , 1998(Higgins et al , 1999. Monsoon onset occurs later with higher latitude (Carleton 1985), and northern Arizona, for example, may see monsoon onset dates that are closer to the middle of July.…”

“…During the North American monsoon season, the Southwest experiences strong diurnal variability in cloud cover, which is linked to diurnal cycles of surface heating and convection (Sellers & Hill 1974, Carleton 1985. Precipitation variation is due in part to shifts in the surface layer circulation, specifically the change from daytime cyclonic circulation to nighttime anticyclonic circulation .…”

Synoptic‐dynamic character of ‘bursts’ and ‘breaks’ in the South‐West U.S. summer precipitation singularity

“…A previous paper (Carleton, 1985) presented the cloud climatology of the south-west U.S. summer precipitation singularity (the 'Monsoon') based on analysis of GOES-W satellite infrared imagery for three seasons (1980)(1981)(1982). Strong diurnal, intraseasonal and interannual variations in convective cloudiness were identified and related to several circulation indices.…”