Spring season Colorado cyclones. Part I: use of composites to relate upper and lower tropospheric wind fields.

Abstract

A set of 70 cases of spring season Colorado cyclone events is used to form composites which describe the upper (300 mb) and lower (850 mb) tropospheric wind fields during the early stages of cyclone formation. The 70 cases are partitioned into those which persist beyond 72 h (developing) and those which fill after 24 h (nondeveloping). The developing sample reveals a well-defined 300- mb wind maximum embedded in a short wave trough which propagates eastward during the six time periods studied. However, the nondeveloping sample composite exhibits little structure to the 300-mb wind pattern. A subsample of the developing cases, chosen on the basis of the presence of a net streak over the New Mexico- Texas area, shows a stronger 300-mb wind maximum. The 850-mb composites show southerly flow in the southern Great Plains in the two samples and the subsample. The developing sample and jet streak subsample exhibit a marked increase in 850-mb wind speed as the exit region of the 300-mb jet maximum propagates over the region. -from Authors