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 jet 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. Composites of the departure of the 300-mb observed wind from the geostrophic wind show that in the developing and jet streak cases the normal component of the ageostrophic wind is directed to the right of the wind in the exit region of the jet. This is consistent with the upper transverse branch of an indirect circulation. The 850-mb ageostrophic winds are directed toward the north in all three composites; however, the developing sample and jet streak sub-sample exhibit slightly stronger values and a greater north-south geographical extent than the nondeveloping sample. The relatively stronger 850-mb ageostrophic flow in the presence of a composite 300-mb jet may reflect the existence of the lower transverse branch of an indirect circulation in the exit region of the jet. The sites of the 39 developing cases are nearly all located in the left front quadrant of the composite 300-mb jet. This favored location is discussed in terms of upper tropospheric divergence resulting from the viewpoints of vorticity advection and a transverse indirect circulation in the exit region of the 300-mb jet.