A Composite Analysis of Airstreams within Cold-Season Colorado Cyclones

Abstract

Relative wind isentropic analysis was employed to investigate the evolution of airstreams and airstream boundaries within midlatitude cyclones that formed in the Colorado cyclogenesis region of the United States. This study attempts to verify and expand upon existing conceptual models of three-dimensional airflow, while describing how such models vary at different times during cyclone development and when the intensification history of the storm is considered. Forty-nine cyclone events were first divided into three categories: early-developing cyclones (those that intensify with 24 h of cyclogenesis), late-developing cyclones (those that intensify 24?48 h after cyclogenesis), and nondeveloping cyclones (those that either display little change in intensity or weaken with time). Composite isentropic surfaces for multiple levels (315?290 K, separated by 5 K) were constructed by cyclone category for six 12-h time periods within the cyclone life cycle. Three distinct airstreams and four types of airstream boundaries were identified on the composite isentropic surfaces. Two of the airstreams closely resemble the ?drystream? and ?warm conveyor belt (WCB)? described in previous studies. The third airstream is referred to here as the cyclonically turning moist airstream (CMA). Until approximately 24 h after cyclogenesis, the CMA and WCB originate at similar latitudes although the CMA occurs at a lower elevation. Later in the storm life cycle, the CMA originates at a more northerly latitude than the WCB and in comparison is a relatively cold airstream. Airstream boundaries separating the WCB and the drystream are seen at almost all time periods. This feature acquires a forward-leaning orientation with time with only the lowermost boundaries being accompanied by a modest to strong temperature gradient. Two airstream boundaries involve the CMA. The first separates the CMA and the drystream and is a lower-tropospheric feature, particularly late in the storm life cycle. The second boundary is located north or northwest of the cyclone center and separates the CMA from northerly descending air. This midtropospheric feature occurs along a relatively weak temperature gradient. The fourth type of airstream boundary is referred to as a southwest confluence zone and separates northerly, descending airflow southwest of the cyclone center from easterly, rising airflow to the southeast. At the middle and later stages of the cyclone life cycle, this boundary is a lower-tropospheric feature. It is often associated with a relatively strong temperature gradient. The composites indicate that the evolution of the airstreams and airstream boundaries is remarkably similar for the three cyclone types, except that they are out of synchrony by one or more 12-h time steps. In particular, all three airstreams are evident on the precyclogenesis (time t ? 12) composite surfaces for the nondeveloping cyclones, whereas the full suite of airstreams does not appear until 12 h later for the developing cyclones.