A composite analysis of airstreams within cold-season Colorado cyclones

Relative wind isentropic analysis was employed to investigate the evolution of airstreams and airstream boundaries within midlatitude cyclones that fo rmed in the Colorado cyclogenesis region of the United States. This study a ttempts to verify and expand upon existing conceptual models of three-dimen sional airflow, while describing how such models vary at different times du ring cyclone development and when the intensification history of the storm is considered. Forty-nine cyclone events were first divided into three cate gories: early-developing cyclones (those that intensify with 24 h of cyclog enesis), late-developing cyclones (those that intensify 24-48 h after cyclo genesis), and nondeveloping cyclones (those that either display little chan ge in intensity or weaken with time). Composite isentropic surfaces for mul tiple levels (315-290 K, separated by 5 K) were constructed by cyclone cate gory for six 12-h time periods within the cyclone life cycle. Three distinct airstreams and four types of airstream boundaries were ident ified on the composite isentropic surfaces. Two of the airstreams closely r esemble the "drystream" and "warm conveyor belt (WCB)" described in previou s studies. The third airstream is referred to here as the cyclonically turn ing moist airstream (CMA). Until approximately 24 h after cyclogenesis, the CMA and WCB originate at similar latitudes although the CMA occurs at a lo wer 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 airs tream. Airstream boundaries separating the WCB and the drystream are seen a t almost all time periods. This feature acquires a forward-leaning orientat ion with time with only the lowermost boundaries bring accompanied by a mod est to strong temperature gradient. Two airstream boundaries involve the CM A. The first separates the CMA and the drystream and is a lower-tropospheri c feature, particularly late in the storm life cycle. The second boundary i s located north or northwest of the cyclone center and separates the CMA fr om northerly descending air. This midtropospheric feature occurs along a re latively weak temperature gradient. The fourth type of airstream boundary i s referred to as a southwest confluence rone and separates northerly, desce nding 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 wit h a relatively strong temperature gradient. The composites indicate that th e evolution of the airstreams and airstream boundaries is remarkably simila r 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 o n the precyclogenesis (time t - 12) composite surfaces for the nondevelopin g cyclones, whereas the full suite of airstreams does not appear until 12 h later for the developing cyclones.