The Interactions among Cyclone Dynamics, Vertical Thermodynamic Structure, and Cloud Radiative Forcing in the North Atlantic Summertime Storm Track

Abstract

This study investigates the changes in the vertical thermodynamic structure of the troposphere associated with the passage of cyclones and the corresponding influence on cloud radiative forcing (CRF). The focus is the synoptic-scale evolution of lower-tropospheric sounding structure in the extratropical North Atlantic during July 1985?89. The type and amount of low-level cloud cover, which has a strong impact on total CRF in this region, is sensitive to these variations. Composite profiles at ocean weather stations L and C are constructed for soundings with low-level temperature inversions, soundings with surface-based inversions, and soundings without inversions, separately for conditions of large-scale subsidence and ascent. In addition, collocated European Centre for Medium-Range Weather Forecasts analyzed fields and Earth Radiation Budget Experiment CRF data are averaged for each composite sounding. These composites correspond to distinct dynamical regimes associated with the movement of cyclones in the storm track. Shortwave CRF (Cs) can differ by about 50 W m?2 between these composite profiles, and these differences can be linked to dynamically forced changes in sounding structure. In particular, factors such as increased low-level static stability and a shallower boundary layer are consistent with a more stratiform low-level cloud type, greater total cloud cover, and larger magnitude Cs, while the presence of large-scale ascent rather than subsidence tends to moisten the troposphere above the boundary layer and leads to the formation of mid- and upper-level clouds, which also contribute to larger Cs magnitudes.