Extratropical Cyclone Characteristics and Cloud Processes

Clouds associated with extratropical cyclones (ECs) lie at the intersection of well-developed theory of dry baroclinic waves and the nonlinear feedback on their dynamics due to the effects of precipitation and cloud-altered radiation. Among all cloud regimes, EC clouds are the most effective at blocking short-wave radiation at the top of the atmosphere, but they actually enhance atmospheric cooling. The net modulation of the transient waves by moist processes and the resulting climatological effects due to modified poleward transports of energy and water remain unclear. A cyclone tracking algorithm (NASA's MCMS) is used to identify ECs in the ERA-I reanalysis and collect the properties of each disturbance. The database has over a million extratropical disturbances covering 33 years. Considering storm intensity as related to wind speeds, which depend on the pressure gradient, the distribution of cyclone properties is investigated using groups — defined by their local pressure depth (local anomaly) and the radius of the region encompassed by closed pressure contours — to monitor variations with longitude (especially ocean and land), hemisphere and season. Using global data products providing estimates of the cloud radiative effects (CRE) on atmospheric radiative heating/cooling rate (ISCCP FD) and precipitation (GPCP), composites are assembled for each cyclone group and for ``not-stormy'' locations. On average the precipitation rate and the CRE are approximately the same among all cyclone groups and do not strongly differ from ``not-stormy'' conditions. However, the variance of both precipitation and CRE increases with cyclone size and depth. Thus more extreme values at both ends of the distribution are found in larger, deeper storms; namely, maximum precipitation and CRE increase but so do the amounts of non-precipitating and clear sky conditions thereby maintaining the averages. We discuss some of the implications of these findings in the context of the integrated atmospheric energetics.