Impact of Increased Environmental Aerosols on the Development of Extratropical Baroclinic Disturbances

Abstract:

Extratropical baroclinic disturbances, or cyclones, are an important component of the atmospheric general circulation and effectively determine winter hydroclimate variability in regions such as western U.S. and western Europe. Interactions between environmental aerosols and cyclones are often observed over East Asia and western North Pacific. In this study, the SP-CAM (Super-parameterized Community Atmosphere Model, version 5 of CAM) is used to examine the aerosol direct and indirect effects on the development of an extratropical cyclone. Specifically, ensemble simulations with SP-CAM are conducted by increasing aerosol concentrations at slightly different times during the initial growth of a cyclone sampled from a 3-month winter control experiment made also with the SP-CAM. In the polluted environment, the cyclone becomes weaker in association with anomalous heating below the mid-tropospheric trough and anomalous cooling below the ridge. Further investigation indicates that the heating anomaly below the trough is mainly contributed by the weakening of cold advection behind the surface cold front while the cooling anomaly underneath the ridge is mainly caused by suppressed condensational heating in the stratiform clouds ahead of the surface warm front. Radiative heating anomalies associated with aerosol increase are generally weak but have an overall positive contribution to the weakening of the cyclone. Of particular interest is the strengthening of convection ahead of the surface cold front that happens at the same time when the cyclone itself weakens. This behavior suggests fundamental differences in the response to environmental aerosol change between a tropical and an extratropical cyclone with diabatic heating effects of aerosols playing only a secondary role in the development of the latter.