What controls rain rates and cloud properties in midlatitude cyclones?

  Precipitation in Earth’s temperate regions is strongly controlled by the distribution and intensity of midlatitude cyclones, but there is little consensus among climate models about how precipitation will change in response to increasing greenhouse gases. We use NASA satellite observations from over 1500 midlatitude cyclones from 2003/2004 to examine how precipitation in midlatitude cyclones is affected by both storm strength and water vapor. Our multisensor approach uses measurements of precipitation and vertically-integrated water vapor from the Advanced Microwave Scanning Radiometer (AMSR-E), surface winds from Quikscat, and cloud properties from the Moderate Resolution Imaging Spectroradiometer (MODIS). We use NCEP-NCAR reanalysis pressure fields to identify the storm centers and then transform the satellite data onto a common cyclone-relative grid 4000x4000 km in extent. Then we use satellite-derived measures to determine both strength (the mean surface wind speed from Quikscat averaged over the storm) and moisture (the vertically integrated water vapor from AMSR-E, averaged over the storm) and generate composite storms of different strength and moisture categories.
Our results (Fig. 1) clearly demonstrate that the mean precipitation rate in midlatitude cyclones increases both with the storm strength and the amount of moisture, and is explained using a simple warm conveyor belt model. We also demonstrate that water vapor is largely thermodynamically controlled by the sea surface temperature (SST) in these storms, with the mean relative humidity averaged over the storm not being strongly dependent on either storm strength or moisture. Given that the SST is expected to increase in response to increasing greenhouse gases, the observations suggests that if storm strength remains constant under a warmed climate and any migration of the storm tracks is not sufficient to offset the rise in SST, then the precipitation associated with these storms would increase by approximately 7.5% for every 1 K rise in SST, in line with arguments based on the Clausius-Clapeyron equation. In contrast, the change in high cloud cover is mainly controlled by storm strength rather than moisture, increasing by approximately 50% for a similar increase in strength. We suggest that the composite fields constitute useful observational metrics for evaluating cloud feedbacks and for testing the behavior of large scale numerical models.

Paper to appear in Journal of Climate, Paul Field (NCAR) and R. Wood (University of Washington), “Precipitation and cloud structure in midlatitude cyclones”.
 

FIG 1: Composites of rain rate for midlatitude cyclones jointly conditioned on cyclone strength
(left to right panels) and moisture (lower to upper panels). The composite mean surface pressure and Quikscat surface wind vectors are also shown.