Title: Extratropical cyclone clouds and precipitation in GCMs: biases, causes, and implications


Presenter:  James Booth

Abstract:
Using Lagrangian tracking and compositing, we have generated a phenomena-oriented diagnostic of extratropical cyclone clouds and precipitation. First, I will discuss the metrics: (1) a comparison of modeled clouds in southern hemisphere extratropical cyclones against MODIS, CloudSat and CALIPSO cloud and precipitation observations, and (2) a comparison of modeled extratropical precipitation generated over the ocean with the NASA IMERG product. Second, I will discuss results from our recent application of the metrics to MERRA and ECMWF reanalysis and the GFDL and NCAR models. The details are in the next two paragraphs. Finally, I will discuss our preliminary analysis of a developmental version of GISS ModelE3.

 

We examine versions of the GFDL AM4 model in which we alternately change the convection scheme or the cloud tuning. The configurations that are in radiative balance predict a realistic top-of-atmosphere cloud cover in the southern oceans, within 5% of the observations. However, an examination of cloud cover transects in extratropical cyclones reveals a tendency in the models to overestimate high-level clouds and underestimate cloud cover at low-levels, and especially in the post-cold frontal (PCF) region. Focusing on only the models, their differences in high and mid-level clouds are consistent with their differences in convective activity and relative humidity (RH), but the same is not true for the PCF region. In this region, RH is higher in the model with less cloud fraction. These seemingly contradictory cloud and RH differences can be explained by differences in the implementation of the convection scheme as well as differences in tuning parameters that had to be changed to maintain radiative balance. Extratropical cyclone precipitation is evaluated in two reanalyses and two climate models. By decomposing the composites into frequency of occurrence and intensity (mean precipitation rate when precipitating), the analysis reveals a tendency for all four models to overestimate frequency and underestimate intensity, with the former issue dominating in the western half and the latter in the eastern half of the cyclones. Differences in frequency are strongly dependent on cyclone environmental moisture, while the differences in intensity are strongly impacted by the strength of ascent within the cyclone. These biases have consequences when evaluating the changes in precipitation characteristics with changes in cyclone properties: the models disagree on the magnitude of the change in precipitation intensity with a change in environmental moisture and of the change in precipitation frequency with a change in cyclone strength. This complicates accurate predictions of precipitation changes in a changing climate.