Process Oriented Diagnostics of Tropical Cyclones In Climate Models

“The simulation of tropical cyclone (TC) activity in climate models is still a challenging problem. While some models are able to simulate TC activity with characteristics very similar to those observed, many models have very strong biases. While increasing horizontal resolution often improves the characteristics of model TCs, resolution alone is not sufficient for high skill in simulating TC activity. We propose here a suite of process-based diagnostics to identify model characteristics that are responsible for a good simulation of TCs in global atmospheric and ocean-atmosphere coupled climate models, including high-resolution global models which have become increasingly used for studies of the relationship between TCs and climate.

First, we will examine the role of large-scale environmental variables, such as vertical wind shear and potential intensity as well as various integrated genesis indices, with standard TC activity measures. Second, we will develop and test process-based diagnostics to investigate the influence of model physics on TC formation in the models. As there are some similarities between the process involved in the Madden-Julian Oscillation (MJO) and tropical cyclogenesis, our proposed diagnostics are heavily influenced by the process-based diagnostics that were developed for the MJO simulation. These diagnostics focus on how convections, moisture, clouds and related processes are coupled at individual grid points, and give information about how the convective parameterizations interact with resolved model dynamics. Our working hypothesis is that similar interactions are important for tropical cyclone genesis and intensification in models, and we will test that hypothesis. Third, we will use standard wave diagnostics to identify the Madden-Julian Oscillation and various convectively coupled tropical in the models, and related these to standard TC activity diagnostics, to determine how well the models simulate the association between TCs and these disturbances which are known to modulate TC genesis in observations. In the fourth and final part of our project, we will perform parameter sensitivity studies in which we will modify the physics of a climate model, the NASA Goddard Institute for Space Studies (NASA GISS) model, and analyze how the simulated TC activity is affected. This approach will allow us to explore the physics parameter space in a more controlled fashion than is possible with existing model ensembles of opportunity. In the first three phases, the project will use existing multi-model databases, including those form the Coupled Model Intercomparision Project Phase 5 (CMIP5) and US CLIVAR Hurricane Working Group, as well as additional simulations and model output from our collaborators from NASA GISS, National Oceanographic Atmospheric Administration – Geophysical Fluid Dynamics Laboratory (NOAA-GFDL), in Princeton, NJ and the Istituto Nazionale di Geofisca e Vulcanologia – Centro Euro-Mediterraneo sui Cambiamenti Climatici (INGV-CMCC), in Bologna, Italy.

This project fits well within the MAPP process-oriented evaluation of climate models, by developing new diagnostics to evaluate model perforce in simulating tropical cyclones. The knowledge gained with these diagnostics can then be used in improving the next generation of climate models. This project fits well with NOAA’s long-term climate goals, given the large impacts of tropical cyclones in the U.S. and the importance of making robust projections of future tropical cyclone activity.”