Multiple Perspectives on the Effect of Latent Heat Release in an Extratropical Cyclone

Accurate modeling of mid-latitude cyclones is a key to making relevant and informative predictions, as well as understanding climate-induced changes in the Earth's hydrologic cycle. While current models realistically simulate extratropical cyclone structure at the synoptic scale, these cyclones are also affected by processes at the meso- and microscales, such as the absorption and release of latent heat during the evaporation and condensation of water vapor. Although localized on shorter time scales, these smaller processes affect the cyclone as a whole through upscale linkages. These effects are not immediately obvious, and must be deconvolved with the aid of numerical models, which provide a virtual laboratory to explore both intra- and interscale linkages. In addition, though models have utility as a standalone tool, a more complete view of the entire system is possible when model results are used in combination with observations.

Our study utilizes this dual perspective to examine the effect of latent heat release in an extratropical cyclone off the East Coast of the United States between 21-26 November 2006. This storm was sampled multiple times by various satellites in NASA's A-Train constellation, as it remained stationary for approximately 3 days. During this time, it was characterized by a persistent warm conveyor belt airflow that stretched from the tropics to the Canadian Maritime provinces. Given the condensation and subsequent latent heat release of the tropical air as it was drawn northward, this storm is an optimal case to isolate the effects of latent heat release on cyclone and frontal structure. To deconvolve these effects, we modelled this case with the Weather Research and Forecasting (WRF) model run at a cloud system resolving horizontal grid spacing of 4 km, under both control conditions and with latent heat release removed. Furthermore, we have leveraged observations obtained from NASA's A-Train suite of satellites to inform and evaluate our control model results. Using these resources, we performed a three part analysis. Firstly, we conduct a traditional synoptic analysis, allowing for direct comparison with reanalysis and observations of the event. Secondly, we compare the differences between model runs and observations via simulated satellite data, allowing for comparison to space-borne A-Train observations. Thirdly, we probe the cyclone dynamics by conducting a potential vorticity analysis, focusing on the anomalous areas, and specific changes due to the presence of latent heat release.

We find that, contrary to previously reported results, in this case the surface-level pressure gradient around the low pressure center is stronger when the effects of latent heat release are removed from the model run. While the potential vorticity anomalies associated with the system are both weakened and more isolated in the no-latent heat release run, the minimum sea level pressure in both model runs is similar. Our results indicate that latent heat may not have had a large role in strengthening the surface cyclone in this particular storm. Instead, the location of maximum condensational heating along and poleward of the surface warm front served to modify the thermal structure east of the surface cyclone and to enhance the warm conveyor belt circulation. The implication is that cyclogenesis may be more sensitive to placement of latent heat release than to the total integrated amount.