Evaluation of the Effect of Latent Heat Release on an Extratropical Cyclone by Simulation and Observational Comparison

Accurate modeling of mid-latitude cyclones is key to making relevant and informative forecasts, as well as understanding climate-induced changes in the Earth's hydrologic cycle. Current models realistically simulate extratropical cyclone structure at the synoptic scale, where large-scale dynamics play a pivotal role. However, these storms 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. While localized in the short term, these smaller processes affect the cyclone as a whole through upscale linkages. These effects are not immediately obvious, and must be deconvolved with the help of research models. Modelling the synoptic-scale storms provides a virtual laboratory to adjust the physical parameters of the environment, and better explore both intra- and interscale linkages. However, a better sense of the entire system can be obtained when model results are combined with observational results. Using both data sources allows for an analysis by comparison, and a fuller view of the system than either view could provide on its own.

Our study utilizes this dual perspective to examine the effect of latent heat release in the warm conveyor belt of a winter storm 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, drawing in significant amounts of warm moist air from the tropics. Given the condensation and subsequent latent heat release of the tropical air as it was drawn northward, this storm is an optimal to isolate the effects of latent heat release on cyclone and frontal structure. To deconvolve these effects, we modeled this case with the Weather Research and Forecasting (WRF) model run at a cloud system resolving horizontal grid spacing of 4 km, with both a control simulation, which included the effects of latent heat release, as well as a simulation in which latent heat release was turned off. Furthermore, we have interrogated the observations of NASA's A-Train suite of satellites in order to verify and enlighten our control model results. In addition, we examine differences between the “no latent heating” run, the control run results, and observations. In comparing these differences through both traditional analyses and via simulated satellite data, we can directly compare with both terrestrial analyses as well as space-borne observations, such as those in NASA's A-Train constellation.

We find that 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 this particular storm. The implication is that cyclogenesis may be more sensitive to placement of latent heat release than to the total integrated amount.