Sunday, 22 January 2012
The Effect of Warm Frontal Latent Heat Release on the Structure and Evolution of An Extratropical Cyclone
Hall E (New Orleans Convention Center )
In order to improve public forecasting accuracy, increased understanding of mid-latitude cyclones is an important step along the way. Already, the synoptic scale structure and motion of such storms has been well documented, beginning with Bjerknes in 1922. However many of these synoptic–scale processes are not limited to large scales. Upscale linkage from mesoscale and microscale processes creates a convoluted web of causation. Because it is difficult to attribute any single effect to a particular process, it is difficult to unravel this web using observational analysis alone. Modeling has long been used to bridge the gap between observation and theory, and can provide an experimental framework within which theory can be evaluated. Synthesizing both components in a case study framework allows for a more comprehensive view of the system than can be obtained from either modeling or observations alone. Accurate and timely observations illuminate what actually happened in real-time, including how the dynamics and structure of a system evolved. With modeling, these analyses can be taken to another level, deconvolving and isolating individual processes, in the hopes of tracing their effects back upscale to synoptic scales. This study examines the effect of latent heat release in the warm conveyor belt of a winter storm that occurred between 21-26 November 2006 off the east coast of the United States. Because this storm remained nearly stationary for approximately three days, it was sampled multiple times by various satellites in NASA's A-Train constellation. To deconvolve the effects of latent heating, we modeled this case with the Weather Research and Forecasting (WRF) model run at cloud system resolving horizontal grid spacing of 4 km. We ran 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. We verified the accuracy of our control simulation by comparing the WRF model output to data from the North American Regional Reanalysis (NARR) and satellite observations from the NASA A-Train suite of satellites. We find that removing the effect of latent heat release on the surrounding environments had a profound effect on the structure of the low pressure system, and that many of the results run counter to what has been found in previous studies. Both the low pressure and adjacent high pressure systems are stronger in the absence of latent heating, as is the upper-tropospheric jet stream. The simulation without latent heat release produced a larger cloud shield and similar amounts of accumulated precipitation as in the full-physics run. Our results suggest that, in certain cases, rather than adding to cyclone strength and intensity, latent heat release may in fact act to restrict storm motion via amplification of a downstream blocking ridge. The resulting increase in stationarity of the system produces a more barotropic structure, and consequently a weaker surface cyclone.