Polar Mesocyclone Energy Flux Representation in High-Resolution Reanalysis and Model Data

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Wednesday, 7 January 2015
Kyle Ahern, Florida State University, Tallahassee, FL; and M. A. Bourassa

In the polar regions of the globe, maritime mesocyclones form throughout the year, often near or embedded within cloud streets associated with massive cold air outbreaks. Such storms appear on the 100–1000km horizontal scale; and their maximum sustained winds can exceed gale force, at which point they are sometimes referred to as “polar lows.” However, research (Harold et al. 1999) suggests that polar mesocyclones tend to exist on the lesser end of the horizontal scale. As a storm's size decreases, the likelihood that they will be represented well—or at all—in data also decreases. Underrepresentation of polar mesocyclones in reanalyses will affect climatological forecasts and research that utilize such data (Condron et al. 2008). Namely, the air-sea interactions associated with polar mesocyclones will be undercut, thereby impacting estimates of ocean circulation. Harsh polar conditions make regions of scientific interest unfavorable for in situ data collection, which compounds the aforementioned issues.

At the time of this writing, the Arctic Surface Reanalysis dataset produced by The Ohio State University's Polar Weather Research and Forecast Model (PWRF) is available to the community with 30km grid spacing every three hours from 1 January 2000 to 31 December 2010. As its name implies, the dataset is optimized for the arctic and centered over 90°N. Included in ASR are upward energy fluxes, wind speed and direction, sea ice extent, sea-surface temperature, and an atmospheric temperature profile—all as computed from the PWRF. Because ASR is newly released and still developing, it is to be determined how reliable the data is for use in meteorological and climatological studies that include polar regions.

To study the performance and reliability of ASR, high-resolution (4km grid spacing) Weather Research and Forecasting (WRF) model simulations of polar mesocyclone events will be completed and compared to ASR. ERA-Interim data will provide initial and boundary conditions to the model. Prior to in-depth analysis of WRF output and ASR, the model output will be validated against satellite-based SeaFlux global datasets, which includes global turbulent flux information over open water. Validated model output will be compared against ASR with a focus on energy flux representation in polar mesocyclones of variable size.

A necessity in modeling mesocyclones of variable size for this research is emphasized, as a key point of intrigue is the extent to which the ASR dataset can sufficiently represent a given polar mesocyclone. In discovering the limitations of ASR to resolve progressively smaller storms, insight is acquired regarding the reliability of its utilization in meteorological and climatological study that includes polar regions.