Cold Air Damming Erosion – an Opportunity to Use Previous Collaborative Research and SREF Ensemble Differences

HPC surface analysis valid 18Z 2012/01/22

What I believe is the first significant cold air damming (CAD) event of the winter was ongoing across the Mid Atlantic on Sunday. These episodes are often frequently associated with high impact weather and numerous forecast challenges and the current event is no exception. Portions of western Virginia, Maryland, and points northward were under a freezing rain advisory Sunday night. An approaching cold front will spread showers and thunderstorms across the Tennessee Valley and the Carolinas on Monday with some of the convection becoming potentially severe on the eastern perimeter of the wedge air mass. In addition, forecasting the erosion of the CAD air mass and the impact to sensible weather was acknowledged as a challenge in several Area Forecast Discussions on Sunday afternoon.

This forecast problem has been examined in several angles through collaborative research between NC State University and the NWS. The results have lead to improvements in the understanding of the physical processes in play, the development of conceptual models used by forecasters, and improvements to NWP.  Various forecast discussions noted the reduced confidence in the eventual erosion of the CAD on Monday. The two links below from a previous CSTAR project provide some conceptual models and resources regarding erosion of CAD.

One other interesting aspect of this problem is how the configuration of NWP impacts the way the guidance erodes CAD events. This can be visualized through the image at the bottom of this post which shows the 21 hour BUFR forecast soundings for Greensboro NC (KGSO) from the 22 January 2012 / 21Z run of the SREF, valid at 18Z 23 January. The ensemble display in Bufkit provides the ability to toggle off and on various SREF members, either by model family, convective parameterization or perturbation type.

The example image below shows the consistency within each the of the four SREF model families in how they forecast the CAD erosion at KGSO while also highlighting the variability between the various SREF families (ARW, Eta, NMM, and RSM). For example, the 5 RSM members warm the boundary layer the fastest while the other WRF/Eta members show a more similar, stable setup. Among the WRF/Eta members, the ARW has a less stable boundary layer, while the NMM and Eta members who both use the MYJ PBL scheme, have the most stable boundary layer. More information on the SREF members is available in a table at the following link: http://www.erh.noaa.gov/rah/science/rah.nwp.reference.pdf

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