Sunday, April 10, 2011

NSSL WRF performance

The more I looked at the NSSL WRF forecast from yesterday the more interesting it was.

While the model initiated storms off the warm advection cloud band in Iowa after 0100 UTC, which did turn out to be correct, it was not severe nor was it the major player. Observations indicate this was part of an alto-cumulus castellanus area.

The area in NE where storms initiated in the real world along the warm front sharpened until 23-00 UTC where in model land it weakened considerably. There was an indication in the model that reflectivity was small but non-zero when the convergence was strong. This is a good albeit weak signal.

Another area under consideration for convection initiation was along the dryline in KS and northern Oklahoma where one storm formed around 23 UTC. The model had little in the way in convection here until 2 hours later, along the dryline.

Another area was in SW OK, where observations indicated a small, weak storm developed around 01 UTC and quickly died off. The closest model storm was at 05 UTC. Of course, the whole forecast goes awry in these latter two isolated storms as the model initiates convection all along the dryline from KS through Southern Oklahoma. The last 6 hours of the forecast looks little like what happened in terms of storms. I would caution that the model is not entirely wrong, just very aggressive. The cloud fields in the model develop into convection but closely resemble the cloud fields observed.

We are just beginning to harvest the wealth of information contained within such forecasts. I believe we will learn a lot more about these types of forecasts when we get down to looking at cloud fields (not at the grid scale but over substantive areas) and use these to compare the model with observations. This perspective should give forecasters more confidence in the overall appearance, and solidify what to look for when examining fine resolution forecasts. I think information extraction will be much more successful than reflectivity alone.