Day 270: Urban Heat Island Effect and Lake Effect in a High Resolution Model

Posted by pmarsh on 27 Sep 2010 | 1 comment

Under certain atmospheric conditions, typically clear skies and calm winds, the overnight low temperature at a given location is often affected by non meteorological conditions. The more urban an area, the warmer the overnight low temperature will be (given the aforementioned quiescent conditions). This is known as the Urban Heat Island Effect. Also, because water heats and cools slower than the air temperature, proximity to large bodies of water also play a significant role in the overnight low temperatures (once again, under quiescent conditions).

What’s amazing is that our numerical weather prediction models are being run at spatial scales that can now capture these trends. Above (and annotated below) is the forecast low temperatures for the southern plains tomorrow morning from the NSSL Weather and Forecast Model (WRF). (It’s a 36 hour forecast, valid at 12 UTC, or 7 AM, Tuesday.) This model is run using a spatial grid length of 4 kilometers in the horizontal direction. If you look carefully at the forecast, you can see what appear to be spurious areas of warmer low temperatures than the general pattern of low temperatures. These areas are not spurious, but actually the model forecasting Urban Heat Island or Lake effects on low temperatures. Overnight low temperatures near the lakes will be highly dependent on what the lake temperature is. Warmer lakes result in warmer overnight lows.

Urban areas are circled and labeled in red, whereas the lake effects are circled in yellow.