Summer midtropospheric perturbations over the U.S. northern plains: Climatology and NAM forecasts

In the United States northern plains, summer progressive convective storms that occur in weakly forced environments are often coupled with short wave perturbations which are embedded in the mid-level northwesterly flow. These midtropospheric perturbations (MPs) are capable of inducing propagating convection that contributes to a majority of the rainfall over the northern plains, particularly during July and August. MPs are also found closely linked to Northwest Flow Severe Weather Outbreaks. There is a possibility that the difficulties of numerical weather prediction models in forecasting summer convective rainfall over the northern plains are partly attributed to their deficiency in forecasting MPs. The present study tests this possibility through examining operational forecasts by the North American Mesoscale (NAM) model during the summers of 2005 and 2006.

Forecasted MPs exhibit slower propagation speeds and weaker relative vorticity than the observations leading to systematic position errors. Underpredicted vorticity magnitudes weaken horizontal vorticity advection that influences vorticity tendency throughout the MP life cycle and, in turn, slows the propagation speed of MPs. Biases of weak ambient flow speed and vortex stretching contribute to the magnitude and propagation speed errors of MPs, as well. Skill scores of precipitation forecasts associated with MPs are low, but can be considerably improved after removing the MP position error that displaces the rainfall pattern. The NAM also tends to underpredict precipitation amounts as the NAM insufficiently generates atmospheric humidity over the central United States. The shortage of moisture in the forecast reduces the water vapor flux convergence that is part of the precipitation process. The precipitation bias may feedback to affect the MP growth through the bias in heating, thus further slowing the perturbation.