Evaluations of NAM Forecasts on Midtropospheric Perturbation-Induced Convective Storms over the U.S. Northern Plains

Abstract

In the U.S. northern plains, summer progressive convective storms that occur in weakly forced environments are often coupled with short-wave perturbations that are embedded in the midlevel northwesterly flow. These midtropospheric perturbations (MPs) are capable of inducing propagating convection that contributes to a majority of the rainfall over the northern plains during July and August. 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 the vorticity tendency throughout the MP life cycle and, in turn, slows the propagation speed of MPs. Moreover, biases of weak ambient flow speed and vortex stretching contribute to the magnitude and propagation speed errors of MPs. 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. A modified water vapor budget analysis reveals that 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 feed back to affect the MP growth through the bias in heating, thus further slowing the perturbation.

Neighborhood-Based Verification of Precipitation Forecasts from Convection-Allowing NCAR WRF Model Simulations and the Operational NAM

“…Furthermore, using a modified version of the entity-based EbertMcBride technique, Grams et al (2006) also found north and west displacement errors for convective systems in three different 12-km grid-spacing model configurations used during the International H 2 O Project (IHOP; Weckwerth et al 2004). Finally, the results are also consistent with those of Wang et al (2009), finding westward displacement errors in NAM forecasts of precipitation areas related to midtropospheric perturbations over the central United States.…”

The Effects of Great Plains Irrigation on the Surface Energy Balance, Regional Circulation, and Precipitation

“…Initial conditions and 3-hourly boundary conditions for the simulations were provided by the 12-km North American Mesoscale Model (NAM, formerly known as the Eta model [34]). Although the NAM exhibits biases in moisture [35] and MCS propagation characteristics [35,36], we judged the 12-km grid spacing of NAM beneficial in forcing convection on the WRF grid, as compared to employing coarser datasets such as the 2.5-degree National Centers for Environmental Prediction (NCEP) global reanalysis [37]. With respect to the NAM dry bias, our WRF simulations produce sufficient precipitation over the 1-month period of our simulations.…”

Weather Research and Forecasting model simulations of a rare springtime bow echo near the Great Salt Lake, USA

“…In the Midwest during midsummer, when it is relatively dry under the prevailing northwesterly upper‐level winds, up to 60% of rainfall and 80% of storm reports are linked to midtropospheric perturbations (Wang et al , ). Wang et al () pointed out that, in the Midwest, the performance of forecasting models on simulating midtropospheric perturbations is crucial in capturing the propagation and development of progressive MCSs including bow echoes; it will be shown in Section 3.4 that this is also the case for NAM simulations in the Intermountain West.…”

“…There is a discernable position shift (or delay) in the trough location at f12 and further at f24 compared with f00. This result suggests that the speed bias of midtropospheric perturbations in the NAM, similar to that documented in Wang et al (), is also present in the Intermountain West.…”

“…By comparing the mixing ratio (q) of the Salt Lake City sounding (KSLC) at 1200 UTC 21 April with model simulations, it was found that the simulated middle-to-lower troposphere was too dry, with a 13-22% deficit in q throughout the layer of 850-600 hPa (results not shown). It was noted by Wang et al (2009a) that NAM has a dry bias in warm-season climatology. Thus, in the next experiment q was increased artificially by 50% over the desert area west of the GSL (114.3-112.5 • W, 39.5-41.3 • N).…”