Numerical simulations of convective initiation failure: 12 June 2002 International H₂O Project case study

Abstract

<p>Convective initiation (hereinafter denoted CI), defined as the development of cumulus convection into a categorical convective mode, is a particularly difficult forecasting and modeling issue in meteorology. Understanding when and where CI occurs and the mechanisms that cause CI are vital to forecasters, especially in regards to severe weather during the spring and summer months in the United States. If CI occurs earlier than was forecasted, it may force the issuance of warnings to be much sooner than expected for rapidly developing severe thunderstorms. Forecast errors in CI timing could also result in incorrect quantitative precipitation forecasts (QPFs). Decreases in QPF skill during the summer months have been due to difficulty in forecasting convective rainfall (Fabry 2006).</p> <p>For this thesis, I will test hypotheses given by Markowski et al. (2006) for the development of convection on 12 June 2002 by introducing a cold pool to increase moisture upwelling and surface convergence near the triple point of the 12 June case and to decrease dry air entrainment by increasing relative humidity between 1400 and 4000m AGL.</p> <p>The key findings of this thesis include: 1) CI failed to occur near the triple point for each simulation (control, cold pool, and reduced dilution experiments). All clouds that formed near the triple point above the LFC did not develop into a sustained convective mode, for all three simulations. The cold pool did produce clouds along its corridor of ascent, but these clouds were short-lived as the corridor of ascent propagated southward, causing the forcing that formed these clouds to be brief temporally and spatially. The decreased dilution experiment resolved a larger number of clouds that also covered a larger area, compared to the control simulation. However, none of these clouds that formed above the LFC formed into a sustained mode of convection.</p>