Sensitivity of convective initiation and subsequent convection based on environmental parameters using 500m resolution WRF ARW

Convective initiation (CI), or the initiation of thunderstorms, is a rather difficult forecasting challenge in terms of its location, timing, and nature. Many studies have observed CI sensitivity to the environment, but few studies have looked at the sensitivity of CI mechanisms to the environment. This study will test the sensitivity of CI mechanisms for a particular case that was observed during the IHOP 2002 field project, which 12 June 2002 was chosen. This case featured numerous CI mechanisms in the vicinity of northwestern Oklahoma, where CI occurred. Potential CI mechanisms include: dryline, internal gravity waves, outflow boundary, intersection of dryline/outflow boundary, and horizontal convective rolls (HCRs). The convection that formed for this case caused primarily wind and hail damage in northern and central Oklahoma. Three papers in particular have diagnosed this CI case (Markowski et al. 2006, Weckwerth et al. 2008, and Liu and Xue 2008). Each paper attempted to diagnose which mechanism caused the initial convection to occur. Weckwerth et al. 2008 suggested HCRs were the primary CI mechanism, which was located east of the dryline by ~40 km. This was where CI occurred on that day (Liu and Xue, 2008). However, Liu and Xue 2008 suggested the intersection of the dryline/outflow boundary had a more direct role on CI. Markowski et al. 2006 attempted to determine why certain mechanisms played a more crucial role in CI than others, especially the outflow boundary in the vicinity. They had hypothesized that the development of convection did not occur by the outflow boundary due to a lack of moisture upwelling and a lack of substantial moist air being transported due by mesoscale circulations. The goals of this study are to 1) test Markowski's hypotheses of the lack of moisture upwelling contributing to the lack of convection along the outflow boundary, and 2) determine the sensitivity of each CI mechanism based on perturbations in the environment, including potential temperature and water vapor mixing ratio. The perturbation in these parameters took place in the planetary boundary layer (PBL) to ensure that the perturbations affected the CI mechanisms. The main aspects of CI that will be observed with most significance will be CI timing, location of CI, and areal coverage of the subsequent convection. To gauge the sensitivity of CI, this study will employ the Weather Research and Forecasting (WRF, Skamarock et al. 2001) Advanced Research WRF (ARW) numerical model to perturb the environmental parameters to simulate the change in CI location and timing. This model will run at 500-meter resolution with a spatial domain of 100 km² centered over the CI region. The model is initiated with 20 km resolution Rapid Update Cycle (RUC) data for the lateral boundary conditions, and the simulation period of the model begins at 2000 UTC and ends at 2300 UTC. First, a control simulation was run to get data as close to the actual event as possible, in terms of CI timing and location. This simulation did not have any parameters perturbed (i.e., the simulation was initialized by the RUC analysis data and allowed to run until 2300 UTC). Then, additional simulations were run, with the perturbed environments, to compare with the control simulation to observed differences in CI location, timing, and areal coverage of subsequent convection. The preliminary results show that an increase in water vapor mixing ratio of 2 g kg⁻¹ initiated convection sooner than the control simulation (2020 UTC compared to 2025 UTC). The increase in water vapor mixing ratio also helped to develop additional convection along the outflow boundary, which agrees with Markowski et al. hypotheses. The convection generated from the increase in water vapor mixing ratio along the outflow boundary did not appear in either the control simulation or another test simulation in which the potential temperature perturbation was applied. In fact, an increase in potential temperature in the boundary layer did very little in generating additional convection along the outflow boundary; CI even occurred later, compared to the control simulation (2040 UTC compared to 2025 UTC). This is hypothesized to be because of the warming in the PBL, which would help to raise the lifted condensation level (LCL), and make initiation of convection more difficult. At the workshop, additional results will be discussed with regards to additional perturbations of other parameters, and their comparisons of CI timing and location.

References Liu, H., and M. Xue, 2008: Prediction of Convective Initiation and Storm Evolution on 12 June 2002 during IHOP_2002. Part I: Control Simulation and Sensitivity Experiments. Mon. Wea. Rev., 136, 2261–2282. Markowski, P., C. Hannon, and E. Rasmussen, 2006: Observations of convection initiation “failure” from the 12 June 2002 IHOP deployment. Mon. Wea. Rev., 134, 375–405. Skamarock, W. C., J. B. Klemp, and J. Dudhia: Prototypes for the WRF (Weather Research and Forecasting) model. Preprints, Ninth Conf. on Mesoscale Processes, Fort Lauderdale, FL, Amer. Meteor. Soc., J11—J15, 2001. Weckwerth, T. M., H. Murphey, C. Flamant, J. Goldstein, and C. Pettet, 2008: An Observational Study of Convection Initiation on 12 June 2002 during IHOP_2002. Mon. Wea. Rev., 136, 2283-2304.