At last month’s CSTAR workshop, I shared the results and analysis of my thesis project as well as some thoughts on related future work. I would like to provide a summary of my and Dr. Lackmann’s work with this post for those who could not be there and as a refresher for those who were.
The problem I focused on is that severe convection that forms in High Shear, Low CAPE (HSLC) environments is difficult to predict and is dangerous. There are two approaches to improve HSLC severe convection predictability: the first is the use of environmental predictors such as the SHERB (Sherburn and Parker 2014) and the MOSH (Sherburn et al. 2016) and the second is using Numerical Weather Prediction (NWP) models to predict convection which is what myself and Jessica King did (King et al. 2017). This research project sought to answer two research questions:
1. At what horizontal grid spacing, if any, does an NWP model provide operationally useful information about explicit low instability severe convection?
2. Can a NWP model properly differentiate between a HSLC event case and a HSLC null event case given the proper initial conditions?
Dr. Lackmann and I looked at two cases, a event case February 24-25, 2011, a null case, December 25-26, 2009. I ran these cases using the WRF model at convection-permitting grid spacings (Figure 1) and then evaluated how well each domain represented the severe weather using severe proxy metrics: half-hourly maximum updraft helicity (1-4km), half-hourly maximum 10-meter wind speed, half-hourly maximum updraft speed, and composite radar reflectivity (Figure 2). In order to compare the different domains on an “equal playing field” so to speak, we conformed the 1.2-km and 400-m domains to a grid with a 3.6-km grid spacing over the area encompassed by the 400-m domain. The severe proxy metrics were calculated on the higher resolution domain first before this interpolation. This process is effectively sub-sampling. The distributions of the severe proxy values of these interpolated domains were compared to one another both in the same case and between the two cases (Figure 3).
The results of this analysis are as follows:
– Based on this study, the recommended horizontal grid spacing to run a NWP model in a way that provides operationally useful information about low instability convection is 3.6-km.
– A larger gain in detail is observed between the 3.6-km and 1.2-km domains than between the 1.2-km and 400-m domains.
– Overall, the event case and null event case are statistically significantly different. It is important to note that there is a difference between differentiating and distinguishing; the model had difficulty distinguishing between the two cases.
– Thresholds of severe proxy parameters, especially updraft helicity, should be adjusted to handle finer model resolution.
I look forward to receiving your questions and comments as well as participating in any discussion that follows. Please feel free to ask me questions in the comments section or to send an email to firstname.lastname@example.org. Thank you!
Figure 1: Model domain set up for the event case February 24-25, 2011
Figure 2: NEXRAD and conformed model domain composite radar reflectivity for the event case. Time shown is 03:00 UTC on 02/25/2011.
Figure 3: The severe proxy conformed half-hourly maximum updraft helicity distributions for the event case and null event case at each convective permitting domain.
King, J.R., M.D. Parker, K.D. Sherburn, and G.M. Lackmann 2017: Rapid Evolution of Cool Season, Low CAPE Severe Thunderstorm Environments. Wea. Forecasting. doi: 10.1175/WAF-D-16-0141.1
Sherburn, K.D., and M.D. Parker, 2014: Climatology and Ingredients of Significant Severe Convection in High-Shear, Low-CAPE Environments. Wea. Forecasting, 29, 854-877, doi: 10.1175/WAF-D-13-00041.1
———-, ———–, G.M. Lackmann, and J.R. King, 2016: Composite environments of severe and non-severe high-shear, low-CAPE convective events. Wea. Forecasting. Doi: 1175/WAF-D-16-0086.1