Tuesday, 24 January 2017
4E (Washington State Convention Center )
Updraft helicity (UH) serves as an effective proxy for supercells, and forms the basis for probabilistic forecasts of multiple severe convective hazards. However, establishing a relationship between UH and observed storm rotation has proven difficult, in part due to a lack of a large database characterizing storm rotation. This work seeks to answer the question, ‘what can an ensemble of simulated storm characteristics tell us about observed storms?’ by using the 4 km National Severe Storms Laboratory (NSSL)-WRF ensemble and an extensive hand-analyzed dataset established by the Storm Prediction Center (SPC). Through comparison of these datasets, the relationships between high-resolution modeled and observed storm characteristics can be explored via examination of ensemble-generated values within a certain space-time window around the report occurrence. In particular, the relationships between UH and rotational velocities are characterized, as well as relationships between the model-produced and analyzed environmental parameters such as the Significant Tornado Parameter (STP). Additionally, the relationship between the analyzed STP and rotational velocity and the model-derived STP and UH are compared, to determine if the ensemble is reflecting storm characteristics that would be expected in a given environment (i.e. stronger rotation in an environment with higher STP). Comparisons are drawn by locating the closest model grid point to a given report, and then searching the ensemble within various space and time neighborhoods.
The model/observational relationships are also explored through the lens of the hazards they produce: the tornadic and non-tornadic storms are examined separately to determine whether or not the strength of various model parameters discerns between observed tornadic and non-tornadic storms. UH integrated over different layers is shown to have a statistically different relationship with rotational velocity in tornadic storms than in non-tornadic storms, across multiple space and time windows. Ongoing work at the SPC establishes probabilities of a tornado given a right-moving supercell with a certain value of rotational velocity; if information about the rotational velocity can be taken from the ensemble members’ UH, an unconditional model-generated tornado probability with an observational basis is possible.
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