Thunderstorm initiation and evolution during IHOP: implications for aviation thunderstorm nowcasting

The International H2O Project (IHOP) was conducted in 2002 over a 400,000 km2 area in Kansas, Oklahoma and Texas. A primary goal of this project was to study thunderstorm initiation using a very impressive collection of radars, lidars, surface mesonet stations, soundings and rapid scan satellite. All convective storm initiations (radar cells exceeding 40 dBZ) occurring during the 44 days of the experiment were identified and grouped into 112 initiation episodes. A special effort was made to identify and track all surface convergence lines occurring during IHOP so that their impact could be examined on storm initiation and evolution. A variety of environmental variables were then used to characterize the conditions in the area of each initiation episode. Environmental variables included surface convergence, CAPE, CIN, low-level shear, and relative motion between cells and convergence lines.

Day time initiation occurred mostly along frontal regions and gust fronts. Initiation by dry lines was less than expected. Half of the storm initiations were not associated with surface convergence rather the updraft origins were above the boundary layer. These events are characterized as elevated storm initiation and occurred primarily during the night. They are generally less intense than the day time events but provide a very challenging forecast problem. However, often there was clear evidence in the RUC10 analyzed wind fields of convergence zones aloft associated with the elevated initiation.

The evolution of the initiation episodes covered a great range of situations including intense supercells, squall lines, mesoscale convective systems, short lived convective lines, and short lived unorganized groups of storms. The evolution and lifetime of the initiation episodes was closely tied to the development of gust fronts. Elevated systems were much less likely to develop gust fronts thus they were less likely to become well organized with long lifetimes.

A 10 km version of the operational Rapid Update Cycle (RUC10) numerical forecast model was specifically run by the NOAA Forecast System Laboratory for IHOP. The ability of the RUC10 to make 3 and 6 hr forecast of thunderstorm initiation, movement and evolution is examined. The RUC10 generally captured the initiation although it was often too early and too extensive. Once the gust fronts developed and the system propagated with the gust front the RUC failed to move the precipitation accordingly or it was far too slow.

Implications of these observational and numerical studies for the 0-6 hr forecasting of thunderstorms for aviation interests are provided. For example because of the high frequency of elevated convection in the central U.S. it is important to develop very short period forecasts of their initiation and evolution. Also because of the importance of gust fronts on storm evolution, motion and lifetime it is important to both detect them and anticipate their development.