January 19, 2012 Meeting Summary
Da-Lin Zhang began the meeting with his presentation titled, "On the Rapid Intensification (RI) of Hurricane Wilma (2005)." Previous RI studies are limited to our understanding of environmental conditions with recent studies beginning to study inner core processes. Using a quadruply nested WRF-ARW model with 55 vertical levels, Da-Lin found that the model predicted a 4mb/hour deepening similar to what was observed. To explain this, Da-Lin had two hypotheses: first, that an intense warm core tends to form within the upper outflow layer and second, that convective bursts tend to play an important role in RI through detraining stratospheric air into the eye of the storm. To explain the first hypothesis, using the hydrostatic equation, Da-Lin showed that a higher-level warm core is more effective than a lower-level warm core when inducing surface pressure falls. This could be shown in a plot of height versus forecast time. An upper level warming core is shown in the outflow layer, which coincides with the onset of RI. Looking at a cross section of height versus distance from the eye of the storm, cooler air can be seen moving from points further from the eye to closer to the eye. Da-Lin concluded with the main points of his talk including that RI of a tropical cyclone (TC) can be closely related to convective bursts, upper level warm core flow structures, and sea surface temperatures (SSTs). Also, results from this work suggest that more emphasis be made on upper-level "ventilation effects" to more accurately predict RI of TCs.
Dave Zelinsky next presented a talk titled, "High Temporal Frequency Diagnostics: 2012 Products." Using the 2011 HFIP Stream 1.5 triple-nested HWRF, Dave looked at output from every 5 seconds. In plots of maximum wind and minimum pressure, some variability is seen from point to point. Sources of this include axisymmetric spin up/down of the storm, growth/decay of convective elements, fluctuation of wind at individual gridpoints, and nest movement. To help forecasters with this variability, Dave introduced new products and diagnostics. These include bounded intensity plots, which use the maximum and minimum +/- 3 hour value of max winds as a guideline for the model-output max winds. Values inside the bounds can be considered reasonable. Other new products include track plots which feature a color code for the category of max wind that follows the storm center provided by the track. For the radius of maximum winds (RMW), plots of +/- 3 hour average RMW, RMW storm-relative position, and RMW x- and y-components. were shown.
Ligia Bernardet concluded the meeting with a presentation titled, "Operational HWRF 2011 RI/RW (Rapid Intensification/Rapid Weakening) Forecast Evaluation." Motivation for this work included HFIP's focus on improving intensity forecasts and a lack of previous studies on HWRF's RI/RW. Overall, Ligia showed that using statistical scores, the HWRF over-forecasted intensity and under-forecasted intensity change. The HWRF also forecasted less RI and RW events than were observed, with better forecast skill for RW than RI. For 2011, the HWRF showed comparable forecast skill for the Atlantic and East Pacific basins. Relaxation of matching time windows led to better skill scores.