Study on the rapid intensification of Hurricane Dorian near the Eastern Caribbean published in Atmosphere

Summary:

We look at why Hurricane Dorian was able to intensify in the Eastern Caribbean when many forecasts suggested that it would stay weak and perhaps die out. To help answer this question, we use a forecast model called the Hurricane Analysis and Forecast System (HAFS). Specifically, we use an ensemble, a group of forecasts from the same model with slightly different starting conditions. We use 80 different ensemble members and compare those that (correctly) showed Dorian intensifying with those that (incorrectly) kept it weak. This shows some of the key factors that allowed Dorian to intensify early, and also motivate further work to understand how and why other storms in similar conditions can intensify while others don’t.

Hurricane Dorian undergoing the intensification described in the study.

Important Conclusions:

The large-scale weather pattern around Dorian, specifically moisture and wind at the top of the hurricane, was more favorable in the ensemble members that showed a strong storm. Specifically, there was more outflow (air flowing out away from the storm at the top, which helps lower the central pressure of the storm, see Fig. 1 below) and more moisture (which favors more thunderstorms and precipitation, which also helps to lower the pressure and lead to a stronger storm) in the strong ensemble members.

Figure 1: 36-hour forecasts of Hurricane Dorian’s upper-level outflow from a weak ensemble member (left) and strong ensemble member (right). Wind speed is in kt (n mi per hour) and is represented by color. Note that the wind speed in the strong ensemble member are larger than those in the weak member.

The ensemble members that produced a strong Dorian had a more developed eyewall, with the precipitation forming a symmetric ring (which previous studies have shown is important for allowing rapid intensification), earlier on in the life of Dorian (see Fig. 2 below).

Figure 2: 48-hour forecasts for Dorian’s precipitation structure from (top) a weak ensemble member and (middle) a strong ensemble member. Observed rainfall from one of the NOAA P-3 flights into Dorian, and the aircraft flight tracks and times, are shown on the bottom. Wind speed is shown by the barbs (long barb is 10 kt, short barb is 5 kt). Rain is represented by radar reflectivity (dBz), with higher values meaning harder rainfall. The arrows show the vertical wind shear (change in wind between the surface and region near the top of the hurricane). Note that the strong member has a ring of convection, the eyewall, whereas the weak member does not.

The large-scale (upper-level outflow) and small-scale (eyewall formation) processes working together were important to Dorian’s unforecast intensification.