DISCUSSION: On September 1, 2016, WCNC (Charlotte, NC) Chief Meteorologist Brad Panovich acknowledged a peculiar feature inside of Hurricane Hermine while using the ZDR Dual-Polarization Differential Reflectivity Doppler Radar. Meteorologists operate Doppler Radar (RAdio Detection And Ranging) to detect precipitation, thunderstorms and other weather characteristics. It helps forecasters to predict tornadoes and pinpoint the exact location of precipitation. Dual-polarization radar exhibits unique abilities that allow it to distinguish precipitation from non-meteorological objects such as insects, birds, ground clutter and debris. The radar transmits pulses that are oriented in the vertical and horizontal directions. Once the pulses hit an object, it returns two-dimensional information back to the radar. Differential reflectivity (ZDR) is utilized to identify the shape and size of an object. It can detect airborne tornado debris, hail, storm updrafts and rotation. Thunderstorms accompanied with deep rotating updrafts are often a good indication of storm-producing tornadoes.
The Dual-Polarization radar image (below) detected a cluster of objects that was situated in the core of the northeastern part of the hurricane’s eye (i.e. the center of the tropical cyclone). The eye is the region of calm winds (less than or equal to 15 mph), which dynamically organizes beneath a vacuum of sinking air that is characterized by pleasant weather conditions. Also, the eye is encircled by an eye-wall, which is composed of strong-to-severe thunderstorms whose impacts often consist of heavy rainfall and excessively strong winds.
Simultaneously, Hurricane Hermine was drifting north-northeast at approximately 14 mph while maintaining maximum sustained wind speeds of up to 80 mph. Birds often travel inside of hurricane eyes to avoid the devastating winds which are situated within and beyond the eye-wall. Inside the eye of Hermine, a bird flock attempted to seek refuge in the core of this storm. More interestingly, the flock migrated in the same direction as the hurricane was moving just to survive throughout the duration of the storm.
Nonetheless, this isn’t the first time a Dual-Polarization radar detected birds inside of a hurricane. In July 2014, the University of Alabama Huntsville Severe Weather Institute Radar and Lightning Laboratories used ZDR and Dual-Polarization reflectivity to distinguish birds versus hurricane precipitation within Hurricane Arthur. In the eye of Arthur, scientists concluded that a pink cluster of non-meteorological objects on the radar was a flock of birds seeking refuge. Similarly, as Chief Meteorologist Brad Panovich reminisced on the Hurricane Arthur case, he used radar reflectivity and ZDR to comparatively identify that a flock of birds were present on the radar within Hurricane Hermine.
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©2016 Meteorologist Jordan Rabinowitz
Classic Hand Analysis of Hurricane Hermine Just Prior to Landfall! (credit: NWS Little Rock, Arkansas)
DISCUSSION: Just a little over an hour ago (i.e., in the vicinity of 10:00 PM CDT), Hurricane Hermine was positioned just to the south of Saint Marks, Florida (located in the eastern part of the Florida Panhandle). One of the neater features associated with the circulation of Hermine is the particularly tight pressure gradient which is evident in the hand-drawn analysis attached above (courtesy of forecasters at the National Weather Service office located in Little Rock, Arkansas). This notably strong pressure gradient is especially noticeable from the center of Hermine and outwards both towards the northwest, southwest, and southeast. As far as a meteorologically-based explanation, the best reason for why there was an anomaly located just northwest of the core of this tropical cyclone at that point in time was the possible movement of drier air from the mid-levels towards the surface within the core of the circulation of Hermine. As a consequence, this naturally led to an inconsistency in the magnitude of the pressure gradient in the vicinity of the northeast quadrant of Hermine's circulation center as noted in the hand-drawn analysis above.
In addition, you will note now on the east and north easterly side of the circulation associated with a currently weakening Hurricane Hermine, there is a plethora of deep tropical moisture flowing around the eastern half of the tropical storm based on widespread dew point values in excess of 70°F. This is a classic feature often found with intensifying tropical cyclones based on their ability to transport large quantities of moisture towards the center as it wraps around the center of the circulation both over the ocean and onshore during the post-landfall phase. You can also denote the classic cyclonic flow occurring within the larger-scale wind field as denoted by the southwesterly-to-southeasterly flow (from south-to-north) occurring within the eastern half of this tropical storm and the northeasterly-to-northwesterly flow (north-to-south) occurring within the western half of this weakening tropical storm!
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©2016 Meteorologist Jordan Rabinowitz