Hurricane Irene made landfall on the morning of August 27th near Cape Lookout in eastern North Carolina as a strong Category 1 hurricane. Heavy rainfall was observed with this TC which provided a unique experience to analyze differences between Dual-Pol and Legacy storm total precipitation estimates. The KMHX radar went through the Dual-Pol upgrade in mid June (learn more here) and was the first to observe a TC landfall across the Southeast/Mid-Atlantic region.
Storm total precipitation amounts associated with Hurricane Irene were particularly high along and just west of the TC track. Totals ranged from around 5 inches over the northern Outer Banks, to around 15 inches in Beaufort, Pamlico, and northern Craven Counties. A COOP station in Bayboro (Pamlico County) observed the highest rainfall with 15.74 inches. The Bunyan RAWS in Beaufort County came in second with 15.66 inches. Several CoCoRaHS reports were also valuable in reporting heavy precipitation across the region especially in Martin county (14.27 inches), Lenoir county (13.61 inches), and Craven county (12.64 inches).
Two Doppler radar images below illustrate the estimated storm total accumulations from Dual-Pol (Figure 1) and Legacy (Figure 2). Note that both estimate over 17 inches of precipitation in portions of Beaufort, Craven, and Pamlico Counties. The Dual-Pol minus Legacy (DSD) accumulation graphic is also given (Figure 3) to demonstrate the underestimation of Legacy precipitation accumulation by 2-3 inches in locations where highest rainfall amounts were observed.
Figure 1: KMHX Dual-Pol storm total accumulation for Hurricane Irene (Aug. 26-27).
Figure 2: KMHX Legacy storm total accumulation for Hurricane Irene (Aug 26-27).
Figure 3: Storm total accumulation difference product (DSD) between Dual-Pol and Legacy. Note that Legacy accumulation underestimated precipitation around 2 to 3 inches for the region with heaviest precipitation.
The Dual-Pol “digital” storm total accumulation product has greater spatial resolution than the legacy storm total product (0.25 km vs. 1 km) which can be seen in comparing the figures above (e.g. Legacy appears “blocky”). Dual-pol determines a new ZR relationship each volume scan based on the Hybrid-Scan Hydroclass (HHC) which is specific to the dominant hydrometeor type for a given location. This process is used to potentially improve precipitation estimation (i.e. different rainfall rate for large meteors vs. small meteors). In determining the ZR relationship for legacy products, the ZR relationship was changed from normal convective mode (300 R^1.4) to tropical mode (250 R^1.2).
Carin Goodall-Gosnell, MHX Science Operations Officer, analyzed observed precipitation against pixel values of both Dual-Pol and Legacy storm total precipitation and found lower errors in Dual-Pol vs. Legacy (Figure 4). The image below also illustrates that Dual-Pol and Legacy contained similar errors at low values but Dual-Pol demonstrated much lower errors for greater precipitation amounts.
Figure 4: Dual-Pol vs. Legacy storm total accumulation error trends. A linear fit was applied to an analysis of pixel value difference from observations (y-axis is error and x-axis amount of rainfall).