One of the applications of radar data is rainfall estimation, which is very important for agriculture and hydrology. In fact, the opportunity to improve these estimates was one of the driving motivations for the recent dual-polarization upgrade. Because of the relatively good spatial coverage and high resolution of the data, radars are particularly well-suited for quantitative precipitation estimation (QPE).
Over the winter, WDT has developed POLARIS Quantitative Precipitation Estimation (PQPE), which utilizes the dual-pol variables differential reflectivity (Zdr) and specific differential phase (Kdp) in addition to reflectivity (Z) to estimate precipitation amounts, in contrast to standard techniques that only use reflectivity.
Within the past week, very heavy rain fell in several parts of the United States, providing an excellent opportunity to compare traditional reflectivity-only QPE to PQPE. This blog post will highlight five of those areas.
Area 1: Localized rainfall near and just east of Austin, TX on May 5-6
The following image shows the 24-hour precipitation totals ending near 7 am local time reported near and just east of the Austin, TX metro area by a dense network of volunteer observers. The central and northern Austin metro area generally saw 3-5” of rain, with the highest totals near Elgin, including a maximum of 7.09”.
The following images are the corresponding radar-based precipitation estimates for the same area and time. PQPE is on the left and traditional QPE is on the right. Note that both images use the same color scale, with shades of red indicating 3-6” and pink indicating 6-8”. In this case, PQPE has maximum values between 6” and 8” that correspond well with the highest gauge observations near Elgin. Over the Austin metro, PQPE has values mostly between 3” and 5”, with a few pockets of 5-6”, which are also consistent with gauges. The traditional QPE has values that are much lower in these regions, with generally 1.5-2.5” over Austin, and maximum amounts near Elgin of only 3-4”, only about half of what was observed.
Area 2: Southern Nebraska on May 6
During the evening of May 6, numerous thunderstorms tracked over the same narrow strip of southern Nebraska, resulting in tremendous rainfall and several reports of severe weather. The following image shows the National Weather Service precipitation analysis (using radar, gauges, and expert quality control) for the 24 hour period ending at 7 am local time on May 7 over the state of Nebraska. Maximum amounts exceeded 10”.
Below are the corresponding PQPE (left) and traditional QPE (right) images. In this case, both methods agree quite well with each other and with the gauge observations (maximum amount of 10.47”, several observations above 8” within the heavy rain swath).
Area 3: Oklahoma and neighboring states May 5-11
Heavy rain occurred over much of Oklahoma and parts of Texas and Arkansas on multiple days during the week, leading to widespread flooding. Rainfall amounts observed by the Oklahoma Mesonet for the 7 days ending at about 9 am local time on May 11 exceeded 10” at several locations, with a maximum of 12.57” at Minco, just west of Oklahoma City, as shown in the image below.
The corresponding 7-day rainfall estimates from PQPE (left) and traditional QPE (right) are shown below (note the different color scale for the longer accumulation interval, with shades of red now representing 5-10” and pink showing 10-15”). The two techniques agree reasonably well regarding widespread 5”+ rainfall in southern Oklahoma. PQPE appears to be perhaps a bit closer on the very high amounts along I-40 east of Oklahoma City, though there are some underestimates in SW Oklahoma (this likely had to do with the raw data feed from the Frederick, OK radar having some outages/delays during the period).
Area 4: Coastal North Carolina on May 11 associated with TS Ana
This past week also saw the first named tropical storm of the year in the Atlantic basin, with Ana making landfall in the Carolinas and producing some localized heavy rain, with some areas of eastern North Carolina getting up to 5”. The following image shows the National Weather Service rainfall estimate for the 24 hours ending at 8 am local time on May 12.
Rainfall with tropical characteristics is one of the situations in which traditional QPE often significantly underestimates rainfall, and dual-polarization methods perform better, and this was no exception. Below are the PQPE (left) and traditional QPE (right) images for the corresponding time period. The maximum amounts and areal coverage of amounts above 2” are clearly better represented by PQPE.
Area 5: Southeast Wyoming on May 9-10
Even the high plains of southeastern Wyoming got in on the heavy rain action in the past week. As the following image shows, some volunteer observers east of Cheyenne saw over 3” on May 9-10, an unusually heavy amount of rain for an area with an average annual rainfall of around 16”.
In this case, the precipitation mostly fell as a cold rain, with the freezing level not far above the surface. Because the radar beam gets higher as it moves away from the radar, a low freezing level means it will be observed relatively close to the radar (located close to Cheyenne). In addition to the reduced correlation coefficient seen near the freezing level that was described in a previous blog post, the area near the freezing level often contains higher reflectivity than other areas. Again, because the radar beam gets higher off the ground as it goes out, this often shows up as a ring of higher reflectivity known as the “bright band”. An example of this is shown below, with a ring of higher reflectivity between the black rings on the 3.3° scan from KMPX. The outer ring, representing the location where snowflakes are beginning to melt, is at a height of about 3 km above the radar, which is very close to the environmental freezing level.
If these enhanced reflectivity values are persistent over the same areas on the lowest radar scan, rainfall can be considerably overestimated by traditional QPE methods. That appears to have happened in this Wyoming case, because traditional QPE (right) is severely overestimated just east of Cheyenne. Because PQPE (left) takes advantage of dual-polarization data to identify areas that are likely part of the “bright band” and apply a compensating adjustment to the rain rate, it provides a more realistic estimate.
These examples illustrate the potential for improved estimates of heavy rainfall by using dual-polarization variables in addition to reflectivity. These improved estimates would be beneficial for hydrology and flood warnings/forecasts, as well as agriculture.