Curious Precipitation Curtains within the Inflow Region of a Supercell Thunderstorm: A Status Report

Rodger A. BrownNOAA/National Severe Storms Laboratory,

Norman, OK

BP-9

Background• Phased Array Radar (PAR) measurements were made in a

tornadic supercell thunderstorm on 10 February 2009 from the National Weather Radar Testbed in Norman, OK

• As the supercell storm moved northeast, it skirted the NW side of Oklahoma City where it produced two short-lived tornadoes

• The PAR completed 90o-wide volume scans through the storm every 70 s

• The frequent radar sampling revealed a series of Precipitation Curtains (PCs) that formed just outside the storm and moved into the storm along with the right-flank inflow

• The reflectivity features are called Precipitation Curtains because they are tall (up to 4-5 km), long (5-12 km), and narrow (1.5 km)

• This presentation gives some of the preliminary findings concerning the PCs as well as raising questions about their physical mechanisms

Overview of Precipitation Curtains (PCs)• PAR data collection began at 2022:58 UTC after a series of

PCs had begun flowing into the storm; five individual PCs were identified during the ensuing 20 min

• The PCs initially were evident in the low altitude reflectivity scans (right panel Fig. 1), but reconstructed vertical cross-sections revealed that PCs had 3D structure that tilted in the downstream direction with height (upper left panel Fig. 1)

• The PCs formed beneath—and apparently were not attached to—a midaltitude layer (4-6 km ARL) having reflectivity values of 25-35 dBZ that extended to the east through south of the storm’s right flank overhang (left panels Fig. 1)

• The PCs moved with the flow into the right flank of the storm at ground-relative speeds of 20-25 m s-1 (storm-relative speeds of 15-20 m s-1)

• Storm-relative positions of the five PCs are shown in Fig. 2; the separation distance between successive PCs at a given time averaged 3.5-4.0 km

• The evolution of PC 3 (Fig. 3) is presented as an example of typical PC evolution; the vertical profiles are of maximum reflectivity values at each azimuth and height along the PC

• Peak reflectivity values are found at relatively low heights (2-3 km) and remain at those heights throughout PC evolution—raising questions about the origin and maintenance of the PCs

• As the PC moved beneath the lowered right-flank overhang, the reflectivity in its upper portions merged with the overhang’s reflectivity

• It is curious that the reflectivity pattern sloped from one end of the PC to the other, while reflectivity increased in magnitude at all heights with time

• The line of radial nondivergence (red line in Fig. 3)—marking the division between radial convergence below and radial divergence above—also sloped as well as generally increasing in height with time

Fig. 1. PAR reflectivity data at 2026:28 UTC on 10 February 2009. (Upper left) Vertical cross-section is along the dashed line (azimuth 332o) in the right panel. Sloping PCs 1, 2, and 3 beneath the anvil overhang are indicated, with PC 3 not yet extending to the ground. (Lower left) Horizontal cross-section at 5.4 km ARL through the midaltitude layer identified in upper left panel. (Right) Reflectivity data at 0.5o elevation angle. Horizontal projections of PCs 1 and 2 are indicated on top of the portions of the PCs evident at 0.5o elevation angle. The squares and circle along the dashed line are separated by 6.25 km.

Fig. 2. Storm-relative positions at 70 s intervals of the horizontal projections (of lowest detectable heights) of the five PCs. The reflectivity pattern is a composite of the slightly-varying individual reflectivity patterns at 0.5o elevation angle. The dashed line is an approximation of the 30 dBZ edge of the anvil overhang.

Fig. 3. Undistorted vertical cross-sections of maximum reflectivity at 70 s intervals within PC 3 obtained at 1.00 azimuthal intervals and 0.5 km height intervals from vertical cross-section plots like those in the upper left panel of Fig. 1. Black curves indicate either the 25 dBZ base of the midaltitude layer or the 30 dBZ base of the anvil overhang. The red line separates radial convergence below the line from radial divergence above. Nagging Questions

• How did the Precipitation Curtains form?

• Why was there a series of them?

• Why did they seem not to be attached to stronger reflectivity aloft?

• Why did maximum reflectivity stay at 2-3 km height?

• Why did reflectivity become stronger at all heights as a function of time?

• Why did the reflectivity pattern slope upward to the east?

• Why did the red line of radial nondivergence (separating radial convergence below from radial divergence above) also slope upward to the east?

• Why did the depth of radial convergence increase with time?

National Weather Association Annual Meeting, Oklahoma City, OK, 19-22 October 2015

Contact: Rodger.Brown@noaa.gov