M.S. Research Endeavors
(supported by an AMS fellowship, CAPS and DRARSR)



1. Upshear development of a thunderstorm.
2. Warm rain development in a continental thunderstorm.
3. Coalescence freezing.
4. Hail processes within a supercell.





UPSHEAR DEVELPMENT OF A THUNDERSTORM








This is a series of radar images of radar reflectivity factor Z (roughly the intensity of the storm) at a height of 3.3 km above ground level. The colors indicate the Z values, with the color tables at the bottom of the figures providing reference. The white lines are Zdr (differential reflectivity) contours at 1.0, 2.5, and 4.0 dB. The images are oriented with North at the top and West on the left. This series of images illustrates the evolution of this storm. It originated as a weak cell, labelled C0. With time, new cells initiated. These new cells arose primarily on the "upshear" side of the storm. Cells on the upshear side are labelled with a U, while cells on the downshear side are labelled with a D, with numbers indicating successively identified cells. This series of figures covers approximately 25 minutes of this storm's life.

Shear in this context is the increase of wind with height. Actually, it is slightly more complicated than that. Consider two winds, one at the surface and one 10 feet above the surface. If the wind at 10 feet is the same as at the surface except it has a slight Southerly component, then the shear is said to be from the South to the North. If the wind at 10 feet is 0, then the shear "vector" points in the direction opposite the surface wind (assuming the surface wind is not 0). On this day, the wind became increasingly strong from the West. Thus, the shear vector pointed from West to East. Cells growing on the West side of the original cell, then, grew on the upshear side of the original cell.

The images above show how this storm initiated as a weak cell, evolved through a succession of stronger cells that grew upshear of the original cell, and then established supercellular structure (last figure, cell UC4). Others have noted preferred upshear thunderstorm development, although it has not received much attention recently. If this is a preferred growth mechanism, understanding it could help improve severe weather warnings, especially for severe weather events associated with the early stages of thunderstorms.





WARM RAIN DEVELOPMENT IN A CONTINENTAL THUNDERSTORM






The top image is a "slice" through this same storm at about 3:25 p.m. CST (Sept. 19, 1993). The color indicates radar reflectivity factor Z (roughly intensity) with the ramp at the bottom providing a reference, while Zdr (differential reflectivity) is overlaid in white. This is very early in the storm, a time when precipitation-sized particles are starting to form.

To understand this figure, a little background concerning Zdr is needed. Zdr is a measure of the relative return between hoizontally and vertically polarized waves. When raindrops fall, they are deformed by air resistance. Thus, they are wider than they are tall. Zdr detects this. When the radar is sensing raindrops, Zdr is positive (sources of errors and mixtures of hydrometeors not being considered here). If something is taller than it is wide, on the other hand, it could result in negative Zdr values. Large hailstones, for instance, sometimes orient themselves with their longest axes vertically, resulting in negative Zdr values.

With that background, consider the first figure again. At a very early stage in the storm's life, positive Zdr values exist in a column (this type of feature is known as a Zdr column ). The second figure further illustrates the Zdr column at this time. The second figure is a 3D shade plot of the 1 dB isosurface of Zdr. It is giving a 3D view of the locations of the positive Zdr values (indicating the presence of raindrops). Analysis of the radial wind information indicated that this Zdr column was coincident with updraft. This, and other information, led me to conclude that given the available data, the most likely process that could have caused these signatures is the warm-rain process . Warm-rain refers to the production of raindrops without the intervention of the ice phase. It has been expected that rain production in mid-latitudes depends upon the presence of ice, especially in continental storms (storms away from seas or oceans). Some investigators have provided evidence that this is not always the case. This evidence may be some of the most convincing to date. Improved comprehension of microphysical processes is needed to improve modeling and forecasting capabilities.





COALESCENCE FREEZING

Coalescence freezing is a process in which raindrops develop via the warm-rain process , are lofted above the freezing level, freeze, and then grow as graupeln or hail. Dr. Roscoe Braham named this physical process in 1986, although this process was known to exist before then.

Traditionally, precipitation processes have been envisaged as split into two camps, warm and cold. In coalescence freezing, however, it is the interaction of those two camps that is crucial.

Considering the above discussion ( Zdr), the pattern associated with the first branch of coalescence freezing is predictable. One would expect a Zdr column to develop. Subsequent evolution of the Z and Zdr fields depend upon the behavior of graupeln and hail. As graupeln and hail fall beneath the freezing level and melt, they develop a liquid-water torus around their equators. This makes them look wider than they are tall and gives rise to what I call the melt-out signature. Graupeln and hail that are not melting tend to have low Zdr values. Thus, a melt-out signature is characterized by low Zdrs aloft and high values below the freezing level. Check out what coalescence freezing looks like by clicking on this: See radar evidence of coalescence freezing. This sequence of cross-sections actually shows coalescence-freezing in two different cells. The one furthest to the left is the most complete example of the process. First, a Zdr (which is overlaid in white) column develops. With time, that weakens and eventually disappears as a melt-out signature predominates.





HAIL PROCESSES WITHIN A SUPERCELL

Since frozen raindrops serve as hailstone embryos (frozen raindrops are found in the center of hailstones, implying that some hailstones initiate when raindrops freeze), it has been proposed that raindrops from Zdr columns can serve as hailstone embryos. My master's research supports that hypothesis. Furthermore, data from this storm indicates that a hail embryo source region different from those previously identified in earlier research may be active in supercells.



The above figure is a 3D shade plot of where Zdr values exceed 2 dB. The time is about 3:50 p.m. CST and the storm is now a supercell. Immediately apparent are are Zdr column on the west side of the storm and a ridge of Zdr extending eastward. The Zdr ridge signifies melting graupeln and hail. Because the Zdr column and the Zdr ridge are connected and because the Zdr column is indicative of updraft, it appears as if shed and melt water from graupeln and hail stones may be recirculated into the Zdr column, helping to sustain the Zdr column and acting as a source for hailstone embyos. Thus, this hailstone embryo source region is the left-front flank of the storm. Earlier suggestions for hailstone embryo source regions include the right-rear flank of the storm and the region immediately downstream of the updraft maximum.







|Home |