Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: Lee Grenci , 4:34 PM GMT on January 29, 2013
The Day 1 Convective Outlook issued by the Storm Prediction Center at 13Z this morning (January 29) calls for a moderate risk of severe thunderstorms (interpretation) over portions of the lower Mississippi, Missouri, and Tennessee Valleys later this afternoon and tonight as a cold front approaches from the west (HPC surface forecast, valid at 00Z). The dominant mode of thunderstorms will be a squall line that develops along the cold front, where low-level convergence is strong (12-hour NAM forecast for 1000-mb streamlines, valid at 00Z this evening). Discrete to semi-discrete supercells (storms with rotating updrafts) are also possible ahead of the cold front, where weaker low-level convergence occurs in concert with the relatively subtle confluence of streamlines.
Tornadoes, damaging straight-line winds, and large hail are possible (run your cursor over "Tornado," "Wind," and "Hail" to gauge the probabilities for these three types of severe weather.
The 12-hour NAM forecast for 500-mb isotachs, expressed in knots, valid at 00Z this evening. Strong mid-tropospheric winds associated with a short-wave 500-mb trough will produce strong vertical wind shear over the region at risk for severe weather. Larger image. Courtesy of Penn State.
Later this afternoon and this evening, a vigorous short-wave 500-mb trough over Texas will advance eastward from the southern Rockies (check out the 12-hour NAM forecasts of 500 heights and absolute vorticity at 500 mb, valid at 00Z). Given the large height gradient to the east of the vigorous short-wave trough, it's not surprising that there will be a wind maximum at 500 mb, with wind speeds exceeding 100 knots (see the 12-hour NAM forecast for 500-mb isotachs above; larger image). As a result of these strong winds at 500 mb, the vertical wind shear between the earth's surface and an altitude of six kilometers is expected to sufficiently strong (vertical wind shear in this layer is a standard tool for storm prediction). Keep in mind that a representative height of 500 mb is 5500 meters, so I'm using 500-mb winds as a proxy for winds at six kilometers.
Meanwhile, a low-level jet stream zipping northward from the Gulf of Mexico will intensify, with 850-mb wind speeds topping 60 knots by this evening (see the 12-hour NAM forecast for 850-mb isotachs and streamlines below; larger image). As a result, the atmosphere will become moderately unstable as relatively warm, moist air gathers ahead of the cold front (check out the 12-hour NAM forecast for surface-based convective available potential energy in Joules per kilogram, valid at 00Z this evening). More important to this discussion, the vertical wind shear from the surface to six kilometers, which serves as one of the standard tools tool that forecasters use when they expect severe storms to be surface-based.
The 12Z NAM forecast for 850-mb isotachs and streamlines (valid at 00Z this evening), showing a robust low-level jet stream blowing from the Gulf of Mexico at speeds exceeding 60 knots. Larger image. Courtesy of Penn State.
Back to the topic of vertical wind shear. Why 0-6 kilometers? Good question! As it turns out, model simulations conducted by the Weisman and Klemp in the 1980s helped to identify the layer between the ground and an altitude near six kilometers as pivotal for predicting thunderstorm mode (here's their 1982 paper). Specifically, their simulations indicated that thunderstorms tended to be short-lived whenever model environments lacked deep vertical wind shear (shear that didn't extend to altitudes near six kilometers). Later empirical research confirmed that vertical shear needs to be relatively strong through the lowest five or six kilometers of the troposphere in order for supercells to form. Essentially, strong winds near six kilometers (and higher altitudes) insure that precipitation is carried farther downstream rather than falling back into the updraft and thereby weakening the storm. I'll add that many references describe the role of strong vertical wind shear as "tilting" the updraft. Such a notion is incorrect because most updrafts in supercells are upright (or nearly so). My objections aside, let's look more closely at the vertical wind shear from the ground to an altitude of six kilometers.
Why choose the earth's surface as the bottom of this layer (sfc-6 kilometers)? Easy. The observed and predicted surface winds are readily available (these data commonly appear on most skew-T output; see my previous blog). To seal the deal, surface winds contribute to the low-level shear available for surface-based supercells (I'll discuss the role of low-level shear in the formation of supercells in later blogs).
For the record, vertical wind shear is a change in wind speed and/or wind direction with altitude. To get your quantitative bearings, this vertical profile of winds (courtesy of A World of Weather: Fundamentals of Meteorology) represents an environment with relatively strong vertical shear between the ground and six kilometers. Granted, the change in wind direction with height is not substantive, but the overall increase in wind speed with height should be obvious to you. Compare this example of strong wind shear to an example I would categorize as weak.
Given that the wind is a vector (it has both direction and magnitude), we can calculate vertical wind shear in any given layer of air by performing vector subtraction between the wind at the top of the layer and the wind at the bottom of the layer. Specifically, the change in wind speed and wind direction between the bottom and top of a layer of air (in other words, the vertical wind shear) equals the wind vector at the top of the layer minus the wind vector at the bottom of the layer. Right off the bat, you should see that vertical wind shear is also a vector (the difference between two vectors is a vector). As a vector, vertical wind shear has both magnitude and direction. Seems pretty scary, eh? Don't get nervous. I realize that many of you aren't accustomed to working with vectors, but we can simplify the vector subtraction by plotting the winds on a polar coordinate system. Still scary? Don't worry. In a moment, I'll introduce a nifty flash animation that does all the work for you (copyright by the Penn State Certificate Program).
The black arrow represents the magnitude (in knots) and direction (314 degrees) of the wind shear vector in a given layer of air. The green vector indicates a wind at the bottom of the layer blowing from the southwest (250 degrees) at 10 knots. The blue vector represents the wind at the top of the layer blowing from the northwest (300 degrees) at 40 knots. Copyright by Penn State.
For starters, check out the polar coordinate graph above. The circles represent wind speed expressed in knots (the interval between successive circles is 10 knots). The horizontal and vertical axes serve as references for a wind compass so that we can also take wind direction into account. Okay, let's see how it works. Let's assume that we want to calculate the vertical wind shear vector in a layer of air where the wind at the top of the layer blows from the west-northwest (300 degrees) at 40 knots, while the wind at the bottom of the layer blows from the west-southwest (250 degrees) at 10 knots. To plot the wind vector at the top of the layer, I estimated 300 degrees on the wind compass and judiciously placed a small dot (not shown) on the fourth concentric circle from the origin. Then I drew the vector corresponding to the wind at the top of the layer (bluish) from the origin to the dot. Now for the wind at the bottom of the layer. I estimated 250 degrees on the wind compass and placed a dot (not shown) on the innermost circle and drew the vector (in green).
To subtract the lower wind vector from the upper wind vector, simply draw a vector from the arrowhead of the lower wind vector to the arrowhead of the upper wind vector. Yes, the black vector represents the vertical wind shear vector in the layer. It has magnitude (34 knots) and direction (314 degrees). How did you get those answers, Grenci? As it turns out, subtracting wind vectors involves some trigonometry, so the math gets a little cumbersome. So here's a handy-dandy flash animation that automatically calculates the vertical wind shear vector for any given layer of air. I recommend investing some time and exploring the animation so that you get comfortable with treating vertical wind shear as a vector. Such a vector subtraction leads to the bulk vertical wind shear; essentially, it does not take into account any variations of wind speed and wind direction between the top and the bottom of the layer of air.
I hope you learned something new about vertical wind shear.
I should add here that forecasters use effective bulk shear, which takes into account storm depth, as a tool for predicting elevated thunderstorms (as well as surface-based). Perhaps this topic will be fodder for a later blog.
Given the risk of tornadic storms over portions of the Tennessee and lower Mississippi Valleys later this afternoon and tonight, stay safe everybody.
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