Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: 24hourprof , 4:04 PM GMT on January 03, 2013
Before the Internet became such a popular medium to communicate weather forecasting and analysis, Penn State forecasters regularly gathered in front the "Map Wall" in the PSU Weather Station to discuss (and debate) the prevailing and future weather patterns. The old Map Wall was divided in half...paper analyses of observed data on the left, paper copies of numerical weather forecasts on the right (old photograph). Stu Ostro probably remembers those good old days, when we waited anxiously beside the electronically chattering teletype for the hourly SAOs (Surface Aviation Observations), and then regrouped at the map wall to resume the discussion. Penn State now has an awesome electronic map wall (photograph), but the age of personal computers has all but made the group gatherings in front of the PSU map wall essentially obsolete. One of my most vivid memories of the "socials" of weather forecasters was the night Hurricane Hugo made landfall in South Carolina (1989)...the Penn State Weather Station was all abuzz and teeming with students and faculty well past midnight (on September 22).
As a young forecaster (a long, long time ago), I always spent a great deal of time in front the map wall where observations were posted. There, I attempted to become "one with the atmosphere" (I'm sure Stu Ostro remembers that we sometimes drew our own surface and upper-air analyses based on observations). Then, and only then, would I walk to the forecast side of the map wall and look at the LFM and AVN models (and later the NGM and ETA models).
My tradition of first looking at weather observations continued yesterday morning when I started my day by looking at the observed data at all the mandatory pressure levels (I confess that, in my old age, I've stopped looking at the raw data listed on the TTAA and TTBB tables). At any rate, the 250-mb rawinsonde observations over the Northeast Seaboard caught really my attention, with wind speeds exceeding 200 knots (not too shabby... check out the 12Z rawinsonde observations over Chatham, Massachusetts, and Upton, New York). The 12Z NAM model analysis of 250-mb isotachs (color-filled in knots) revealed a robust polar jet stream with an embedded jet streak (larger image).
The NAM model analysis of 250-mb isotachs (color-filled in knots). Larger image. Courtesy of Penn State.
No doubt that the gradient in heights at 250 mb (standard height is 10500 meters) was strong. For confirmation, check out the 12Z NAM model analyses of 250-mb heights (here's the NAM model analysis of 250-mb streamlines so you can gauge wind direction as well). The strong gradient at 250 mb is only part of the story because the robust polar jet stream over the eastern half of the contiguous states was linked to the vertically integrated horizontal temperature gradients from the ground to jet-stream level (these horizontal temperature gradients were associated with a frontal zone over the southeast quarter of the nation (and adjacent seas).
To see this frontal zone, check out the 12Z surface analysis and the 12Z NAM analyses of isotherms at 1000 mb, 850 mb, 700 mb, 500 mb and 400 mb, etc. (all upper-air isotherms in degrees Celsius). Note how the relatively large horizontal temperature gradients slant upward to the north behind the surface front (more generally, frontal zones slant upward back into the cold air mass). This observation is a very important key to the ultimate position of the jet stream near 250 mb.
By vertically integrated, I mean that the wind speeds at jet-stream level were generated by all the horizontal temperature gradients below 250 mb. Actually, the level of the jet stream over the Northeast Seaboard yesterday likely lay a little bit above 250 mb...note the wind of 215 knots between 250 mb and 200 mb on the 12Z skew-T for Upton, New York. Note that westerly wind speeds reached a maximum at roughly 220 mb and then decreased higher up.
Let's examine the notion of vertically integrated temperature gradients more closely without using differential equations. Check out the idealized cross section of temperature across a frontal zone, with a warm air mass to the south and cold air mass to the north. The horizontal white line segments indicate the height of the mandatory pressure level. Note that pressure decreases faster in the cold air mass compared to the warm air mass (this observation is a rule that can be derived mathematically). The bottom line is that the spacing between mandatory pressure levels is greater in the warm air mass, especially in the upper troposphere.
If I connect the corresponding heights of the mandatory pressure levels (see this annotated idealized cross section with the connectors indicated in green). Note how the slope of the connectors increase with increasing altitude (decreasing pressure). In essence, the horizontal temperature gradients in the frontal zone translate to horizontal gradients in heights that increase with increasing altitude (decreasing pressure). Given that we can treat horizontal height gradients on constant pressure surfaces as if they were horizontal pressure gradients on constant height surfaces, we should expect that wind speeds to increase with increasing altitude (decreasing pressure).
An idealized cross section of temperature through a frontal zone, with a cold air mass (blue) to the south and a warm air mass (red) to the south. White line segments represent the height of the mandatory pressure level. Larger image. Courtesy of A World of Weather: Fundamentals of Meteorology.
Indeed, the idealized schematic above (larger image) summarizes what I've discussed so far. Note the vertical profile of generally westerly winds on the right...speeds increase with increasing altitude (decreasing pressure) up to a layer near 300 mb and 250 mb. As a general rule, the altitude of the "jet-stream layer," which is typically a couple of kilometers thick, varies with season and latitude...the jet stream typically lies at higher altitudes (lower pressures) and is much weaker during the summer season (compared to winter).
Why do wind speeds decrease above the "jet-stream level?" Good question. Keeping in mind that pressure decreases faster with altitude in cold air (compared to a warm air mass), the tropopause typically lies at a lower altitude in the cold air mass. This observation means that temperatures remain constant or start to increase with altitude above the tropopause over the cold air mass. Meanwhile, in the warm air mass, temperatures continue to decrease with altitude until we reach its more lofty tropopause. In other words, horizontal temperature gradients start to decrease in the frontal zone above the "jet-stream layer." In turn, height gradients in the frontal zone decrease (note the flattening slopes of the 200 mb and 150 mb pressure levels across the frontal zone. As a result of the decrease in the height gradient across the frontal zone at 200 mb, 150 mb, etc., wind speeds also decrease.
Ladies and gentlemen, I give you the polar jet stream.
Here endeth the lesson.
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