Monday, October 6, 2014

Forecast Tools: The Time-Height Section

There are many ways to slice and dice atmospheric analyses and forecasts.  One of my favorites, especially for forecasting in mountainous regions, is the time–height section.
0600 UTC 26 March 2014 NAM Time-Height Section
Time-height sections consist of a time series of profiles at a given location, such as the Salt Lake City International airport shown above (from March since the forecast for the next few days is so boring).   Although one is often used to time increasing to the right in most graphs, meteorological time-height sections are often created with time increasing to the left.  Thus, in the chart above, the forecast begins on the right at 6Z Wed and ends on the left at 18Z on Saturday.  Pressure is usually used for the height coordinate.  The units displayed above are hectoPascals (hPA), which is the equivalent of a millibar (mb).  In northern Utah, 850-mb is roughly at the elevation of the east bench and 700-mb is just below the highest peaks in the central Wasatch Mountains. 

The beauty of the time-height section is that it shows how he atmosphere is forecast to evolve over the site of interest.  One can see, for example, the distribution of relative humidity in the vertical, where the atmosphere is stable or unstable, and how the wind direction and speed changes with height.  All of these are potentially important for forecasting mountain precipitation.  

In the time-height section above, the forecast begins with southwesterly flow and dry air at low levels (see 6Z Wed at the right).  By 12Z Wed, however, the low-level winds have shifted to northwesterly and there has been a drop in equivalent potential temperature (θe, a thermodynamic variable that is used to identify airmass characteristics).  The wind shift and drop in θe are the results of a shallow frontal passage.  Note how at 12Z Wed the wind shifts abruptly from northwest to southwest from 700 to 650 mb.  So, in this case, the frontal passage is very shallow and relatively dry.  Higher relative humidity air does not move in at low levels until 3-hours later, after which there is a prolonged period of relatively moist westerly to northwesterly flow through a fairly deep layer.  

θe can also be used to assess stability.   Layers in which θe decreases with height are considered potentially unstable in that lifting of the layer by a front or flow over a mountain can lead to the generation of convection that can enhance precipitation rates.  Note that I'm using "can" rather than "will" as there are a number of nuances to consider in assessing the triggering of convection in this manner that I'm going to ignore here.  

Curiously, there are very few web sites where you can find time-height sections, but fortunately we have them at [we are currently having a problem with our GFS time-height sections, so ignore them until you see more recent dates appearing on the graphics].  For the NAM, we have time-height sections for Alta (UT), Jackson (WY), Mammoth Mountain (CA), and Kalispell (MT). 

For more on time-height sections and the use of θe see:

Steenburgh, J., and E. Greene, 2004: Intermountain winter storm evolution during a 100-inch storm cycle. The Avalanche Review, 22(4), 13-16.

Which is available on page 13 of the May 2004 issue of The Avalanche Review

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