The Importance of Short-Wave Troughs and How to Identify Them.

Short-wave (S/W) toughs are features in the general flow of the atmosphere that are very important when it comes to forecasting for convective and winter weather.  Short-waves tend to be more of "weather-makers" where as Long-waves (Rossby Waves) are "trend-makers." Multiple S/W troughs can be imbedded with in a Rossby Wave trough.  Shortwave troughs tend to move faster then its longer wavelength counterparts.

The images below are from a 500 mb analysis at 1200 UTC 22 September 2003.  The first image shows 6 Long-Waves around the Northern Hemisphere denoted by the blue line across each long-wave trough axis.  The second image depicts the 500 mb analysis across North America only.  Embedded with in the Rossby Wave trough across the central US are 3 S/W troughs moving around the it, denoted by blue axes.

Here is a quick table to show the differences between Long-waves and Short-waves.  This table is assuming the observer is at 40º latitude.  

General Characteristics:	Long-waves	Short-waves
Number of Waves	3-7 across hemisphere; typically 4-5	~ 15-40° (1275 - 3400 km) longitude wide, move through long wave troughs
Amplitude	Meridionally, on the order of several 1000 km	Meridionally, on the order of several hundred km, up to 1000 km
Wavelength	~50-120° longitude (4250 - 10,200 km)	~10-40° longitude (1275 - 3400 km)
Diagnosed best at	At 500 mb and above	At 500 mb and below
Movement	Generally eastward at 10-15 knots; but can remain quasi-stationary or even retrogress	Dominantly eastward with a northerly or southerly component; faster than the long wave troughs
Energy Regime	Barotropic or Equivalent Barotropic	Definitely baroclinic

*At 40º latitude, 1º longitude ~ 85 km and at 45º latitude, 1º longitude ~ 79 km.

Identifying a S/W trough at mid-levels of the troposphere is not always simple or straight forward, especially during the warm season (May-September). Here are some guidelines to help one to identify S/W troughs.

1)  Look for a trough in the height field.  In the cold season S/W troughs are usually easily diagnosed through a good contour analysis.  In the warm season, you may need to decrease the interval between heights to better define the trough (example, using a 30 gpm interval at 500 mb instead of the standard 60 gpm).

2)  Locate a wind shift across the trough axis.  Normally, winds should back as the S/W trough approaches an area and then veer after passage of a trough.  A 12-h wind shift change can be used over a station to help individuals to spot these trends (example, V30, B40).

3)  Analyze the isotherm pattern.  Isoplething isotherms at a 2ºC interval on upper-level maps can help to spot subtle cool pools or thermal troughs that may not show up in the height analysis.  Remember, heights of pressure surfaces are based on the mean temperature of the atmospheric column. Shallow cool pockets of air aloft may be masked by the height field since it is more sensitive to the mean temperature and not the level of analysis' temperature. 

4) Look at the mid-level dew points or dew point depressions.  Downstream from the S/W trough, you should observe higher dew points and lower dew point depressions than upstream from the trough axis. This assumption is based on the fact that there is generally upward vertical motion (UVM) downstream from the trough axis and downward vertical motion (DVM) upstream from the trough axis.  

5)  Review the surface cloud reports.  Typically, SW troughs aloft create mid-level cloudiness (i.e., altocumulus, altostratus, altocumulus castellanus) downstream from the trough axis.  This is associated with mid-level UVM associated with the S/W trough.

6)  Look at visible (VIS) and infrared (IR) satellite imagery. As noted in point (4), mid-level clouds are associated with S/W trough.  

7)  Analyze the water vapor (WV) channel.  Often jet streak can be found coming around the base of a S/W trough.  This feature may show up as a dark slot on the WV imagery.

8)  Review a sounding.  Soundings downstream from the S/W trough should show a middle tropospheric layer of higher dew points and low dew point depressions even if clouds are not see in the satellite imagery or surface cloud report.

9)  Look for vorticity maximum or "lobe" (elongated curved region) of vorticity.  As noted in point (2), S/W trough have cyclonic wind shift across their axis which will show up on a vorticity analysis. Remember, when performing vorticity analysis, be sure to use absolute vorticity and not relative vorticity.

10)  Analyze the height change field (isallohypses).  Similar to a frontal passage where an incoming front will have lower pressure tendency values ahead of it; the height field ahead of an approaching S/W trough should have negative values in the isallohypses field.

Hope these basic pointers about short-wave troughs will help you to become a better armchair forecaster.  

Buoyantly yours,

@docwx