Trough Identification

Following Knippertz (2003), I ran calculations on 500 hPa height data for 1 Jun through 30 Sept from 1980 to 2008.

This identification scheme uses calculated zonal geopotential height gradient data to detect occurrences of troughs. For an in-depth methodology refer to the above paper. My domain covers 10 to 60N and 145 to 65W. Setting these domain boundaries, specifically the longitudinal boundaries, produces data for trough occurrences located between 130 to 80W, which is of importance because of its influence on the North American monsoon system.

I ran a moving-grid box calculation across the domain for each day of each year in order to locate troughs within the region. The grid-box calculation consisted of three boxes. Z2, the central of the three, was a 3x4 grid domain. Z1 and Z3, the boxes westward and eastward adjacent, respectively, were comprised of 3x5 grid domains. For a given day the mean of the averaged Z1 and Z3 was compared to the mean of Z2. The resulting value, P, comparable to the zonal geopotential height gradient, was then assigned to a central point within Z2. Values of P greater than 25 are indicative of a location experiencing troughiness. Values of 100 or greater relate to more extreme, a.k.a. deep, troughs.

Below is an example of results for one day, 1 Jun 2001, from these calculations. The 500 hPa height field is shown in black, solid lines. We can see a clear pattern of troughiness off the western coast of the United States and the Mississippi River Delta. A well defined ridge is located between these two locations. The results from the P calculations are colored-coded, values ranging from 25 to 100, and plotted under the 500 hPa height field. The location of the P values that are associated with troughs are indeed co-located along the trough axis. After running loops of one week to one month, it seems clear that the calculations for P have worked quite well, and can be considered accurate, at least empirically.
Next, ridges will also be defined. This will be a simple task though, as I will use the same P calculations that have already been done, expect I will look for large negative values to indicate ridge occurrences.

-jamie