4.3 Estimating Mixed Layer Depths




From the basic trajectory calculation shown in the previous section, it was clear that although the trajectory starting height was similar to the tracer release height, the low-level trajectory did not correspond to the tracer footprint or was representative of the levels that the tracer was transported. We can use several tools through the GUI to evaluate the best transport level.

  1. The first step is to determine the mixing depth at the time and location of the tracer release. Go to the main menu and select the Meteorology / Display Data / Text Profile menu tab. This will call a program to read the values from the meteorological data file at that point and time and write the information to a text file.

  2. The Text profile tab opens a menu where you need to select the appropriate meteorological data file captex2_narr.bin and set latitude and longitude of the release point: 39.90 -84.22. Setting the correct time can be a little confusing using the two radio-buttons: offset and increment. This particular meteorology file contains data every 3 hours, so we could just select an increment of 3 hours and the contents of the entire file at that location will be extracted. However, we already know that the file starts at 15 UTC and the tracer release started at 17 UTC. Therefore by setting the offset=3 hours and the increment=0, only one time period, 18 UTC will be extracted from the file.

  3. The profile results will be shown in its own window and also written to the file profile.txt. In addition to the data contents at the selected grid point, the file also contains information about the first and last time periods. At this point all we need to know is the Height of the Planetary Boundary Layer (HPBL) which is shown to be 1716 meters (MSL). Information about each of the variables will be discussed later as needed. More information about these variables can be found in the Users Guide (see hysplit documents directory).

  4. Note that several other meteorological variables are consistent with this result. Both turbulent kinetic energy (variable=TKEN) and the specific humidity (SPHU) start to drop at the first level (825 hPa) above the boundary layer, while the potential temperature (TPOT) starts to increase at that level.

Not all meteorological data files contain a variable to represent the mixed layer depth. In these situations one can estimate the depth to be the height at which the potential temperature starts to increase. There are other ways to let HYSPLIT automatically compute the depth, which will be shown in the next section.