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. Review Exercise #3 Problem #2 before proceeding with this section.

  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_wrf27uw.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 hour, so if we select an increment of one hour, the contents of the entire file at that location will be extracted. However, we can reduce the number of time periods extracted. We already know that the file starts at 00 UTC and the tracer release started at 17 UTC. Therefore, by setting the offset=12 hours and the increment=6, 18 UTC will be the second time period 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 1222 meters (AGL). 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 (TKEN) and the specific humidity (SPHU) drop sharply at the first level above the boundary layer (843 hPa), while the potential temperature (TPOT) starts to increase at that level. The WRF data files do not explicitly contain the height of the fields. The PBL height can be estimated assuming that in a standard atmosphere the height changes by about 9 m per hPa. In this case the estimated pressure for HPBL=1222 would be about 852 hPa (988-1222/9).The actual sounding at Dayton suggests that the PBL height should be around 1200 m AGL.

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. The methods used by HYSPLIT to estimate the mixed layer depth is discussed in more detail in a later section. There is an automated way to set the computational starting height with respect to the mixed layer depth, which will be shown in the next section.