19.1. Simple Calculations

The simple calculations cab be done with the simple mathematical symbols and then add that calculation to a dataset.

ds['variable_name'] = <mathematical calculation>

A new variable can be added to a dataset (ds) by defining the name within quote marks on the left-hand side of the equal sign in the same way as we did with a Pandas DataFrame earlier in the semester. Then the mathematical calculation can be completed on the right-hand side using at least one variable already in the dataset.

THICKNESS

To understand the function and how it works, let’s calculate the geopotential thickness between 500 and 1000-hPa. Recall from previous material:

\[\text{1000-500-hPa Thickness} = z_{500} - z_{1000}\]

So, to compute thickness in Python is a straightforward geopotential height (z) difference between two pressure levels, the most common thickness is the 1000-500-hPa Geopotential Height Thickness. This parameter can be calculated using the following code:

hght_500 = ds.Geopotential_height_isobaric.metpy.sel(time=plot_time,
    vertical=500 * units.hPa).metpy.quantify()

hght_1000 = ds.Geopotential_height_isobaric.metpy.sel(time=plot_time,
    vertical=1000 * units.hPa).metpy.quantify()

ds['thickness'] = (hght_500 - hght_1000).metpy.dequantify()

This code would add the 1000-500-hPa thickness to the dataset and be able to be readily plotted. The first two lines extract the 500 and 1000-hPa Geopotential heights from our Dataset and quantify them (make them unit arrays). The third line computes the thickness from the variables that we just created, then we dequantify the variable to make it back into a DataArray style for use in our Dataset.