atmqty Manual: AtmQty Object Instantiation

Minimally, an AtmQty object requires 3 positional keywords to be instantiated. Thus, the following line will instantiate an AtmQty object:

>>> x = atmqty.AtmQty(lat, lon, lev)

This object is in pressure coordinates in the vertical (lev_type = "pressure") and has a missing value (missing) value of 1e+20. Latitude, longitude, and levels are each Numeric vectors, lat, lon, and lev, respectively. The lat and lon vectors must be monotonically increasing with increasing vector index. The lev vector must be either monotonically increasing or monotonically decreasing with increasing vector index. The values of lev_type and missing are set to their defaults because they were not given in the keyword argument list; of course, we can set them explicitly in the calling line:

>>> x = atmqty.AtmQty( lat, lon, lev, lev_type="pressure" \
...                  , missing=1e+20 )

All domain-describing attributes must not change for the life of the object. In the case above, this means lat, lon, lev, lev_type and missing are fixed. In fact, for all pressure coordinate domains, these 5 attributes, and only these 5, must be fixed.

If the levels are isentropic, besides the 5 attributes held fixed for pressure coordinate domains, the p0 attribute is also set on object instantiation and remains fixed for the life of the instance. p0 can be passed via the argument list:

>>> x = atmqty.AtmQty( lat, lon, lev, lev_type="isentropic" \
...                  , p0=1000.0, missing=1e+20 )

or, if not explicitly included, will be set to the default value of 1000. These 6, and only these 6, attributes must be fixed for the life of an isentropic levels instance.

To reiterate, this means that p0 is not defined upon the instantiation of a pressure level domain instance. That attribute only comes into existence when potential temperature is calculated and can be changed whenever potential temperature is recalculated to a different reference pressure. When the domain is on isentropic levels, however, p0 is defined on instantiation and does not change for the life of the instance.

The summary list of attributes provides a quick reference sheet as to what attributes are fixed and what values attributes take on object instantiation.