Create a client side method to reduce complex mixed interval variables

[lesserwhirls]

Complex mixed interval variables are a pain. This issue shows up in GRIB. For an example, check out the variable Total_precipitation_surface_Mixed_intervals_Accumulation as found in the GFS Global 0.25 degree GRIB collection.

The valid forecast time associated with this variable has values of:

3, 6, 9, 12... hours since the model runtime.

Looks like three hour accumulations of precipitation, yes? Well, no, that would be too easy. This is a mixed interval accumulation, so one must look at the time bounds associated with each valid forecast time. What do those bounds look like?

valid forecast time	interval start	interval end
3	0	3
6	0	6
9	6	9
12	6	12

Which translates into the following:

Total accumulated precipitation over the past three hours valid for a forecast time three hours into the model run.

Total accumulated precipitation over the past six hours valid for a forecast time six hours into the model run.

Total accumulated precipitation over the past three hours valid for a forecast time nine hours into the model run.

Total accumulated precipitation over the past six hours valid for a forecast time twelve hours into the model run.

Wait, what? See the issue? [hint: where is total accumulated precipitation over the past three hours valid for a forecast time six hours into the model run?]

In order to get the missing 3 hour accumulations, one must do some subtraction - no big deal, and GEMPAK does this behind the scenes. In the example above, we would need to do:

T_{v:6, I:3} = T_{v:6, I:6} - T_{v:3, I:3}

where v is the valid forecast time, and I defines the interval width in time.

The proposal is to create a client side method to reduce these "complex mixed intervals" into something that is more simple (i.e. all three hour intervals instead of a mixture of three and six hour intervals).

[lesserwhirls]

Summary of discussion from 2015/06/11 THREDDS group meeting:

1. Don’t allow subsetting before reducing complex mixed interval on first pass of writing code.
2. Add a GridHelper class with a reduceComplexMixedInterval method that takes a grid and returns a grid.