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The GRIB file fc_pf.grib contains the 50 perturbed forecast member of the ENS run at 7 August 00 TC (four days before the event). We will compute and visualise the mean of the ensemble members to see how the ENS predicted the event. The computations will be done in Macro.

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g=read("fc_pf.grib")

Since this Our GRIB contains several time steps we need to write a loop to (84, 90 and 96 hours, respectively)and we would like to compute the ensemble mean for each time step individuallyone separately. To achieve this we will write a loop going through the time steps. First, define the fieldset that will contain the resulting means:

e_mean=nil

We will process the fields in a loop going through Next, add this piece of code for the loop (we store the time steps in a list).:

tsLst=[84,90,96]

loop step in tsLst

    ...your code will go here ...

end loop

Within the loop, first, read all the 50 members for the given timesteptime step

f=read(data: g,
		step: step
	)	

then compute their mean with the mean() macro function

f = mean(f)

and add it to the resulting fieldset:

e_mean = e_mean & f

We finish the macro by returning the resulting fieldset:

return e_mean

By using the return statement our Macro behaves as if it were a fieldset (GRIB file). Drag it into the bottom left map and customise it with the wgust_shade Contouring icon and the title_mean Text Plotting icon. You will see that the ensemble mean hints that high wind speed can happen.

Filedset f now contains 50 fields. Next, compute the probability in as follows:

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Next, we define the threshold for wind gust (in m/s units). We will store it in a variable:

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tsLst=[84,90,96]

loop step in tsLst

    ...your code will go here ...

end loop

Within the loop, simply read all the members for the given timestepNext, compute the probability in as follows:

f=read(data: g,
		step: step
	)	

then compute their mean with the mean() macro function

f = f > val	
f=100*mean(f)

Here the first line turned set ech gridpoint hiegher value than val 1 and all the others zero, Last, add it to the resulting fieldset:

e_meanprob = e_meanprob & f

We finish the macro by returning the resulting fieldset:

return e_meanprob

By using the return statement our Macro behaves as if it were a fieldset (GRIB file). Drag it into the bottom left map and customise it with the wgustprob_shade Contouring icon and the title_mean prob Text Plotting icon. You will see that the ensemble mean hints that high wind speed can happen.

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