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Best practise to iterate over all hindcastDates of several hindcastYears for CMA

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Table of Contents

 

What is the objective of this page?

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titleThe objective:

To help users to improve S2S CMA MARS requests performance via the WebAPI.

(lightbulb) A good understanding of the MARS efficiency issues is essential especially for users that are interested in downloading large amounts of data.

How the S2S data is organised in MARS?

...

In general it is organised, as a huge tree, with the indentation below, showing different levels down that tree:

  • centre (CMA, ECMWF, NCEP, JMA, ...)
    • realtime or reforecast
      •  type of data (control forecast or perturbed forecast)
        • type of level (single level or pressure level or potential  temperature)
          • model version date (2014-05-01 or ...)
            • hindcast dates (2014-01-01 or 2014-01-02 or 2014-01-03, ...)
              •  time-steps
                • members (for perturbed forecast)
                  • levels (for pl or pt)
                    • parameters

...

What would be the natural way to group requests?

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Following the previous paragraph,  the natural way to group requests would be:
all parameters, all levels, all members, all time-steps for 1 hindcast date.

(warning) Note the following:

  1. 'all' means 'all' that the user wants. It doesn't have to be all parameters.
  2. If a user is interested only on z500,  he may request more hindcast dates in one go, since the overall request will not be so big.

What is the best approach to loop over several HindcastDates  for a CMA request?

...

titleThe main idea in brief:

...

  • The best approach is to iterate over the

...

  • hindcastYears.
  • For each

...

  • hindcastYear iterate over all

...

  • the available hindcastMonths and for each

...

  • hindcastMonth iterate over all

...

  • the available hindcastDays.

(lightbulb)At this point you may wish to check CMA availability and to view a CMA request

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for hindcastYear in hindcastYears
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titleThe main idea in brief:
for HindcastYear in HindcastYears
    for HindcastMonthhindcastMonth in HindcastMonthshindcastMonths        
        for HindcastDayhindcastDay in HindcastDayshindcastDays             
            HindcastDatehindcastDate = HindcastYearhindcastYear-HindcastMonthhindcastMonth-HindcastDayhindcastDay               
                S2S-request(HindcastDate)

An example to request Control forecast, sfc, for HindcastYears 2010 to 2014 for 2 HindcastMonths  (eg April and June)

hindcastDate)

Web-API examples:

A CMA reforecast request for all the available hindcastDates

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  • The objective of this example is to demonstrate how to iterate efficiently over all the available hindcastYears, hindcastMonths and hindcastDays for a CMA reforecast request
  • It can be used as a starting point, however you need to keep in mind that you have to adapt it to your needseg to set the keyword values according to your requirements ("param", "levtype", "step" etc).
  • In this way you can extend this request to download the whole S2S CMA reforecast.

Please note:

  • you can set the variable "target"  to write each hindcastDate on a separate file (lightbulb)
  • taking under consideration your request's size (eg Nr of fields and volume)  you can merge several hindcastDates on the same "target" (smile)
Code Block
languagepy
#!/usr/bin/env python
import calendar
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()

origin = "babj"
modelVersionDate = "2014-05-01"

def retrieve_cma_reforecast():
    """
Info
titleThe main idea in brief:
for HindcastYear from 2010 to 2014    
    for HindcastMonth in 04, 06      
  
     A function to fordemonstrate HindcastDayhow into HindcastDaysiterate efficiently over all hindcastYears, hindcastMonths etc     
  
     for a particular cma_reforecast_request.    HindcastDate = HindcastYear-HindcastMonth-HindcastDay
       Change the variables below to  
  adapt the iteration to your needs 
    """
    hindcastYearStart = 1994
    S2S-request(HindcastDate) (see the web API request example below)

A  simple web API example, requesting Control forecast, sfc for one hdate

Code Block
languagepy
#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()
server.retrieve({
    "class": "s2",
    "dataset": "s2s",
    "date": ModelVersionDate, (ie "2014-05-01"),
    "expver": "prod",
    "hdate": HindcastDate, (ie the selected HindcastDate eg "2014-04-01"),
    "levtype": "sfc",
    "origin": "babj",
    "param": "165",
    "step": "0",
    "stream": "enfh",
    "target": "CHANGEME",
    "time": "00",
    "type": "cf",
})

(lightbulb) If the request is "small" you may request more HindcastDates in one go.

 (info) Do you need more CMA re-forecast examples?

A web API example requesting data for several hindcastDates (iterating over several hindcastYears, hindcastMonths and hindcastDays)

(lightbulb) Let's see how we can change the script above to iterate over several hindcastYears, hindcastMonths and hindcastDays efficiently

(grey lightbulb) Note that each hindcastDate is written into a separate file

...

Code Block
languagepy
#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()
def retrieve_data(hindcastDate):
    target = "target_s2s_%s.grb" % hindcastDatehindcastYearEnd = 2014
    hindcastMonthStart = 1
    hindcastMonthEnd = 12
    
    for hindcastYear in list(range(hindcastYearStart, hindcastYearEnd + 1)):
        for hindcastMonth in list(range(hindcastMonthStart, hindcastMonthEnd + 1)):
            numberOfDays = calendar.monthrange(hindcastYear, hindcastMonth)[1]
            for hindcastDay in list(range(numberOfDays)):
                hindcastDate = '%04d%02d%02d' % (
                    hindcastYear, hindcastMonth, hindcastDay + 1)
                target = "%s%s.grb" % (origin, hindcastDate)
                cma_reforecast_request(hindcastDate, target)

def cma_reforecast_request(hindcastDate, target):
    """       
        A CMA reforecast request. Change the keywords below to adapt it to your needs.   
    """
    
    server.retrieve({
        "class": "s2",
        "dataset": "s2s",
        "date": "2014-05-01"modelVersionDate,
        "expver": "prod",
        "hdate": hindcastDate,
        "levtype": "sfc",
        "origin": "babj"origin,
        "param": "165/166",
        "step": "024/to/1440/by/24",
        "stream": "enfh",
        "target": target,
        "time": "00",
        "typenumber": "cf",
})

for hindcastYear in ["2012", "2013"]:1/2/3",
    for hindcastMonth in ["08", "09type"]:
        for hindcastDay in ["01", "02"]:"pf",
    })

if        hindcastDate = hindcastYear+hindcastMonth+hindcastDay
        __name__ == '__main__':
    retrieve_cma_datareforecast(hindcastDate))


Useful links

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