Introduction
Some users are interested on geopotential (z) of the different model levels (ml). This Python script computes the geopotential on model levels, using as inputs NetCDF files containing geopotential and the pressure (z and lnsp) on the surface, and creating as output a NetCDF file containing the geopotential in m^2/s^2 for each level. Note that you can get the height in meters if you divide the geopotential by the gravity of Earth (9.80665 m/s^2).
- This script is intended only as a workaround for users who can not work with the ECMWF GRIB API. If you can use the GRIB API, use the script available here.
- The script requires Python and the ECMWF WebAPI to access ECMWF public datasets
The script only works correctly for ECMWF ERA-Interim data, do not use it with other datasets
Input data has to be gridded, not spectral
Get input data
For this use case, you need both temperature (t) and Specific humidity (q) for each model level. You also need both surface geopotential (z) and logarithm of surface pressure (lnsp) for model level = 1. Input files have to be synchronized using the same date, date, time, step and gridType. Afterwards the calculation of geopotential will iterate through the date/time/step parameters, allowing you to get the values for multiple times.
First get temperature (t) and Specific humidity (q), using the ECMWF Web API:
#!/usr/bin/env python from ecmwfapi import ECMWFDataServer server = ECMWFDataServer() server.retrieve({ "class": "ei", "dataset": "interim", "date": "2015-08-01", "expver": "1", "grid": "0.75/0.75", "levtype": "ml", "levelist": "all", "param": "t/q", "step": "0", "stream": "oper", "time": "0", "type": "an", "format": "netcdf" "target": "tq_ml_20150801_00.nc" })
#!/usr/bin/env python from ecmwfapi import ECMWFDataServer server = ECMWFDataServer() server.retrieve({ "class": "ei", "dataset": "interim", "date": "2016-12-01", "expver": "1", "grid": "0.75/0.75", "levelist": "1", "levtype": "ml", "param": "z/lnsp", "step": "0", "stream": "oper", "time": "0", "type": "an", "format": "netcdf" "target": "zlnsp_ml_20150801_00.nc", })
Compute geopotential on model levels
The following script was written by Mark Jackson from Cambridge Environmental Research Consultants Ltd., based on a similar script for data in GRIB format.
# Copyright 2016 Cambridge Environmental Research Consultants Ltd. # # This software is licensed under the terms of the Apache Licence Version 2.0 # which can be obtained at http://www.apache.org/licenses/LICENSE-2.0 # # ************************************************************************** # Function : compute_geopotential_on_ml_netcdf # # Author (date) : Mark Jackson (1/12/2016) # # Category : COMPUTATION # # OneLineDesc : Computes geopotential and height on model levels using netCDF files # # Description : Computes geopotential on model levels using netCDF files. # Based on the Python script by Cristian Simarro which uses GRIB files: # https://software.ecmwf.int/wiki/display/GRIB/Compute+geopotential+on+model+levels # Which was based on the code of the Metview function mvl_geopotential_on_ml: # https://software.ecmwf.int/metview/mvl_geopotential_on_ml # This in turn was based on code from Nils Wedi, the IFS documentation: # https://software.ecmwf.int/wiki/display/IFS/CY41R1+Official+IFS+Documentation # part III. Dynamics and numerical procedures # and an optimised implementation by Dominique Lucas. # Ported to Python by Cristian Simarro # # Parameters : FileA.nc - netCDF file with the levelist of t and q. Does not require all levels # but does require a contiguous set of levels all the way to the bottom. # FileB.nc - netCDF file with levelist 1 for params z and lnsp # # Return Value : outputs CSV files # with geopotential and height (relative to terrain) on each model level. # # Dependencies : netCDF4, numpy, scipy # from __future__ import print_function # make sure print behaves the same in Python 2.7 and 3.x import netCDF4 from netCDF4 import num2date import numpy as np from scipy import interpolate import datetime import sys import io import math #Variable names in the netCDF #(File A - model levels) # level = model level numbers, posssible values 1-60 # t = temperature K # q = specific humidity kg/kg #(File B) # lnsp = log surface pressure # z = surface geopotential #arguments #ONEDAY - read these from the command line FILE_A_PATH="G:\\MiscProjects\\ERA-Interim-Python\\20160922-InvestigateProfiles-netCDF\\FileA.nc" FILE_B_PATH="G:\\MiscProjects\\ERA-Interim-Python\\20160922-InvestigateProfiles-netCDF\\FileB.nc" GRID_LAT=55.875 #(degrees N) GRID_LONG=-4.5 #(degrees E) OUT_DIR_PATH="G:\\MiscProjects\\ERA-Interim-Python\\20161101-Check MarkJ geopotentials against ECMWF geopotentials\\output\\" #Routine to Read File A data file for t, q values at a particular grid point def readfa(fileapath): #Connect to data file for reading print() print("==================================================") print("File A information") print("Filename{}".format(fileapath)) fa = netCDF4.Dataset(fileapath, 'r') #Variables as netCDF variable objects print() print("--------------------------------------------------") print("Variables") print(fa.variables.keys()) # get all variable names fanclongs = fa.variables['longitude'] print(fanclongs) fanclats = fa.variables['latitude'] print(fanclats) fanclevels = fa.variables['level'] print(fanclevels) fanctimes = fa.variables['time'] print(fanctimes) fancts = fa.variables['t'] print(fancts) fancqs = fa.variables['q'] print(fancqs) #Get level values (either model levels or pressure levels) and number of levels falevels=fanclevels[:] print() print("--------------------------------------------------") print("Model levels") fanlevels=falevels.shape[0] print("There are {} levels: {} - {}".format(fanlevels, falevels[0], falevels[fanlevels-1])) #Get time values and number of times fatimes=fanctimes[:] print() print("--------------------------------------------------") print("File A Times") print(fatimes) fantimes=fatimes.shape[0] #Get python datetime for each time fapydts=num2date(fatimes, fanctimes.units) #Output first 10 datetimes print() print("First 10 times as date-time") print([pydt.strftime('%Y-%m-%d %H:%M:%S') for pydt in fapydts[:10]]) #Get lat and long values falats = fanclats[:] print() print("--------------------------------------------------") print("File A Latitudes") print(falats) falongs = fanclongs[:] print() print("--------------------------------------------------") print("File A Longitudes") print(falongs) #Get index of grid point of interest failat=np.where(falats==GRID_LAT)[0] failong=np.where(falongs==GRID_LONG)[0] print() print("==================================================") print("Grid point location: latitude and longitude indexes for lat {} and long {}".format(GRID_LAT, GRID_LONG)) print(failat) print(failong) #Get t, q values for specified grid point for all levels (slicing) #The result is still a 4D array with 1 latitude and 1 longitude print() print("==================================================") print("Get t, q values for all levels") fats=fancts[range(fantimes),range(fanlevels),failat,failong] faqs=fancqs[range(fantimes),range(fanlevels),failat,failong] return (fats, faqs, falevels, fapydts) #Routine to Read File B data file for z, lnsp values at a particular grid point def readfb(fbfilepath): #Connect to file for reading print() print("==================================================") print("fb File information") print("Filename{}".format(fbfilepath)) fbf = netCDF4.Dataset(fbfilepath, 'r') print(fbf) print() #Variables as netCDF variable objects print() print("--------------------------------------------------") print("Variables") print(fbf.variables.keys()) # get all variable names fbnclongs = fbf.variables['longitude'] print(fbnclongs) fbnclats = fbf.variables['latitude'] print(fbnclats) fbnctimes = fbf.variables['time'] print(fbnctimes) fbnczs = fbf.variables['z'] print(fbnczs) fbnclnsps = fbf.variables['lnsp'] print(fbnclnsps) #Get time values and number of times fbtimes=fbnctimes[:] print() print("--------------------------------------------------") print("fb Times") print(fbtimes) fbntimes=fbtimes.shape[0] #Get python datetime for each time fbpydts=num2date(fbtimes, fbnctimes.units) #Output first 10 datetimes print() print("fb First 10 times as date-time") print([pydt.strftime('%Y-%m-%d %H:%M:%S') for pydt in fbpydts[:10]]) #Get lat and long values fblats = fbnclats[:] print() print("--------------------------------------------------") print("fb Latitudes") print(fblats) fblongs = fbnclongs[:] print() print("--------------------------------------------------") print("fb Longitudes") print(fblongs) #Get index of grid point of interest fbilat=np.where(fblats==GRID_LAT)[0] fbilong=np.where(fblongs==GRID_LONG)[0] print() print("==================================================") print("Grid point location: fb latitude and longitude indexes for lat {} and long {}".format(GRID_LAT, GRID_LONG)) print(fbilat) print(fbilong) #Get z, lnsp values for specified grid point (slicing) #The result is still a 3D array with 1 latitude and 1 longitude print() print("==================================================") print("Get z, lnsp values ") fbzs=fbnczs[range(fbntimes),fbilat,fbilong] print('shape of z slice: %s' % repr(fbzs.shape)) fblnsps=fbnclnsps[range(fbntimes),fbilat,fbilong] return (fbzs, fblnsps, fbpydts) #Read File A file fats, faqs, falevels, fapydts = readfa(FILE_A_PATH) fanlevels=falevels.shape[0] fantimes=fapydts.shape[0] #Read File B file fbzs, fblnsps, fbpydts = readfb(FILE_B_PATH) print() print("==================================================") print("Running...") #Calculation of geopotential and height def calculategeoh(z, lnsp, ts, qs, levels): heighttoreturn=np.full(ts.shape[0], -999, np.double) geotoreturn=np.full(ts.shape[0], -999, np.double) Rd = 287.06 z_h = 0 #surface pressure sp = math.exp(lnsp) # A and B parameters to calculate pressures for model levels, # extracted from an ECMWF ERA-Interim GRIB file and then hardcoded here pv = [ 0.0000000000e+000, 2.0000000000e+001, 3.8425338745e+001, 6.3647796631e+001, 9.5636962891e+001, 1.3448330688e+002, 1.8058435059e+002, 2.3477905273e+002, 2.9849584961e+002, 3.7397192383e+002, 4.6461816406e+002, 5.7565112305e+002, 7.1321801758e+002, 8.8366040039e+002, 1.0948347168e+003, 1.3564746094e+003, 1.6806403809e+003, 2.0822739258e+003, 2.5798886719e+003, 3.1964216309e+003, 3.9602915039e+003, 4.9067070313e+003, 6.0180195313e+003, 7.3066328125e+003, 8.7650546875e+003, 1.0376125000e+004, 1.2077445313e+004, 1.3775324219e+004, 1.5379804688e+004, 1.6819472656e+004, 1.8045183594e+004, 1.9027695313e+004, 1.9755109375e+004, 2.0222203125e+004, 2.0429863281e+004, 2.0384480469e+004, 2.0097402344e+004, 1.9584328125e+004, 1.8864750000e+004, 1.7961359375e+004, 1.6899468750e+004, 1.5706449219e+004, 1.4411125000e+004, 1.3043218750e+004, 1.1632757813e+004, 1.0209500000e+004, 8.8023554688e+003, 7.4388046875e+003, 6.1443164063e+003, 4.9417773438e+003, 3.8509133301e+003, 2.8876965332e+003, 2.0637797852e+003, 1.3859125977e+003, 8.5536181641e+002, 4.6733349609e+002, 2.1039389038e+002, 6.5889236450e+001, 7.3677425385e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 0.0000000000e+000, 7.5823496445e-005, 4.6139489859e-004, 1.8151560798e-003, 5.0811171532e-003, 1.1142909527e-002, 2.0677875727e-002, 3.4121163189e-002, 5.1690407097e-002, 7.3533833027e-002, 9.9674701691e-002, 1.3002252579e-001, 1.6438430548e-001, 2.0247590542e-001, 2.4393314123e-001, 2.8832298517e-001, 3.3515489101e-001, 3.8389211893e-001, 4.3396294117e-001, 4.8477154970e-001, 5.3570991755e-001, 5.8616840839e-001, 6.3554745913e-001, 6.8326860666e-001, 7.2878581285e-001, 7.7159661055e-001, 8.1125342846e-001, 8.4737491608e-001, 8.7965691090e-001, 9.0788388252e-001, 9.3194031715e-001, 9.5182150602e-001, 9.6764522791e-001, 9.7966271639e-001, 9.8827010393e-001, 9.9401944876e-001, 9.9763011932e-001, 1.0000000000e+000 ] levelSize=60 A = pv[0:levelSize+1] B = pv[levelSize+1:] Ph_levplusone = A[levelSize] + (B[levelSize]*sp) #Get a list of level numbers in reverse order reversedlevels=np.full(levels.shape[0], -999, np.int32) for iLev in list(reversed(range(levels.shape[0]))): reversedlevels[levels.shape[0] - 1 - iLev] = levels[iLev] #Integrate up into the atmosphere from lowest level for lev in reversedlevels: #lev is the level number 1-60, we need a corresponding index into ts and qs ilevel=np.where(levels==lev)[0] t_level=ts[ilevel] q_level=qs[ilevel] #compute moist temperature t_level = t_level * (1.+0.609133*q_level) #compute the pressures (on half-levels) Ph_lev = A[lev-1] + (B[lev-1] * sp) if lev == 1: dlogP = math.log(Ph_levplusone/0.1) alpha = math.log(2) else: dlogP = math.log(Ph_levplusone/Ph_lev) dP = Ph_levplusone-Ph_lev alpha = 1. - ((Ph_lev/dP)*dlogP) TRd = t_level*Rd # z_f is the geopotential of this full level # integrate from previous (lower) half-level z_h to the full level z_f = z_h + (TRd*alpha) #Convert geopotential to height heighttoreturn[ilevel] = z_f / 9.80665 #Geopotential (add in surface geopotential) geotoreturn[ilevel] = z_f + z # z_h is the geopotential of 'half-levels' # integrate z_h to next half level z_h=z_h+(TRd*dlogP) Ph_levplusone = Ph_lev return geotoreturn, heighttoreturn #Output all the values for the specified grid point #Iterate over all the times and write out the data for itime in range(fantimes): if fapydts[itime]!=fbpydts[itime]: print("ERROR! Mismatching times in the files. Time number {}: File A = {} and File B = {}".format( itime, fapydts[itime].strftime('%Y-%m-%d %H:%M:%S'), fbpydts[itime].strftime('%Y-%m-%d %H:%M:%S'))) sys.exit() pydt=fapydts[itime] print(" Processing {}/{} : {} ...".format(itime, fantimes, pydt.strftime('%Y-%m-%d %H:%M:%S'))) z, lnsp = fbzs[itime,0,0], fblnsps[itime,0,0] sline="{}, {}, {}".format(pydt.strftime('%Y-%m-%d %H:%M:%S'), z, lnsp) #Calculate geopotentials and heights for the model levels geo, h = calculategeoh(z, lnsp, fats[itime,range(fanlevels),0,0], faqs[itime,range(fanlevels),0,0], falevels) with io.open(OUT_DIR_PATH + pydt.strftime('%Y%m%d-%H') + ".csv", 'w', newline='\r\n') as writer: sheader1 = "lat, long, geopotential, h\n" writer.write(unicode(sheader1)) #Iterate over levels for this time for ilevel in range(fanlevels): sline="{}, {}, {}, {}".format( GRID_LAT, GRID_LONG, geo[ilevel], h[ilevel]) sline+="\n" writer.write(unicode(sline)) print("Finished")
Examples
This example will compute the geopotential on the 2015-10-08 time 00 operational analysis model levels (137). Below you can see the MARS user documentation request used to retrive both files. You can set a different class/stream/type for the input data. The gribType and resolution can also be changed.
python compute_geopotential_on_ml.py tq_ml_20151008_00.grib zlnsp_ml_20151008_00.grib python compute_geopotential_on_ml.py tq_ml_20151008_00.grib zlnsp_ml_20151008_00.grib -o my_grib.grib |
tq_ml_20151008_00.grib
zlnsp_ml_20151008_00.grib