Wrong value for Variable HGT Terrain elevation

[jfqbsm]

Hi,

I would like to produce forecast for a little domain. The domain is defined as follow:

Namelist.wps
&share
 wrf_core = 'ARW',
 max_dom = 2,
 start_date = '2020-01-31_00:00:00','2020-01-31_00:00:00',
 end_date   = '2020-01-31_06:00:00','2020-01-31_06:00:00',
 interval_seconds = 10800,
 io_form_geogrid = 2,
 opt_output_from_geogrid_path = '/home/meteo/WPS/',
 debug_level = 0,
/

&geogrid
 parent_id         = 1,1,
 parent_grid_ratio = 1,5,
 i_parent_start    = 1,26,
 j_parent_start    = 1,22,
 e_we          = 65,66,
 e_sn          = 55,56,
 geog_data_res = '5m','30s',
  dx = 5600,
 dy = 5600,
 map_proj =  'lambert',
 ref_lat   = 45.609,
 ref_lon   = 6.747,
 truelat1  = 45.609,
 truelat2  = 45.609,
 stand_lon = 6.747,
 geog_data_path = '/home/meteo/WPS_GEOG',
 opt_geogrid_tbl_path = '/home/meteo/WPS/',

Every thing works fine but the output data produced seems to be inaccurate. If i have a look on the HGT variable according to the following NCL script:

begin
; Chargement du fichier de sortie WRF
a = addfile("wrfout_d02.nc","r")  ; Open a file
time = wrf_user_getvar(a,"times",-1)
ntime = dimsizes(time)
do it = 0,ntime-1,1
date = time(it)
wlat=a->XLAT(0,:,:)
wlon=a->XLONG(0,:,:)
print("date  " +date)
print("wlat  " + min(wlat) + " ; " + max(wlat))
print("wlon  " + min(wlon) + " ; " + max(wlon))
olat=45.609
olon=6.747
ref=wrf_latlon_to_ij(wlat,wlon,olat,olon)
print("Grille ref " + ref)
lonlat=wrf_user_ij_to_ll(a,48.,43., opt)
print("Lon/Lat " + lonlat)
h = wrf_user_getvar(a,"HGT", it)
hbsm = h(43,48)
print("altitude " + hbsm)
end do
end

Which give me the following result

Copyright (C) 1995-2018 - All Rights Reserved
University Corporation for Atmospheric Research
NCAR Command Language Version 6.5.0
The use of this software is governed by a License Agreement.
See http://www.ncl.ucar.edu/ for more details.
(0) date 2020-01-31_06:00:01
(0) wlat 45.1845 ; 45.7299
(0) wlon 6.06894 ; 6.99255
(0) Grille ref 42
(1) Grille ref 47
(0) Lon/Lat 6.74702
(1) Lon/Lat 45.609
(0) altitude 983.395

Unfortunatly the true elevation is 1120m . Could you tell me what is wrong and how solve this problem
Thanks in advance

 

[kwerner]

Hi,
I think the problem may be with the NCL script. I just did a test with a wrfout* file I have. If I use ncview to look at the HGT field at a certain location, I see a value of 3401.36, but when using the NCL script, I get 2747.8. According to Google Maps, the altitude at that point is 3478 m. The difference between Google Maps and the wrfout* HGT value is likely due to resolution and smoothing, but I think the difference between those, and the NCL script output is related to the NCL script. Use something like ncview to check the HGT field at that point and see if it's at least closer to what you know to be the actual value there. If so, then perhaps you should post your question to the NCL section of this forum so that someone from that team will be able to help.

 

[jfqbsm]

Hi,
Thank you for this information, i'll do that and keep you inform

 

[jfqbsm]

Hi again,
I've checked my wrfout file with ncview and for the same coordinate point i get 1061m which is nearest of the true altitude of 1120m. Due to the cell area it's acceptable. However i've checked some other points and particularly around Mont Blanc top which is part of my domain and even ncview gave me the hightest value of 3034m compared to the 4810m. The altitude range given by ncview for my domain is between 399 to 3280.
Have you any idea about this strange behaviour?

 

[jfqbsm]

Hi,
Finally i've tried to produce new netcdf files with gfs 0.25° while the first one was issued from gfs 0.5°. The altitude is the same for my point of interest but nearest of reality for highest mountain. I got a new range from 299 to 4297.

 

[kwerner]

Okay, that does sound much better. Are you content with that?

 

[jfqbsm]

Hi,
It's OK now, i'll go further in that way and check if i get good forecasts.

Thank you for your help