MAGIC HIRLAM Meteorological Model Archive

The MAGIC HIRLAM Model Archive is currently accessible to members of the MAGIC consortium and contains time series of

• Zenith tropospheric delay and integrated water vapor

• the GPS antenna height
• the elevation of the model orography.

BELL

 Data derived from HIRLAM model analyses and forecasts

    41.59960     1.40110     0.80357

1999   5   1   2

   0.50063   0.80357

 720

 

   0   0   0   0   -9.9 2172.0   82.1   12.9  920.1   -9.9 -9  -9.90  -9.90  -9.90  -9.90 2261.6   95.5   15.1  953.7  286.1  95

   0   2   0   0   -9.9 2171.7   82.1   12.9  920.0   -9.9 -9  -9.90  -9.90  -9.90  -9.90 2261.4   95.6   15.1  953.5  286.1  95

   0   4   0   0   -9.9 2171.5   82.2   12.9  919.8   -9.9 -9  -9.90  -9.90  -9.90  -9.90 2261.1   95.6   15.2  953.4  286.0  96

   0   6   0   0   -9.9 2171.2   82.2   12.9  919.7   -9.9 -9  -9.90  -9.90  -9.90  -9.90 2260.8   95.6   15.2  953.3  285.9  96

   0   8   0   0   -9.9 2170.9   82.2   12.9  919.6   -9.9 -9  -9.90  -9.90  -9.90  -9.90 2260.5   95.6   15.2  953.1  285.8  96

   0  10   0   0   -9.9 2170.6   82.2   12.9  919.4   -9.9 -9  -9.90  -9.90  -9.90  -9.90 2260.2   95.7   15.2  953.0  285.7  97

   0  12   0   0   -9.9 2170.3   82.2   12.9  919.3   -9.9 -9  -9.90  -9.90  -9.90  -9.90 2260.0   95.7   15.2  952.9  285.6  97

Access

HIRLAM ZTD-IWV Data Specification

Doc. No. D05220, Version 1.2, Date: 2 August 1999
Prepared by Henrik VEDEL, DMI, magic@dmi.dk

Purpose:

The Danish Meteorological Institute (DMI) are providing estimates of the zenith total delay, the integrated water vapor and other
quantities for validation of GPS derived estimates of similar quantities. The estimates are derived using data extracted from runs of the numerical weather prediction model HIRLAM.

Model:

We run DMI's version of the HIRLAM model at a resolution of 0.3 degree and with 31 vertical levels for a region defined by
 -33.0 <= longitude <= 39.3 degrees,
  24.0 <= latitude  <= 55.5 degrees.

Analyses (assimilation of new observational data) are made every six hours (00, 06, 12, 18). The maximum forecast length for the data provided are six hours. The boundary fields come from ECMWF. Except for the region and the necessary additions to extract the data for the MAGIC project, the model we run is similar to the operational model being used at DMI.  Further information on the Research activities of the DMI are available at  http://www.dmi.dk/eng/f+u/index.html

Model exceptions:

For the following dates our tape archive was missing the ECMWF boundary files, and the model was instead run with boundary data valid at the proper time from the boundary forecast of the day before:
19990510, 19990524, 19990604, 19990704, 19990705.
 

GPS stations covered:

We extract data for the locations of all GPS stations  participating in the MAGIC project and known by us. Known means that their coordinates, in terms of longitude, latitude, and altitude, are specified on the official MAGIC list of GPS stations or have been given to us directly. Please, send a mail to magic@dmi.dk if a station is missing or the coordinates we use improper. If you mail coordinates, notice  that we need the coordinates as latitude, longitude and height above the geoid.

Data provided:

For each location we provide the surface pressure, the zenith total delay (ZTD), the zenith wet delay (ZWD), the integrated water
vapor (IWV), both at the HIRLAM surface and at the altitude of the GPS antenna. Furthermore the 2 meter temperature and the 2 meter humidity are
provided for the HIRLAM orography. The quantities are calculated for each time-step in the model, currently every two minutes. Notice that for
a period following each analysis there may be a visible odd/even effect on some properties due to the leap-frog integration scheme used in the HIRLAM model. If that is a concern, remove it by filtering.

Derivation of the data provided:

The properties of the atmosphere of the region at a given time is determined by running the HIRLAM model.

The properties of the atmosphere above each GPS site are calculated firstly by a horizontal interpolation within the HIRLAM grid. From this vertical profile the estimates of surface pressure, ZTD, ZWD, and IWV at the HIRLAM surface are derived, as are T2m and RH, the two meter temperature and relative humidity.

Secondly a vertical transformation is applied to the atmospheric profile at the  GPS site to allow for the offset in altitude between the HIRLAM surface and the real surface (here = the specified altitude of the GPS antenna). One should notice that there exist no physically correct way of making this vertical transformation, only educated attempts based on various reasonable assumptions. After all one is trying to go beyond the resolution of the model. Here we adopt a method described by Majewski (Beitr. Phys. Atmosph., 1985, Vol. 58, p. 147), a method which is widely
used for mapping meteorological data from one model grid to another.

After the vertical shift of the atmospheric profile estimates of pressure, ZTD, ZWD, and IWV are found at the GPS antenna level.

Before doing the numerical integrations the integrals are recast in terms of the variables of the HIRLAM model. The change of variables
is done assuming hydrostatic equilibrium. The HIRLAM model itself is based on that assumption too, as are other large scale numerical weather
prediction models. As long as the hydrostatic assumption and the usual equation of state are valid, the forms used are identical to the integrals providing ZTD and ZWD as they are specified in the "MAGIC technical annexe version 2.0", and the IWV estimate provided is similar to the integral of density_of_water_vapor*dz, dz being the increment in altitude.

ZTD is calculated as a sum of two integrals:

 ZTD_hydrostatic = integral_of R_gas/g*k1*dp
 ZWD    = integral_of R_gas/(g*epsilon)*q*((k2-k1*epsilon)+k3/T)*dp,
 ZTD = ZTD_hydrostatic+ZWD

with q being the specific humidity (density of water vapor relative to total density of air), T the temperature, dp the pressure increment,
epsilon is the ratio of the molecular weight of water vapor relative to the mean molecular weight of dry air (approximately 0.622), R_gas is the
gas constant for dry air (we use R_gas=287.04 J/(kg*K) ), g is the net gravitational acceleration at the surface, assumed constant through
the part of the atmosphere modeled by us (we use g=9.80665m/s^2).

The IWV is calculated as an integral over,
    q*dp/g.

For the constants we use:
 k_1= 7.76 *10^-7 K/Pa
 k_2= 7.04 *10^-7 K/Pa
 k_3= 3.739*10^-3 K/Pa
(Bevis at al, 1994, Jour. Apl. Met., vol 33, p379)

Format of data provided.

We use the 'CLIMAP FORMAT' with some necessary additions. The structure of the header is similar to the CLIMAP format. In the body
part we have added some extra items at the end of the lines relative to the CLIMAP format. A file reader able to read CLIMAP formatted files
should thus be able to read our files, BUT NOT the extra data. To read those you'll have to change your file reader a bit.

For each station and each day there will be a single file containing the data. The file is an ascii file. Each file will contain a header and a body. The
header will have the following format:

content   variables   units  format (fortran)
-------------------------------------------------------
station id  name    -  (A4)
GPS equipment  (redundant)   -  (A60)
position  latitude, longitude, altitude deg, deg, km (3f12.5)
time, time increment    year month day tt  yr,mth,day,min (4I4)
altitudes  hirlam_alt, gps_alt  km, km  (2F10.5)
# data points  # records (lines) in body part of file  (I4)
empty line  -    -  (60X)

The year will contain four significant digits.

Each line of the body will have the following content:
------------------------------------------------------
hour, min, sec, msec, ztd_stdv_a,
ztd_a, zwd_a, iwv_a, ps_a, T_a, RH_a,
grad_n/s_a, grad_e/w_a, grad_n/s_stdv_a, grad_e/w_stdv_a,
ztd_h, zwd_h, iwv_h, ps_h, T2m_h, RH_h

written with a fortran format statement like:
format(4i4,6f7.1,i3,4f7.2,5f7.1,i4)

where subscript _a means at antenna level, _h at hirlam surface.

hour, min, sec, msec = the time of the day, measured in hours, minutes, seconds, and milliseconds.
ztd= zenith total delay   in mm
zwd= zenith wet delay,   in mm
iwv= integrated water vapor,   in mm
ps= surface pressure,   in hPa
T2m= temperature (2 meter above ground),  in K
RH= relative humidity 2m above ground,  in %

For the variables ztd_stdv_a (formal error in ztd_a), T_a (temperature used for IWV derivation), RH_a (relative humidity used for IWV derivation), grad_xxx (gradients and formal errors in gradients) we have no data. That is indicated in the files by the missing data indicators -9.9 for reals and -9 for integers.

The names of the files will be yyyymmdd_cccc, where yyyymmdd specifies year, month, and day, and cccc is the station id. For example data for station BELL on 1999 May 1 will be in 19990501_BELL.

The files will be stored in two types of archives, combining them either time wise or station wise.

The time archives will contain data from all stations for one day, each file in the archive being of the type specified above. The archives will be stored as magic/hirl/ztd/recent/yyyydd/yyyymmdd.tar.gz on the ftp server. The time archives will remain on the DMI server for at least 30 days.

The station archives will contain gzipped files of the above type for (up to) one month. They will be stored on the server as magic/hirl/ztd/yyyymm/yyyymm_station.tar

Both types of archives will be updated daily, with each daily run extracting data for "yesterday". The starting date for the extractions is 1999-05-01.

List of GPS stations covered:

A separate file, named ztd.list, will contain a list of all the GPS stations for which we have made available this type of data. The list
will contain for each station, the name of the station, the coordinates used, and the altitude of the MAGIC HIRLAM model at the same location.
The file will be placed as magic/hirl/ztd/ztd.list
 

Access to data via the DMI server:

The data data will be placed on the ftp-server at DMI. At least once a month the data will be copied to the server at ACRI.

To connect to the DMI ftp-server, do a ftp to ftp.dmi.dk, specify magic as userid and the usual MAGIC password as password. You'll be located
at the top MAGIC directory on the server upon being logged in. The ztd data will be in ./hirl/ztd/ and its subdirectories.

The data archives will be copied to the ACRI ftp-server at least once a month. The directory  structure will be similar to the structure at DMI
and the data are foreseen to stay online at ACRI for the duration of the MAGIC project.
 

About HIRLAM model results versus radiosonde reports:

For people not familiar with meteorology it is important to notice the difference between the two types of validation data provided by
the DMI.

The values of ZTD (etc.) produced from the HIRLAM model fields are NOT measurements. They are numerical weather prediction model estimates of ZTD (etc.) at the locations in question, and thus subject to the constraints set by the finite resolution and the quality of the
HIRLAM model itself.  Humidity is a variable in the model and it is assimilated (from e.g. radiosonde reports) along with other meteorological variables at each analysis step, which for these simulations takes place for every six hours.

Contrary to this the radiosonde reports provided are real measurements,  that do not depend on a meteorological model. (Except for the geo potential heights, which are normally derived assuming a hydrostatic atmosphere. That is done  by the  vendor of the rs data, not by the DMI).
 

Questions and comments:

In case of questions or comments feel free to contact the magic group at DMI. This is most conveniently done via email to magic@dmi.dk

Future Versions of the Archive

• Zenith tropospheric delay and integrated water vapor