I am trying to download weather forecast data using the Siphon library (example below). When I specify multiple variables to download, it downloads just some of them (print at the end shows just two of the requested). In the case below, it downloads data only for u-component_of_wind_isobaric and u-component_of_wind_isobaric, ignoring all others. If I comment out those two, it will download the first two. It seems to be limited to either 2 or 3 variables. Is it due to some API restrictions, or do I understand something wrong? I know I could make multiple requests, but it looks strange since it seems to be working in the past.

from datetime import datetime, timedelta

from siphon.catalog import TDSCatalog

variables = [
    'Temperature_surface',
    'Wind_speed_gust_surface',
    'u-component_of_wind_isobaric',
    'v-component_of_wind_isobaric',
    'Total_cloud_cover_entire_atmosphere_Mixed_intervals_Average',
]

best_gfs = TDSCatalog(
    '/'
    'Global_0p5deg/catalog.xml?dataset=grib/NCEP/GFS/Global_0p5deg/Best'
)
print(best_gfs.datasets)

ncss = best_gfs.datasets[0].subset()
query = ncss.query()

start, end = datetime.utcnow(), datetime.utcnow() + timedelta(days=1)
query.lonlat_point(17.918611, 59.651944).vertical_level(100000).time_range(start, end)

query.variables(*variables)
query.accept("netcdf4")

data = ncss.get_data(query)
print(list(data.variables))

Result: ['latitude', 'longitude', 'stationAltitude', 'station_id', 'station_description', 'u-component_of_wind_isobaric', 'v-component_of_wind_isobaric', 'time', 'stationIndex']

As seen most of requested variables missing in the result.

I am trying to download weather forecast data using the Siphon library (example below). When I specify multiple variables to download, it downloads just some of them (print at the end shows just two of the requested). In the case below, it downloads data only for u-component_of_wind_isobaric and u-component_of_wind_isobaric, ignoring all others. If I comment out those two, it will download the first two. It seems to be limited to either 2 or 3 variables. Is it due to some API restrictions, or do I understand something wrong? I know I could make multiple requests, but it looks strange since it seems to be working in the past.

from datetime import datetime, timedelta

from siphon.catalog import TDSCatalog

variables = [
    'Temperature_surface',
    'Wind_speed_gust_surface',
    'u-component_of_wind_isobaric',
    'v-component_of_wind_isobaric',
    'Total_cloud_cover_entire_atmosphere_Mixed_intervals_Average',
]

best_gfs = TDSCatalog(
    'http://thredds.ucar.edu/thredds/catalog/grib/NCEP/GFS/'
    'Global_0p5deg/catalog.xml?dataset=grib/NCEP/GFS/Global_0p5deg/Best'
)
print(best_gfs.datasets)

ncss = best_gfs.datasets[0].subset()
query = ncss.query()

start, end = datetime.utcnow(), datetime.utcnow() + timedelta(days=1)
query.lonlat_point(17.918611, 59.651944).vertical_level(100000).time_range(start, end)

query.variables(*variables)
query.accept("netcdf4")

data = ncss.get_data(query)
print(list(data.variables))

Result: ['latitude', 'longitude', 'stationAltitude', 'station_id', 'station_description', 'u-component_of_wind_isobaric', 'v-component_of_wind_isobaric', 'time', 'stationIndex']

As seen most of requested variables missing in the result.

• python-siphon

1 Answer 1

A limitation of the NCSS service you're trying to use is that it can only return variables on a single coordinate system (e.g. combination of horizontal coords + vertical coord + time). The first two variables use one vertical coordinate, the second two use a different one, and last variable yet another vertical coordinate. You'll need to separate this into 3 different requests.

You can see the variables grouped by coordinate system on the web form for NCSS for that dataset.