我需要在netCDF CF文件中使用xarray的加权纬度计算全球平均值，然后将其转换为Pandas

Question

我需要从netCDF cf data 3D（time，lat，lon）文件计算一个全局时间序列（time），然后将其转换为pandas/dataframe。我需要用cos（lat）来称量纬度。我一直在使用numpy进行平均，但是将pandas/dataframe转换为加权数组不起作用

ds=xr.open_dataset('sample_data.nc')
data=ds.tas
start_time='1980-01-01'
end_time='2018-12-31'
time_slice = slice(start_time, end_time)
nrows=len(data.lat.values)
ncols=len(data.lon.values)
t=len(data.time.values)
weights=np.zeros([len(data.lat.values)])
latsr = np.deg2rad(data.lat.values).reshape((nrows,1))
weight_matrix=np.repeat(np.cos(latsr),ncols,axis=1)
wghtpr=np.zeros_like(data)
for i in range (0,t):
    wghtpr[i,:,:]=data[i,:,:]*weight_matrix
new_data=wghtpr
wtdata=np.average(new_data,axis=1)
da=np.average(wtdata,axis=1)

这将导致一个没有“名称”的numpy数组

如果我做ds，我会得到：

<xarray.Dataset>
Dimensions:    (bnds: 2, lat: 361, lon: 576, time: 477)
Coordinates:
  * time       (time) datetime64[ns] 1980-01-16T12:00:00 ... 2019-09-16
  * lat        (lat) float64 -90.0 -89.5 -89.0 -88.5 ... 88.5 89.0 89.5 90.0
  * lon        (lon) float64 0.0 0.625 1.25 1.875 ... 357.5 358.1 358.8 359.4
    height     float64 ...
Dimensions without coordinates: bnds
Data variables:
    time_bnds  (time, bnds) datetime64[ns] ...
    lat_bnds   (lat, bnds) float64 ...
    lon_bnds   (lon, bnds) float64 ...
    tas        (time, lat, lon) float32 244.15399 244.15399 ... 267.52875
Attributes:
    institution:     Global Modeling and Assimilation Office, NASA Goddard Sp...
    institute_id:    NASA-GMAO
    experiment_id:   MERRA-2
    source:          MERRA-2 Monthly tavgM_2d_slv_Nx
    model_id:        GEOS-5
    references:      http://gmao.gsfc.nasa.gov/research/merra/, http://gmao.g...
    tracking_id:     e77fd4de-19c2-45ad-afe2-ce3f6c1eb148
    mip_specs:       CMIP5
    source_id:       MERRA-2
    product:         reanalysis
    frequency:       mon
    creation_date:   2015-10-11T23:12:34Z
    history:         2015-10-11T23:12:34Z CMOR rewrote data to comply with CF...
    Conventions:     CF-1.4
    project_id:      CREATE-IP
    table_id:        Table Amon_ana (10 March 2011) c3ffdce87438d8df0839620ee...
    title:           Reanalysis output prepared for CREATE-IP.
    modeling_realm:  atmos
    cmor_version:    2.9.1
    doi:             http://dx.doi.org/10.5067/AP1B0BA5PD2K
    contact:         MERRA-2, Steven Pawson (steven.pawson-1@nasa.gov)
#

Answer 1

另一种选择是使用CDO。要获得全局平均值，您只需执行以下操作：

cdo fldmean 'sample_data.nc' out.nc

如果在Linux上，您还可以使用我的Python包nctoolkit，它使用CDO作为后端（https://nctoolkit.readthedocs.io/en/latest/installing.html）。计算全球平均值，然后将其转换为熊猫，需要：

import nctoolkit as nc
data = nc.open_data("sample_data.nc")
data.spatial_mean()
pd_ts = data.to_dataframe()

绘制时间序列需要：

data.plot()

Answer 2

要利用xarray的广播和对齐，可以按如下方式进行加权：

ds=xr.open_dataset('sample_data.nc')
data=ds.tas

#start_time='1980-01-01'
#end_time='2018-12-31'
#time_slice = slice(start_time, end_time)
#nrows=len(data.lat.values)
#ncols=len(data.lon.values)
#t=len(data.time.values)

latsr = xr.ufunc.deg2rad(data.lat)
weights = xr.ufunc.cos(latsr)
weighted = data * weights # broadcasting here
weighted_mean = weighted.mean(['lat','lon'])

# to pandas
df = weighted_mean.to_dataframe()

希望这有帮助