def _do_preprocessing(self, rawfile, varname, s_start_time, s_stop_time, interval='monthly', force_calc=False, valid_mask='global', target_grid='t63grid'):
        """
        perform preprocessing
        * selection of variable
        * temporal subsetting
        """
        cdo = Cdo()

        if not os.path.exists(rawfile):
            print('File not existing! %s ' % rawfile)
            return None, None

        # calculate monthly means
        file_monthly = get_temporary_directory() + os.sep + os.path.basename(rawfile[:-3]) + '_' + varname + '_' + s_start_time + '_' + s_stop_time + '_mm.nc'
        if (force_calc) or (not os.path.exists(file_monthly)):
            cdo.monmean(options='-f nc', output=file_monthly, input='-seldate,' + s_start_time + ',' + s_stop_time + ' ' + '-selvar,' + varname + ' ' + rawfile, force=force_calc)
        else:
            pass
        if not os.path.exists(file_monthly):
            raise ValueError('Monthly preprocessing did not work! %s ' % file_monthly)

        # calculate monthly or seasonal climatology
        if interval == 'monthly':
            mdata_clim_file = file_monthly[:-3] + '_ymonmean.nc'
            mdata_sum_file = file_monthly[:-3] + '_ymonsum.nc'
            mdata_N_file = file_monthly[:-3] + '_ymonN.nc'
            mdata_clim_std_file = file_monthly[:-3] + '_ymonstd.nc'
            cdo.ymonmean(options='-f nc -b 32', output=mdata_clim_file, input=file_monthly, force=force_calc)
            cdo.ymonsum(options='-f nc -b 32', output=mdata_sum_file, input=file_monthly, force=force_calc)
            cdo.ymonstd(options='-f nc -b 32', output=mdata_clim_std_file, input=file_monthly, force=force_calc)
            cdo.div(options='-f nc', output=mdata_N_file, input=mdata_sum_file + ' ' + mdata_clim_file, force=force_calc)  # number of samples
        elif interval == 'season':
            mdata_clim_file = file_monthly[:-3] + '_yseasmean.nc'
            mdata_sum_file = file_monthly[:-3] + '_yseassum.nc'
            mdata_N_file = file_monthly[:-3] + '_yseasN.nc'
            mdata_clim_std_file = file_monthly[:-3] + '_yseasstd.nc'
            cdo.yseasmean(options='-f nc -b 32', output=mdata_clim_file, input=file_monthly, force=force_calc)
            cdo.yseassum(options='-f nc -b 32', output=mdata_sum_file, input=file_monthly, force=force_calc)
            cdo.yseasstd(options='-f nc -b 32', output=mdata_clim_std_file, input=file_monthly, force=force_calc)
            cdo.div(options='-f nc -b 32', output=mdata_N_file, input=mdata_sum_file + ' ' + mdata_clim_file, force=force_calc)  # number of samples
        else:
            raise ValueError('Unknown temporal interval. Can not perform preprocessing!')

        if not os.path.exists(mdata_clim_file):
            return None

        # read data
        if interval == 'monthly':
            thetime_cylce = 12
        elif interval == 'season':
            thetime_cylce = 4
        else:
            print interval
            raise ValueError('Unsupported interval!')

        mdata = Data(mdata_clim_file, varname, read=True, label=self.name, shift_lon=False, time_cycle=thetime_cylce, lat_name='lat', lon_name='lon')
        mdata_std = Data(mdata_clim_std_file, varname, read=True, label=self.name + ' std', unit='-', shift_lon=False, time_cycle=thetime_cylce, lat_name='lat', lon_name='lon')
        mdata.std = mdata_std.data.copy()
        del mdata_std
        mdata_N = Data(mdata_N_file, varname, read=True, label=self.name + ' std', shift_lon=False, lat_name='lat', lon_name='lon')
        mdata.n = mdata_N.data.copy()
        del mdata_N

        # ensure that climatology always starts with January, therefore set date and then sort
        mdata.adjust_time(year=1700, day=15)  # set arbitrary time for climatology
        mdata.timsort()

        #4) read monthly data
        mdata_all = Data(file_monthly, varname, read=True, label=self.name, shift_lon=False, time_cycle=12, lat_name='lat', lon_name='lon')
        mdata_all.adjust_time(day=15)

        #mask_antarctica masks everything below 60 degree S.
        #here we only mask Antarctica, if only LAND points shall be used
        if valid_mask == 'land':
            mask_antarctica = True
        elif valid_mask == 'ocean':
            mask_antarctica = False
        else:
            mask_antarctica = False

        if target_grid == 't63grid':
            mdata._apply_mask(get_T63_landseamask(False, area=valid_mask, mask_antarctica=mask_antarctica))
            mdata_all._apply_mask(get_T63_landseamask(False, area=valid_mask, mask_antarctica=mask_antarctica))
        else:
            tmpmsk = get_generic_landseamask(False, area=valid_mask, target_grid=target_grid, mask_antarctica=mask_antarctica)
            mdata._apply_mask(tmpmsk)
            mdata_all._apply_mask(tmpmsk)
            del tmpmsk

        mdata_mean = mdata_all.fldmean()

        # return data as a tuple list
        retval = (mdata_all.time, mdata_mean, mdata_all)

        del mdata_all
        return mdata, retval