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The GRAPES global four-dimensional variational data assimilation system (GRAPES_GFS 4DVAR) adopts the incremental analysis update scheme (Zhang et al., 2019). Compared with the three-dimensional variational data assimilation system (3DVAR), the amount of observational data that can be effectively ingested in the 4DVAR system is increased by 50%; consequently, the analysis and forecast error amplitude significantly decreases (Zhang et al., 2019). The assimilation window of GRAPES_GFS 4DVAR is six hours, and the observational time slot is 30 minutes, which means the GRAPES_GFS 4DVAR can assimilate the observational data with high temporal resolution. The assimilated observational data in this study include the conventional observational data and the satellite observational data, as summarized in Table 1. The forecast model used in this study is GRAPES_GFS V3.0, which is currently operated at the China Meteorological Administration with a horizontal resolution of 0.25° × 0.25° with 87 vertical layers. The main physical parameterization schemes in GRAPES_GFS V3.0 are shown in Table 2. The long- and short-wave radiation scheme is generated by the Rapid Radiative Transfer Model (RRTMG) (Morcrette et al., 2008). The land surface scheme used is the Common Land Model (CoLM) (Dai et al., 2003). The planetary boundary layer scheme used is the Medium-Range Forecast (MRF) (Hong and Pan, 1996). The deep and shallow cumulus convection parameterization scheme is given by the New Simplified Arakawa–Schubert (NSAS) subroutine (Arakawa and Schubert, 1974; Pan and Wu, 1995; Liu et al., 2015). The cloud physics schemes include a explicitly prognostic cloud cover scheme (Ma et al., 2018) and the CMA double moment microphysics scheme, the macrophysics cloud condensation scheme, the impacts of detrainment of deep/shallow convection on grid-scale clouds to represent the processes of formation and extinction for all hydrometeros. (Tan et al., 2013; Jiang et al., 2015; Chen et al., 2021). In the incremental 4DVAR, the numbers of vertical layers of inner and outer loops and the horizontal resolution of the outer loop are the same as those in the GRAPES operational model. The horizontal resolution of the inner loop is 1.0° × 1.0°. In GRAPES_GFS 4DVAR, the OSW data is directly assimilated as the 10 m neutral wind over the ocean surface. The deblurred zonal wind component (u) and meridional wind component (v) products of HY-2B are also assimilated.
Observation type Instrument Platform Assimilated observation element Conventional observation TEMP Wind, temperature, relative humidity SYNOP Air pressure SHIP Air pressure BUOY Wind AIREP Wind, temperature Satellite observation AMSUA NOAA-15, -18, -19, Metop-A, -B Radiance AMSUB NOAA-15, -18, -19, Metop-A, -B Radiance MWTS-2 FY-3D Radiance ATMS Soumi-NPP Radiance MWHS-2 FY-3D Radiance MWRI FY-3D Radiance HIRAS FY-3D Radiance IASI Metop-A, -B Radiance AIRS AQUA (EOS-2) Radiance GIIRS FY-4A Radiance AGRI FY-4A Radiance S-VISSR FY-2H Radiance GNSS RO COSMIC, Metop-A/B/C GRAS, GRACE-A,
TerraSAR-X, FY-3D GNOSRefractivity GPS-PW Atmospheric column water vapor content AMVs FY-2E, GOES-13, -15, METEOSAT-10,
Himawaii-8Wind (u, v) Table 1. Observations and variables assimilated in the control experiment (CTRL).
Physical process Parameterization Scheme Reference Long-wave radiation RRTMG_SW Morcrette et al., 2008 Short-wave radiation RRTMG_LW Morcrette et al., 2008 Land surface process CoLM Dai et al., 2003 Planetary boundary layer process MRF Hong and Pan, 1996 Cumulus convection process NSAS Arakawa and Schubert, 1974; Liu et al., 2015 Cloud cover CMA cloud cover prognostic scheme Ma et al., 2018 Cloud microphysical processes CMA cloud scheme including the large-scale
condensation, double-moment microphysics,
sub-grid scale convection detrainment processesTan et al., 2013; Jiang et al., 2015; Chen et al., 2021 Table 2. Parameterization schemes for physical processes used in GRAPES_GFS V3.0.
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The HSCAT-B OSW data retrieved by the National Satellite Ocean Application Service (NSOAS) is assimilated in this study, with a horizontal resolution of 25 km. The NWP-based ocean calibration (NOC) algorithm is used for the HSCAT-B OSW retrieval, which considers the influence of sea surface temperature and exerts strict quality control (Verspeek et al., 2012; Wang et al., 2015, 2017; Lin et al., 2017a). The results show that the root mean square errors (RMSEs) of wind speed and wind direction of the HSCAT-B OSW data after strict quality control are 2.26 m s−1 and 16.6°, respectively, evaluated against the buoy wind data. The accuracy of the HSCAT-B OSW data is equivalent to that of the ASCAT-B OSW data (Wang et al., 2020).
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With the wind data observed by oceanographic buoys as a reference, the RMSEs of u and v of the HSCAT-B OSW data are 1.70 m s−1 and 1.76 m s−1, and the biases are 0.03 m s−1 and −0.08 m s−1, respectively (Wang et al., 2020). Although the bias of HSCAT-B is relatively small, previous research showed that the bias is quite sensitive to the actual wind speed. The bias of the HSCAT-B wind speed is particularly obvious when the wind speed becomes larger than 20 m s−1 (Wang et al., 2020). However, there is no bias correction procedure especially for the OSW data in GRAPES_GFS. To mitigate the potential influence of the bias, the observational error should be appropriately enlarged. In addition, the spatial resolution of the u and v components of the OSW retrieval is set at 25 km, which is slightly finer than that of GRAPES_GFS. In general, the correlation between the observational errors of adjacent grids within the current resolution of 25 km should be considered. Previous studies also demonstrated that the spatial correlation of ASCAT-B observational errors approaches zero when the grid interval is beyond 50 km (Valkonen et al., 2017). Nonetheless, in GRAPES_GFS 4DVAR assimilation system, the correlation between observational errors of adjacent grids is not considered in the observational error covariance matrix due to computational cost. Therefore, we process the HSCAT-B OSW observations with the thinning step and enlarge the observational errors in the assimilation system to 2 m s−1 for the u and v wind components.
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There are strict quality controls in the retrieval process of the HSCAT-B OSW (Lin et al., 2017a; Wang et al., 2020). The quality control algorithm for the HSCAT-B OSW L2b products is used to eliminate the data over the areas covered with sea ice and those polluted by precipitation before the GRAPES_GFS 4DVAR assimilation. The quality of observations is controlled by comparing against the background field in GRAPES_GFS 4DVAR. Specifically, the observational data that satisfy the following condition are eliminated:
where
$ {y_{\text{o}}} $ is the observation of u or v,$ {y_{\text{b}}} $ is the background value of u or v,$ \sigma {}_{\text{o}}^{} $ is the observational error of u or v, and$ \beta $ is the threshold constant of quality control. In this study,$ \beta $ is set to 9 in the GRAPES_GFS 4DVAR assimilation system. -
There is a significant correlation between the observational errors of the scatterometer-retrieved OSW across adjacent grids, so it is necessary to thin the pre-processed OSW data before being assimilated (Ochotta et al., 2005; Duan et al., 2017; Valkonen et al., 2017). Previous studies have shown that the assimilation result is optimal when the grid interval of OSW data is 100 km (Bi et al., 2011; Valkonen et al., 2017). Therefore, during the data thinning for HSCAT-B OSW, only one out of four adjacent grid observations is assimilated, which forces the generation of the OSW data to have a horizontal resolution of 100 km.
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Table 3 addresses the experimental configurations. The observational data assimilated in the control experiment (CTRL) are the same as those in the operational GRAPES_GFS (Table 1). The conventional observations include radiosonde data, surface observational data, aircraft-based data, ship data, and buoy data. The assimilated satellite data include the radiance data observed by the Advanced Microwave Sounding Unit-A (AMSU-A), the data from the Microwave Humidity Sounder (MHS) over multiple satellite platforms (NOAA15, etc.), the infrared hyperspectral radiance data observed by the Infrared Atmospheric Sounding Interferometer (IASI) of METOP-A and METOP-B, the infrared hyperspectral radiance data observed by the atmospheric infrared sounder (AIRS) of EOS-2, the radiance data observed by the Micro-Wave Humidity Sounder-2 (MWHS-2) of FY-3D, the radiance data observed by the Micro-Wave Temperature Sounder-2 (MWTS-2) of FY-3D, the Hyperspectral Infrared Atmospheric Sounder (HIRAS) of FY-3D, the radiance data observed by the Microwave Radiation Imager (MWRI) onboard the FY-3D, the brightness temperature observed by the Geostationary Interferometric Infrared Sounder (GIIRS) and the Advanced Geosynchronous Radiation Imager (AGRI) onboard FY-4A, the radiance data observed by the Stretched Visible and Infrared Spin-scan Radiometer (S-VISRR) onboard FY-2H, the refractivity data of navigation satellites (COSMIC, etc.), and the Atmosphere Motion Vectors (AMVs) data of geostationary satellites from multiple satellite platforms (FY-2E, etc.). The HSCAT-B assimilation experiment (HSCAT_EXP) adds the assimilation of HSCAT-B OSW data based on the CTRL.
Exp. name Observations assimilated Assimilation window (h) Simulation period Forecast length (h) CTRL All observations in Table 1 without
HSCAT OSW data6 1 to 30 Sep. 2020 240 HSCAT_EXP CTRL + HSCAT-B OSW data 6 1 to 30 Sep. 2020 240 Table 3. Experimental configurations.
Both experiments have the same assimilation window of six hours and an observational time slot of 30 minutes. The one-month experimental period spans 1 September to 30 September 2020, with a total of 120 data assimilation (DA) and forecast cases.
Observation type | Instrument | Platform | Assimilated observation element |
Conventional observation | TEMP | Wind, temperature, relative humidity | |
SYNOP | Air pressure | ||
SHIP | Air pressure | ||
BUOY | Wind | ||
AIREP | Wind, temperature | ||
Satellite observation | AMSUA | NOAA-15, -18, -19, Metop-A, -B | Radiance |
AMSUB | NOAA-15, -18, -19, Metop-A, -B | Radiance | |
MWTS-2 | FY-3D | Radiance | |
ATMS | Soumi-NPP | Radiance | |
MWHS-2 | FY-3D | Radiance | |
MWRI | FY-3D | Radiance | |
HIRAS | FY-3D | Radiance | |
IASI | Metop-A, -B | Radiance | |
AIRS | AQUA (EOS-2) | Radiance | |
GIIRS | FY-4A | Radiance | |
AGRI | FY-4A | Radiance | |
S-VISSR | FY-2H | Radiance | |
GNSS RO | COSMIC, Metop-A/B/C GRAS, GRACE-A, TerraSAR-X, FY-3D GNOS | Refractivity | |
GPS-PW | Atmospheric column water vapor content | ||
AMVs | FY-2E, GOES-13, -15, METEOSAT-10, Himawaii-8 | Wind (u, v) |