## 2 Used NWP System and Data Setup with Data Latency
### Data setup of LEO IR and MW sounders  
The Binary Universal Form for the Representation of meteorological centers (BUFR) files containing the IR and MW sounder measurements with 20-min and 5-min latency were created from those used in the GDAS run. To determine if observations are delivered on time for the GFS run, the data latency of 20 min or 5 min was added to the actual observation time. If expected data delivery time is earlier than or equal to the data cut-off time, then this observation was written into the respective BUFR file. For example, if the data latency is 20 min and the actual observation time is 0240 UTC 1 July, the data delivery time will be 0300 UTC 2 July. On the other hand, if the data latency is 5 min, the data delivery time for this observation will be 0245 UTC 1 July. For the 0000 UTC GFS analysis cycle, the data cut-off time is 0245 UTC. With the 20-min latency, the observation will not make it into the assimilation system. However, with the 5-min data latency, this observation is just in time for assimilation.  
Figure 1 shows the spatial coverage of Cross-Track Infrared Sounder (CrIS) measurements in a BUFR used by a single analysis cycle (0000 UTC 1 July 2018) depending on the defined data latency. As the GDAS run was triggered about six hours after the analysis time, almost all CrIS measurements observed within the 6-h assimilation window (2100 UTC 30 June 2018 to 0300 UTC 1 July 2018) were timely delivered for the GDAS run (Figure 1a). However, for the operational GFS run (Figure 1b), about 83% of the CrIS measurements used in the GDAS run were delivered before the data cut-off time of 2 h 45 min (0245 UTC 1 July 2018). Compared with the CrIS data coverage at the GDAS run, the CrIS data covering the Western Pacific are missing for the operational GFS run. They were observed before the data cut-off time but not timely delivered due to the data latency. With the 20-min and 5-min data latency, the data usage of CrIS measurements reached about 90% and 94%, respectively (Figure 1c,d). In particular, the CrIS data over the Southwest Pacific are included for both cases. The 5-min latency run provided extra CrIS data over the central West Pacific.  
The time series of total number observations from all sixteen IR and MW sounders for different data latency length for the period from 0000 UTC 7 July 2018 to 000 UTC 22 August 2018 was plotted in Figure 2, in addition to the percentage normalized by the total data number in the GDAS run. As expected, the GDAS run has the largest total number observation available for the data assimilation. For the operational GFS run, the percentage fluctuates between 65% to 75%. At data latency of 20 min and 5 min, the percentage increases to about 88% and 92%, respectively. This trend suggests that reducing the data latency up to 5 min will increase the data available for GFS run by more than 20%.  
Figure 1: Spatial coverage of Cross-Track Infrared Sounder (CrIS) channel 99 (corresponding to the channel wavenumber of 711.15 cm-1) brightness temperatures for (**a**) the global data assimilation system (GDAS) run, (**b**) the operational Global Forecast System (GFS) run, (**c**) the GFS run with the 20-min data latency, and (**d**) the GFS run with the 5-min data latency on 0000 UTC 1 July 2018. Data ratio against the CrIS data for the GDAS run is described in parenthesis.  
Figure 2: Time series of (**a**) total number of the hyperspectral infrared (IR) and microwave (MW) sounder measurements available at the GFS run for the control (black solid), latency_20m (red solid), and latency_5m (blue solid) runs, and (**b**) observation percentage (%) normalized against the number of the sounder measurements used for the GDAS run. The black dashed line indicates the number of the sounder measurements available for the GDAS run.