## 4 Results  
### Comparison of LEO IR and MW Sounder Data Assimilated  
In Figures 1 and 2, the BUFR files indicated an increased number of observations delivered to the GFS assimilation system with reduced data latency. However, as this effort to reduce the data latency ultimately aims to enhance the model forecast by assimilating available satellite measurements as much as possible, it is more reasonable to assess how many additional LEO sounder data were assimilated practically by reducing the data latency. In this section, therefore, we compared the number of assimilated LEO sounder data for three trial runs. As the GDAS run has a very long data cut-off time, it is considered as the benchmark for the maximum number of observations assimilated. Figures 3 and 4 show the percentage of the number of observations assimilated in the GFS run against the GDAS run for five IR sounders and two MR sounders, i.e., CrIS onboard Suomi National Polar-orbiting Partnership (S-NPP) and National Oceanic and Atmospheric Administration 20 (NOAA-20), Infrared Atmospheric Sounding Interferometer (IASI) onboard MetOp-A and MetOp-B, Atmospheric Infrared Sounder (AIRS) onboard Aqua, and the Advanced Technology Microwave Sounder (ATMS) onboard S-NPP and NOAA-20. A comparison of these percentages shows that more observations are assimilated with improved data latency. For the control run, the data usage ratios of the satellite sounder measurements are significantly different depending on the type of IR sounders, fluctuating between 50% and 85% during the whole trial period (Figure 3). In particular, the AIRS instrument onboard Aqua satellite shows the lowest data usage of about 65% in the operational NCEP GFS assimilation system (Figure 3e). The reason is that there is no delivery of the AIRS observations via the DBNet in near real time. Instead, AIRS data are transmitted to NCEP via GTS or FTP after being downlinked at the assigned ground stations. It is also interesting to note that the data usage of the IASI instrument onboard MetOp-A satellite is lower than that of MetOp-B satellite despite being the same type of IR sounder (Figure 3c,d). According to the satellite data acquisition priority determined by the DBNet coordination group, the data direct broadcast of MetOp-A is not currently operating over some regions (e.g., polar region) due to the radiation sensitivity issue of the High Rate Picture Transmission (HRPT) hardware embedded in the MetOp-A satellite [9]. Thus, the length of data latency of MetOp-A is relatively longer than the MetOp-B satellite, leading to the reduction of assimilated data in GFS. Similar to IASI instruments, the data usage of CrIS instrument is also slightly different depending on the type of satellite platform (Figure 3a,b). The data percentage of CrIS data onboard NOAA-20 satellite is 5% higher than that of CrIS sensor onboard S-NPP satellite. The different data usage is because the data from the S-NPP satellite are only downlinked at the CDA but the data from NOAA-20 satellite are downlinked at the CDA and the Antarctic Data Acquisition (ADA) station at McMurdo Station, Antarctica [9]. Even though the length of data latency largely fluctuates depending on the sounding sensor type (shown in Figure 3), it is evident that more observations can be assimilated due to the reduction of data latency of LEO satellites with the use of the DBNet system. However, current average data latency for the IR/MW sounders is still longer than the WSP recommendation of 20 min. This is due to the limited DBNet coverage and sensor-dependent data acquisition issues. The data latency can be further reduced by expanding the number of DBNet stations to the areas that the current local DBNet ground stations do not cover and enhancing the data acquisition/processing capability of the DBNet stations. The results of the two trial runs (latency_20m and latency_5m runs), under the assumption that the data latency is 20 min and 5 min, respectively, for all DBNet stations, show how much the satellite measurements are assimilated in the GFS.