For assimilating the satellite observation data, a quality-control process known as OPS in the operational UM system is performed. Satellite-observed radiances are compared with the radiances simulated with collocated model-generated atmospheric fields (called the background) using a radiative transfer model (e.g., RTTOV). Background fields are the UM 6-h forecasts issued from the previous data assimilation cycle. Through examination of the departure of simulated radiances from satellite observations, the performance of the background fields can be monitored.
The experiments with the new 200 IASI channels had a substantially smaller data volume ratio of IASI observations passing through the OPS than the control run. This significantly reduced volume of IASI observations was due to O3 channels included in the new 200 IASI channels. The use of climatological O3 concentrations in the UM for the O3 channel simulations caused larger departures from the observed radiances, resulting in a convergence failure in the 1D-Var process (e.g., Saunders et al., 2013). Thus, O3 channels were excluded from the 200 IASI channels in the experiment runs, in order to increase the convergence ratio for 1D-Var in the OPS. This resulted in a selection of 189 channels, which is close to the operationally used 183 channels.
Similar problems may arise from the use of shortwave infrared channels over band 3 during the daytime because of solar contamination. Thus, the nighttime band 3 measurements were used, which were defined as those with a solar zenith angle greater than 95°.
In order to examine the impact of the new IASI channels on the background field, atmospheric profiles of the background from the control and experiment runs were used as inputs to RTTOV to simulate channel radiances of the Advanced TIROS Operational Vertical Sounder (ATOVS). The ATOVS consists of three instruments: the High Resolution Infrared Radiation Sounder (HIRS), the Advanced Microwave Sounding Unit-A (AMSU-A), and the Microwave Humidity Sounder (MHS). The differences in first-guess departure between the control and experiment runs for the ATOVS channels in four satellite platforms (i.e., MetOp-A, MetOp-B, NOAA-18, and NOAA-19) are shown in Fig. 8. The characteristics of ATOVS channels are given in Table 1. From the comparison between the control and experiment runs, little difference was found in the temperature sounding channels of ATOVS (i.e., HIRS channels 2-7, given as channel indices of 2-7, and AMSU-A channels 1-14, given as channel indices of 21-34). By contrast, significant differences were noted in the H2O channels (HIRS 12, given as channel index of 12, and MHS 3-4, given as 38-39). Considering that these H2O channels show Jacobian peaks in the upper troposphere, the new channels may provide a bigger impact on the upper-tropospheric humidity field. However, considering the finding that negative biases become larger, especially for the upper-tropospheric channels (e.g., channel indices 12 and 38), the experiment run should induce a relatively drier upper troposphere in the 6-h forecast. The negative biases of the upper-tropospheric humidity channels were persistent throughout the analysis period. Persistent larger biases were also clear in the experiment run.
Such drier biases shown in the experiment run were also evident in the comparison with simulations for other infrared sensors [i.e., the Atmospheric Infrared Sounder (AIRS) and Cross-track Infrared Sounder (CrIS) hyperspectral sounders] with the same background fields. In the UM forecasting, 324 AIRS and 399 CrIS channels over the CO2 and H2O absorption bands, and the window region, are included in the system (Cameron et al., 2005; Gambacorta and Barnet, 2013). However, similar to the IASI channel selection, some of the AIRS and CrIS channels were not used for data assimilation——the unused channels are indicated as empty spaces in Fig. 9. The experiment run showed larger negative mean biases for H2O channels (e.g., AIRS 174-235 and CrIS 185-312 channels), compared to the positive biases shown in the control run. In particular, the larger negative biases were located over higher channel numbers, which represent channels sensitive to the upper-tropospheric humidity. Such relative dry biases for the experiment run are consistent with the ATOVS results shown in Fig. 8.
As shown in the first-guess departures of the ATOVS, CrIS, and AIRS H2O channels, the new IASI channel selection causes larger biases, and the larger biases suggest inferior performance in the assimilated water vapor field. However, considering that the biases between the satellite observations and model backgrounds are subtracted from satellite observations before use for data assimilation (called "bias correction"), the larger H2O channel biases might be related to such bias correction.
Here, for the bias correction, we use the correction scheme employed by the UM OPS, which is based on the bias correction scheme developed by (Harris and Kelly, 2001) for satellite measurements. The scheme uses the geometric thicknesses of the 850-300 hPa and 300-50 hPa layers in the model atmosphere as predictors to regress to the departures between IASI observations and background-derived radiances. Once regression coefficients and an intercept point are determined for each of all the 314 selected IASI channels, the biases for the 189 IASI channels used are predicted by inserting two layer thicknesses into the obtained regression equation. Since our main aim is to examine how the new set of channels improves the forecasts through the improvement of the background field, recalculation of bias correction coefficients for the current experiment may not be necessary and therefore the same bias correction coefficients were used for both the experiment and control runs.
The bias correction mostly removes biases related to instrument errors and uncertain radiative transfer modeling. These biases are variable with time because of sensor degradation or upgrading of the radiative transfer model. However, in the two trial runs (control and experiment run) in this study, the same satellite data were used along with the same version of radiative transfer model. Because sensor-related errors and systemic errors were the same for both runs, the same bias correction coefficients could be used for both runs.
Here, using radiosonde observations, we examine the abovementioned bias correction as a possible cause for the larger negative H2O channel departures shown in Figs. 8 and 9. The mean biases of first-guess departures obtained from radiosonde observations in the assimilation cycle are shown in Fig. 10. Negative biases are clear for both the control and experiment runs, suggesting that the backgrounds (i.e., 6-h forecasts) are humid over almost the entire troposphere, as compared to radiosonde observations (Fig. 10). Much larger humidity biases of the control run over the upper troposphere are significantly different from the near-zero bias shown in the first-guess departures (Figs. 8 and 9), and this is thought to be due to the bias correction made in the OPS. For the satellite-measured radiance assimilation, biases are removed from the observations in order to make observations consistent with model backgrounds. Thus, the apparent near-zero biases of the H2O channels in Figs. 8 and 9 do not necessarily mean a nearly unbiased moisture condition in the control run, as noted in the humid condition in Fig. 10. The bias correction in the control run should have been a positive brightness temperature subtraction from the satellite measurements (i.e., colder H2O channel temperatures representing humid conditions). The experiment run, on the other hand, indicates a less humid condition in comparison to the control run (Fig. 10). Since the coefficients used in bias correction for both the experiment and control runs were the same, the apparent departures should be negative, as in Figs. 8 and 9, because of the less humid conditions as shown in Fig. 10. Thus, we conclude that the larger negative departures shown in the H2O channels (Figs. 8 and 9) are due to the use of the same bias correction for both the control and experiment runs, and the near-zero biases in the control run should not mean an unbiased humidity condition.
In order to examine the impact of the new IASI channels on UM global forecasts, departures of forecast atmospheric parameters obtained from each trial run against radiosonde observations were calculated. The score, called the global NWP index, was used to measure the forecast impact in the UKMO UM system; it combines the scores of particular atmospheric parameters at each forecast hour. The total index is the sum of the scores from all the atmospheric parameters. Details regarding how to derive the scores are found in Appendix A of (Rawlins et al., 2007).
The global NWP index calculated for the experiment shows that the newly selected IASI channels have an overall neutral impact on the forecast improvement for the selected atmospheric parameters, compared to the control run (not shown). Since the main difference between the new IASI channels and the currently used operational IASI channels is the use of different H2O channels and band 3 channels, it is not surprising to find little improvement in the NWP index, based on parameters such as mean-sea-level pressure, 500 hPa geopotential, and winds at 250 hPa and 850 hPa.
In contrast, the forecast temperature and relative humidity parameters, which are not part of the NWP index calculation, show a significant improvement, especially above 500 hPa. Figures 11 and 12 show the mean biases of temperature and relative humidity at the 500 hPa and 850 hPa levels, in the range of T+0 to T+72 forecast hours. The mean biases of the forecast temperature at 500 hPa level were smaller for the experiment run, compared to those from the control run, in both the Northern and Southern Hemisphere, although the mean bias in the Southern Hemisphere at T+36 became larger first. In contrast, the forecast error for the temperature at 850 hPa shows a slight degradation for the experiment run.
In terms of humidity in the control and experiment runs, the positive biases of relative humidity at the 250 hPa level are significantly reduced by up to 4% in the experiment run in both hemispheres, as shown in Figs. 12a and b. However, for the mean bias difference at the 850 hPa level for temperature, the difference in relative humidity also shows a less significant impact. The RMSEs of relative humidity are slightly reduced at the 250 hPa level in the experiment run, but are quite similar at the 850 hPa level (Fig. 13). It is clear that the improvement in the relative humidity forecasts mainly occurred in the upper troposphere from 500 hPa to 200 hPa. A substantial bias reduction is evident in the analysis at T+0. By contrast, in the mid-troposphere from 600 hPa to 500 hPa, the mean biases are larger for the experiment run at T+0 and T+24 (Figs. 14a and b). The humidity RMSEs are mainly reduced in the upper troposphere from 350 hPa to 200 hPa at T+0 and T+24, but the RMSE reduction at T+48 and T+72 appears to be insignificant (Fig. 15). As expected, the humidity biases and RMSEs increase with the forecast hour in both runs, even though the mean biases and RMSEs in the experiment run are smaller than those in the control run.
Considering that the main difference between the new channels and the 183 UKMO operational channels lies in the use of the new IASI H2O channels and band 3 channels, the improvement in the temperature and relative humidity forecasts in the upper troposphere can be attributed to the use of these new additional channels. Since weighting functions of the selected IASI band 3 channels are mostly located in the lower troposphere, we expect improved temperature forecasts in the lower troposphere. However, there is only a small change in the lower troposphere from the use of the new channels, which suggests that the impact of the band 3 channels may be minimal, probably due to the high noise levels in these channels. Thus, the overall improved forecasts for temperature and humidity in the upper troposphere are primarily due to the newly selected H2O channels.
It is also important to note that the difference in the relative humidity bias in the upper troposphere between the control and experiment runs became smaller with forecasting hour, except around the 250 hPa level at T+72 (Fig. 14). The assimilation of additional channels in the experiment run produces a humidity analysis closer to the radiosonde values. However, such bias reduction disappears quickly and the humidity bias tends to return to the level of the control run.