Assimilation of MWHS-2/FY-3C 183 GHz Channels Using a Dynamic Emissivity Retrieval and Its Impacts on Precipitation Forecasts: A Southwest Vortex Case

The dynamic emissivity retrieved from window channels of the microwave humidity sounder II (MWHS-2) onboard the China Meteorological Administration’s FengYun (FY)-3C polar orbiting satellite can provide more realistic emissivity over lands and potentially improve the numerical weather prediction (NWP) forecasts. However, whether the assimilation with the dynamic emissivity works for the precipitation forecasts over the complex geography is less investigated. In this paper, a typical precipitating case generated by the Southwest Vortex is selected and the Weather Research and Forecasting data assimilation (WRFDA) system is applied to examine the impacts of assimilating MWHS-2/FY-3C with the uses of the emissivity atlas and the dynamic emissivity on the forecasts. The results indicate that the use of the dynamic emissivity retrieved from the 89 GHz channel of MWHS-2/FY-3C apparently increases the used data number for assimilation and does improve the initial fields and the 24-hour forecasts (from 0000 UTC 24 June 2016 to 0000 UTC 25 June 2016) of precipitation distribution and intensity except for the rainfall over 100 mm. But these positive impacts are not evidently better than those with the emissivity atlas. In general, these results still suggest that the future use of the dynamic emissivity in the assimilation over the complex terrain is promising.


Introduction
Satellite observation system can provide large quantity observations with high temporal and spatial resolution under all-sky conditions [1], which greatly compensate the sparse observations over oceans and plateaus [2]. With significant improvements in data assimilation algorithm and radiative transfer model, direct assimilation of satellite data has been widely adopted in the numerical weather prediction (NWP) centers and apparently improves the initial atmospheric fields (e.g., temperature, relative humidity, and wind vectors) and the forecasts of high-impact weather processes (e.g., heavy rainfall [3,4]).
Compared with relatively smooth surface over the open ocean, over land and sea ice, the uncertainties in surface temperature and emissivity are higher and the cloud screening is less effective, which makes it more difficult to assimilate microwave observations. Chen et al. [5] compared an approach in which the emissivity is retrieved dynamically from MWHS/FY-3B channel 1 (150 GHz (V)) with the use of an evolving emissivity atlas from 89 GHz observations from the Microwave Humidity Sounders (MHS) on NOAA and EUMETSAT satellites. is study found that the assimilation of the extra data over land with the dynamic emissivity employed improves the fitting of the short-range forecasts to other observations, and the forecasting impacts are mainly neutral to slightly positive over the first 5 days. Recently, the European Centre for Medium-Range Weather Forecasts (ECMWF) has assimilated the observations from FY-3C Microwave Humidity Sounders-2 (FY-3C MWHS-2), which is the second generation of the MWHS/FY-3A & B, in the operational forecasting system since April 4th, 2016 [6]. e Southwest Vortexes (SWVs), a kind of mesoscale cyclonic vortexes, occurring more over the Sichuan Basin and the southeast flank of the Tibetan Plateau (100°E∼108°E, 26°N∼33°N) at 850 hPa or 700 hPa isobaric level [7,8], are generated by the interactions among the leeward slopes of the east-side terrain of the Qinghai-Tibet Plateau, Hengduan Cordillera, Sichuan Basin, and the multiscale atmospheric circulations [9,10]. Migratory SWVs mainly move along either an eastward or southeastward path and the subtropical high is a key factor in determining its direction [11,12]. Meanwhile, moisture transport provides more energy to support the development of SWVs along their trajectories, especially the moisture carried by the Indian Monsoon [11,13]. SWVs produce intense precipitation along the way when they move downstream, affecting Sichuan, Chongqing, South China, North China, and the middle and lower reaches of the Yangtze River, which is the most characteristic precipitating phenomenon in China [7,[14][15][16]. Due to its great intensity and adverse impacts on the safety of the human life and social production, SWVs have been considered as the second most important storm system after the remnant low pressure of typhoon over land [17,18]. ese previous researches mainly used conventional observations, satellite images, and reanalysis data to conduct the diagnostic analysis and numerical simulations on the SWVs. However, there are few studies to evaluate the impacts of assimilating satellite observations, especially FengYun-3 (FY-3) series satellite observations from China Meteorological Administration, on the SWV forecasts. e complex terrain, that is, the Southwest China, always increases the uncertainties and difficulties of forecasts. Sichuan Basin is characterized by a complex terrain with hillock and mountainous areas surrounded as well as broad areas of farmland located. In addition, the northeast trending 500 km long Longmenshan margin marks the eastern border of the Tibetan Plateau with a horizontal distance of 50-100 km from the floor of the western Sichuan Basin to the eastern edge of the Tibetan Plateau [19]. erefore, it is much interesting to investigate the impacts of assimilating MWHS-2/FY-3C observations by employing the dynamic emissivity on the forecasts of precipitation generated by the SWVs over the complex terrain. Dozens of assimilation experiments for precipitation forecasts of the SWVs have been conducted and neutral to slightly positive impacts are found in all of them. erefore, a typical SWV case that occurred from 23 June 2016 to 25 June 2016 is selected as an example in this study, which caused heavy precipitation and affected most areas of the Sichuan Basin. 24-hour accumulated heavy precipitation (100-200 mm) was observed on 24 June 2016 in some regions and the next 24-hour weather report (25 June 2016) suggested that the south and northeast of the Sichuan Basin and Chongqing still had heavy rain. Severe weather phenomena, like hailstorms, strong winds, and thunderstorms, did occur with heavy rainfall causing huge economic losses to local residents. e data and methodology applied in this study are firstly introduced in Section 2. Experiment settings are described in Section 3, and results and discussion are presented in Section 4 before drawing conclusions in Section 5.

MWHS-2/FY-3C Observations.
e first MWHS-2 instrument, flown on the Chinese FengYun-3C satellite, combines 5 traditional 183 GHz humidity sounding channels with novel channels around 118 GHz, which provides further information on temperature and clouds, and two window channels of 89 GHz and 150 GHz. e observations with good quality have been assimilated operationally since 5 April 2016 in ECMWF system [5]. e full list of the characteristics of the MWHS-2/FY-3C channels is displayed in Table 1. Note that in this study only the 183 GHz channels are used in assimilation with applying the 89 GHz window channel for emissivity retrievals over land and the 150 GHz channel for cloud screening.
e WRFDA system and its three-dimensional variational (3D-VAR) component [20] are used in this study, which aims to obtain a statistically optimal analysis through an iterative minimization of a prescribed cost function: where x represents the atmospheric state vector, x b represents the background state, H represents the nonlinear observation operator that maps the model variables to the observation space, and y represents the observation vector.
For satellite radiance data assimilation, the RTTOV is applied as observation operator to simulate brightness temperatures. B and R represent the background and observation error covariance matrices, respectively. e NMC method [21] is employed to obtain case-dependent background error covariance matrices. e complex surface geography largely affects the microwave signal emitted by land surfaces. Compared to 150 GHz channel, the 89 GHz channel is more sensitive to the lower latitudes [6] which cover the Sichuan Basin and other complex terrains, so that the latter is used for emissivity retrieval under the clear-sky conditions. One of the benefits to carry out an emissivity retrieval, rather than use an atlas, is that retrieved emissivity shows variability from one overpass to the next, which is more realistic.
According to Liu et al. [22], for a nonscattering planeparallel atmosphere, assuming a flat and specular surface for a given zenith (θ) angle and frequency (]), the TB (T b ) observed by a satellite sensor can be expressed as follows: where ε (θ,]) represents the surface emissivity at the observation zenith angle θ and frequency ]; T s , T ↓ (θ,]) , and T ↑ represent the surface skin temperature obtained directly from the first guess, atmospheric downwelling radiation at the surface, and upwelling radiation at the top of the atmosphere, which are calculated within the RTTOV, respectively; and Γ (θ,]) represents the net atmospheric transmissivity.
e emissivity can be retrieved from the following equation: e retrieved surface emissivity is used only under the conditions when the surface-to-space transmittance for 89 GHz is larger than 0.5 and when the difference between the retrieved emissivity and the equivalent value from the Tool to Estimate Land Surface Emissivity at Microwaves and Millimeter (TELSEM) waves atlas is less than 0.2. Otherwise, the TELSEM atlas value for the corresponding channels is used.

Quality Control.
Before any process begins, the MWHS-2/FY-3C observations are thinned to a spacing of 60 km. Other quality control procedures [23] include the following: remove observations with brightness temperature lower than 50 K or larger than 550 K; remove pixels with mixed surface types; remove the first five scan angles over the swath edges according to the evaluation report of MWHS-2/FY-3C [24]; and remove the observations if the bias corrected normalized innovation (O-B) exceeds 3σ, where σ represents the specified standard deviation in the observation error. Over land, the orography check is also important to avoid observations that are too sensitive to the surface, that is, where errors in the surface emissivity or the skin temperatures play a large role. e threshold of the orography for channel 11 is 1500 m; that for channels 12-13 is 1000 m; and that for channels 14-15 is 800 m for MWHS-2/FY-3C. ese thresholds reflect the different surface sensitivities of the sounding channels, that is, more rigorous for the lowest channels (channels 14-15) and less tight for the higher channels [6].
To remove observations strongly affected by ice cloud and precipitation, a 5 K check on the absolute value of the first-guess departure of channel 10 (150 (V) GHz) is employed [6]. is definition of "clear sky" will be used in this paper: it does not mean no clouds; rather the radiative impact of clouds is considered to be small and is not analyzed. Note that this criterion will also reject data for which the emissivity or skin temperature estimate is significantly in error. A variational bias correction scheme, known as VarBC [25,26], is also applied to remove the systematic errors. Figure 1 shows the probability distribution function (PDF) of the first guess departures of the MWHS-2/FY-3C channels 11-15 at 0000 UTC 23 June 2016 before and after VarBC. Apparently, after VarBC, the PDFs shift more towards zero to meet the requirements of the data assimilation algorithms for unbiased error models.

Experimental Settings
According to the characteristics of the trajectories, the southwest vortexes can be categorized into the local type and the moving type. e latter can be further divided into the eastward-trajectory type, the northeastward-trajectory type, and the southeastward-trajectory type. For the local type, the southwest vortexes rarely move out and the affected regions are small. Meanwhile, for the moving type, the vortexes can move eastward, northeastward, or southeastward, having a great influence on broad areas and even merging into other weather systems [12].
One typical local type case occurring from 23 June to 25 June 2016 mentioned in Section 1 is selected to show the assimilating impacts by using dynamic emissivity on the forecasts of heavy rainfall in this study. ree experiments were conducted as follows: the first experiment (denoted as CN) does not conduct data assimilation; the second (denoted as AL) and the third (denoted as DY) experiments both assimilate MWHS-2/FY-3C clear radiances, with the former using emissivity atlas and the latter using dynamic emissivities. e Advanced Research WRF (ARW) version 3.  Table 2. e assigned observation errors over land are also listed in Table 2

e Usage of the MWHS-2 Observations.
To examine the impact of the technical implementation of emissivity retrieval, the usage of the MWHS-2 observations is investigated. During the whole assimilation period (i.e., from 0000 UTC 23 June to 1200 UTC 24 June), the amount of the used data in the DY experiment is greater than that in the AL experiment ( Table 3). Note that, compared with channels 12-15, more MWHS-2 observations from channel 11 are assimilated as its peak height is higher and it is less affected by the complex surface geography. On the other hand, as shown in  (Figures 2(c)-2(f )), which would provide more atmospheric information and have a potential to perform better on forecasts.
In order to evaluate the accuracy of the retrieved emissivity, the differences between the retrieved emissivity from 89 GHz channel of MWHS-2/FY-3C and the TELSEM atlas at 0000 UTC 23 June 2016 are displayed in Figure 3(c). e larger differences are distributed over hypsographic areas, which suggests that the retrieved emissivity is more variable and realistic than the TELSEM atlas.

Impacts of the Emissivity Retrievals on the Initial Fields.
e PDFs of the first-guess departures of MWHS-2 observations from each channel during the whole assimilation period in the two experiments are shown in Figure 4. After employing the emissivity retrievals, the patterns of the PDFs in the DY experiment became relatively slimmer and higher as compared with the AL experiment, especially for channels [14][15], which suggests that the emissivity retrievals have an ability to improve the accuracy of the background obtained from the model.
As for the impact of emissivity retrievals on physical fields, the averaged root mean square error differences (delta RMSE) between the analysis departures (O-A) and the firstguess departures (O-B) of the temperature, relative humidity, wind velocity, and geopotential height are calculated ( Figure 5); that is, where N is the number of model grids, y io and y if denote the ith observation from ECMWF ERA-5 analysis and ith data from the analysis or first guess in the two experiments, respectively. Negative values indicate positive impacts. Figure 5 shows not only the impacts of data assimilation on the physical fields in the two experiments and the impacts of the emissivity retrievals employed in the DY experiment on the physical fields as compared with the AL experiment but also which levels reflected the positive impacts. An apparently positive impact has been reflected in temperature and relative humidity in the AL and DY experiments (Figures 5(a) and 5(b)). For the wind velocity and geopotential height as well as mid-low-level relative humidity, negative impacts are detected in the two experiments ( Figures 5(b)-5(d)). e reasons for these negative impacts are not clearly, but some studies [35,36] have also addressed this situation. ey indicated that other advanced data assimilation approaches, such as hybrid (ensemble/3D-VAR) or 4D-VAR method, will deal with this problem as these advanced data assimilations can provide flow-dependent ensemble background covariance error which is not achieved in the 3D-VAR data assimilation. Note that these negative impacts are relatively smaller in the DY experiment as the absolute values of the RMSE differences are smaller than those in the AL experiment, especially the mid-low-level wind velocity and low-level relative humidity. On the other hand, the impacts of emissivity retrievals on each channel are shown in Table 5. Generally, the mean and standard deviation of analysis departures are smaller than those of first-guess departures. e values of mean and standard deviation of the first-guess and analysis departures in the AL and DY experiments are nearly the same, but the number of observations assimilated in the DY experiment is greater than that in the AL experiment. In addition, the variations of values in the DY experiment during the whole assimilation period are slight, suggesting that the extra observations in the DY experiment let through by QC procedures might reinforce model biases.

Impacts of the Precipitation Forecasts.
ere was a band of water vapor flux (>12 g hPa −1 cm −1 s −1 ) over the Yangtze River in the ERA-5 analysis obtained from the ECMWF ( Figure 5(a)), and the moisture was transported from Southwestern China to Eastern China. e band of maximum water vapor flux was nearly east-west direction in the ERA-5 analysis, while in the CN, AL, and DY experiments, the direction was northeast-southwest ( Figure 5). In addition, there were two maximum water vapor flux centers (>18 g hPa −1 cm −1 s −1 ) over the band in the ERA-5 analysis, AL and DY experiments as compared with the CN experiment. However, the locations of two centers in the ERA-5 analysis were evidently different from the experiments assimilating the MWHS-2 observations. e translation speed of a cyclonical vortex (i.e., southwest vortex) located in the east of Sichuan Province in the ERA-5 analysis (Figure 6(a)) was not simulated successfully by the model (Figures 6(b)-6(d)). e 24-hour accumulated precipitation from 0000 UTC 24 June to 0000 UTC 25 June 2016 is shown in Figure 7. e distribution of precipitation was east-west direction (Figure 7(a)), which was the same as the band of water vapor flux as discussed above. e heavy rainfall    Advances in Meteorology (>100 mm) centers were mainly located in Sichuan and Chongqing Provinces in the observations. But in the CN experiment (Figure 7(b)) the amount of precipitation was overestimated, and the distribution of heavy rainfall was more spacious and moved eastward as compared with the observations, AL and DY experiments. e amount of heavy rainfall in the AL experiment (Figure 7(d)) was closer to the observations as compared with the DY experiment (Figure 7(c)), especially in the west of Guizhou Province and north of Hunan Province. e results of the equitable threat score (ETS) show that the two assimilation experiments made much better forecasts of precipitation as compared with the CN experiment except for the rainfall over 100 mm (Figure 8). e score of the precipitation over 100 mm in the CN experiment was slightly higher than the assimilation experiments, but the distribution of rainfall over 100 mm was more spacious than the observations and two assimilation experiments. In addition, the score results from the AL experiment were better than those from the DY experiment; these can be also reflected by the forecasts of the temperature, humidity, wind, and geopotential height (Figure 9). e two experiments assimilating the MWHS-2 observations improved all the variables (i.e., temperature, relative humidity, wind velocity, and geopotential height) significantly as compared with the CN experiment. ese improvements mainly occurred below 600 hPa, and the AL experiment made better performances compared to the DY experiment. So why the results from the dynamic emissivity (DY) experiment could not evidently improve the forecast even though there are some extra observations assimilated as compared with the emissivity atlas (AL) experiment is an obvious question needed to be answered. On one hand, the majority of extra observations are concentrated on the northeast of China, and there are few extra observations over the west-east rainfall band over the Yangtze River where the MWHS-2

Conclusions
In this study, the WRFDA system and its 3D-VAR scheme are employed to evaluate the impacts of assimilating MWHS-2/FY-3C with emissivity atlas and dynamic emissivity applied on the analysis and forecasts of precipitation generated by a representative SWV case over complex terrain within a limited domain. Apparently, the dynamic emissivity could provide more accurate and detailed emissivity over lands and allow more MWHS-2/FY-3C observations to be assimilated. Mainly positive impacts are shown in the initial atmospheric fields, that is, temperature and  relative humidity, but some negative impacts are detected in the wind velocity and geopotential height as well as mid-lowlevel relative humidity.
For the precipitation forecasts, both 700 hPa water vapor flux and 24-hour accumulated precipitation from 0000 UTC 24 June to 0000 UTC 25 June 2016 indicate that the assimilation of MWHS-2/FY-3C observations undoubtedly can improve the precipitation forecast of the SWV and the precipitation distribution and intensity are closer to the observations, regardless of the uses of the emissivity atlas and the dynamic emissivity. e ETSs of the 24-hour accumulated precipitation and the vertical profiles of the 24-hour forecast RMSEs further verify these improvements. Apparently, assimilating MWHS-2/FY-3C data with the dynamic emissivity can increase the amount of assimilated data, but its impacts on this SWV precipitation forecast case are not evidently positive compared to the case with the emissivity atlas. e possible reasons may be the following: firstly, since the main changes in data usage are produced from the channels sensitive to the higher-level water vapor (i.e., channel 11), it is not surprising to see slight improvement for lowerlevel fields with the change of the assimilation strategy; secondly, the extra observations assimilated might reinforce model biases, which makes the results in the DY experiment not better than the AL experiment. Also, the quality control removes the cloud-and rain-contaminated observations and only clear-sky data that are assimilated in this study leading to quite a lot of important information are excluded. erefore, all-sky assimilation (under clear, cloudy, and precipitating conditions) would be a useful approach to make good use of cloud-and rainaffected observations and further study with more SWV cases is also required to investigate the impacts of assimilating MWHS-2 observations on the SWV precipitation forecasts.

Conflicts of Interest
e authors declare that they have no conflicts of interest.