Asian water tower evinced in total column water vapor: a comparison among multiple satellite and reanalysis data sets
The total column water vapor (TCWV) over the Tibetan Plateau (TP) is one important indicator of the Asian water tower, and the changes in the TCWV are vital to the climate and ecosystem in downstream regions. However, the observational data is insufficient to understand the changes in the TCWV due to the high elevation of the TP. Satellite and reanalysis data can be used as substitutes, but their quality needs to be evaluated. In this study, based on a homogenized radiosonde data set, a comprehensive evaluation of the TCWV over the TP derived from two satellite data sets (AIRS-only and AIRS/AMSU) and seven existing reanalysis data sets (MERRA, MERRA2, NCEP1, NCEP2, CFSR, ERA-I, JRA55) is performed in the context of the climatology, annual cycle and interannual variability. Both satellite data sets reasonably reproduce the characteristics of the TCWV over the TP. All reanalysis data sets perform well in reproducing the annual mean climatology of the TCWV over the TP (R = 0.99), except for NCEP1 (R = 0.96) and NCEP2 (R = 0.92). ERA-I is more reliable in capturing the spatial pattern of the annual cycle (R = 0.94), while NCEP1 shows the lowest skill (R = 0.72). JRA55 performs best in capturing the features of the interannual coherent variation (EOF1, R = 0.97). The skill-weighted ensemble mean of the reanalysis data performs better than the unweighted ensemble mean and most of the single reanalysis data sets. The evaluation provides essential information on both the strengths and weaknesses of the major satellite and reanalysis data sets in measuring the total column water vapor over the TP.
KeywordsAsian water tower Total column water vapor Tibetan Plateau Data evaluation Skill weighting
Water vapor plays a key role in Earth’s energy and water cycles through its complex interaction with the clouds and precipitation (Chahine 1992; Oki and Kanae 2006). It is also crucial to climate change, as water vapor is one of the most important greenhouse gases (Lacis et al. 2010). Given the importance of atmospheric water vapor, an international project aimed at the evaluation of the current water vapor data products has been launched, which is called the Global Energy and Water Cycle Exchanges project (GEWEX) water vapor assessment (G-VAP) (Schröder et al. 2016, 2018). There are many acronyms for atmospheric water vapor, such as TCWV (total column water vapor), PWAT (precipitable water vapor), TPW (total precipitable water vapor), IPW (integrated precipitable water vapor) and IWV (integrated water vapor). Consistent with G-VAP, the acronym of TCWV is used in this work. The availability of reliable TCWV data is crucial for understanding the water cycle.
The Tibetan Plateau (referred to as TP hereinafter) is known as the Asian water tower, as it has the most glaciers in the world except for the polar regions (Xu et al. 2008, 2014; Yao et al. 2012). Water vapor as well as its cycle process over the TP is vital to the climate and ecosystem in downstream regions (Yao et al. 2012). Although the water vapor over the TP is important, our knowledge of its changes is limited due to the lack of observational data. Because of the complex terrain, it is a challenge to establish and maintain in situ observations in the TP. Both satellite and reanalysis data are hoped to be useful substitutes, but their quality need to be assessed.
The quality of the satellite data and reanalysis data in the TP has been evaluated in previous studies, but the results are difficult to compare with each other due to the different data used and different time periods and domains considered, as well as the different data processing methods and statistical metrics applied (Wang and Zeng 2012; You et al. 2015; Zhang et al. 2018). These previous studies focused on several data sets, including TRMM and GPM as satellite data, as well as MERRA, NCEP1, NCEP2, CFSR, ERA-40, ERA-Interim, and GLDAS as reanalysis data. In recent years, new versions of satellite data including AIRS version 6 have been released, and more reanalysis data sets including JRA55 and MERRA2 have been updated (Tian et al. 2013b, 2019; Kobayashi et al. 2015; Gelaro et al. 2017). The TCWV derived from 4 satellite data sets including AIRS version 6 and 4 reanalysis data sets including JRA55 has been assessed, but the temporal and spatial coverage of the study was limited, only including the summer season and the southern plateau along the Himalayas (Wang et al. 2017). It is thus urgently needed to provide a comprehensive evaluation of the existing satellite products and reanalysis data sets for the representation of the TCWV over the whole plateau, based on the same observational metrics.
In this paper, we aim to evaluate the quality of two satellite data sets and seven updated reanalysis data sets based on one homogenized radiosonde data set. The scientific questions we considered are: (1) In comparison to the radiosonde data, how reliable are the two satellite data sets (AIRS-only and AIRS/AMSU) in capturing the TCWV features over the TP? (2) When compared to the observational data and satellite product sets, which reanalysis data set performs better in reproducing the annual mean climatology, annual cycle and interannual variability of the TCWV over the TP? (3) Can we construct a skill-weighted ensemble mean of the reanalysis data that outperforms the unweighted ensemble mean and most of the single reanalysis data sets, taking all time scales into account?
The remainder of the paper is organized as follows: the data and method used are introduced in Sects. 2, 3 presents the assessment of the satellite and reanalysis data, as well as the skill-weighted ensemble mean of the reanalysis data, and the major results are summarized in Sect. 4.
2 Data and method description
2.1 Data description
Radiosonde dataIn our analysis, the NOAA homogenized radiosonde data “HomoRaw” is used to assess the quality of both the satellite products and reanalysis data (Dai et al. 2011). The data was processed to produce a more accurate and reliable humidity data set for global water budget and climate change research. The homogenization method introduced in Dai et al. (2011) was applied mainly on the NOAA/NCDC IGRA (Integrated Global Radiosonde Archive) (Durre et al. 2006), containing the CARDS (Comprehensive Aerological Reference Dataset) data and a special radiosonde data set from China (Dai et al. 2011). The data set is available from January 1945, and we used the period of January 2003 to December 2012 in this study. The TP radiosonde stations were defined as stations within the Plateau (26°N–40°N, 70°E–105°E) with an altitude above 2650 m. Eight stations were finally chosen, which are located in the central and eastern parts of the TP (Fig. 1). As in situ observations of the tropospheric total column water vapor, the radiosonde data set will be used as a reference for the quality assessment of the satellite and reanalysis data sets in the following analysis.
Satellite data: AIRS-only and AIRS/AMSU from satellite Aqua
The Atmospheric Infrared Sounder (AIRS) and the Advanced Microwave Sounding Unit (AMSU) are instruments aboard the Aqua satellite, provided by NASA as a part of the Earth Observation System (EOS) (Aumann et al. 2003; Lambrigtsen 2003; Chahine et al. 2006). As a Sun-synchronous polar orbiting satellite, Aqua crosses the Equator from the Southern Hemisphere (SH) to the Northern Hemisphere (NH) at local time 13:30, representing daytime observations, and crosses the equator from NH to SH at local time 01:30, representing nighttime observations (Aumann et al. 2003; Lambrigtsen 2003). The daily ascending and descending observations are used to get the monthly ascending and descending data, which are then arithmetically averaged to produce the AIRS monthly data products (Tian et al. 2019). The diurnal cycle of the TCWV over the TP peaks in the late afternoon to evening, which is consistent with the diurnal cycle of precipitation. The minimum of the TCWV over the eastern TP occurs at about local time 10:00, and the maximum occurs at 18:00 (Ma et al. 2016). Although the diurnal cycle cannot be fully characterized by twice-daily sampling, the Aqua observation is reliable on a monthly scale. The AIRS and AMSU instruments are each cross-track scanning nadir sounders that are coaligned and have a swath approximately 1650 km wide (Tian et al. 2013a). Due to the 45-km granularity of AMSU and 13.5-km granularity of AIRS, the AIRS/AMSU Level 2 data product is generated with a horizontal resolution of 45 km, including a single AMSU spectrum and nine AIRS spectra. The AIRS-only Level 2 data product is generated with a higher horizontal resolution of 13.5 km. The AIRS-only and AIRS/AMSU data sets are both available from 1 September 2002, but only AIRS-only is updated continually, while AIRS/AMSU ended in 24 September 2016. We use the AIRS version 6 Level 3 (gridded) monthly TPW data products from both AIRS-only and AIRS/AMSU which have a spatial resolution of 1° × 1° for the time period of January 2003 to December 2015 (Tian et al. 2013b; Susskind et al. 2014).
There are some studies that have estimated the quality and error sources of a previous version of AIRS/AMSU (Ver. 5) in terms of the global or regional temperature and humidity (Wu 2009; Tian et al. 2010, 2013a; Diao et al. 2013; Wang et al. 2013; Hearty et al. 2014). AIRS/AMSU (Ver. 5) also has the ability to reproduce the total column water vapor distribution and profile over the TP (Zhang et al. 2012, 2013). Compared to the previous version, version 6 has significant improvements in terms of a greater stability, yield, and quality (Susskind et al. 2003, 2011, 2012, 2014). AIRS version 6 is improved compared with version 5 in terms of the temperature profiles and surface-based inversions over Antarctica (Boylan et al. 2015). The AIRS-only product (Ver. 6) can reproduce the mean TCWV on the global scale (Roman et al. 2016). Through a comparison between radiosonde and satellite data sets, our work is a supplement to the assessment of the quality of the AIRS-only and AIRS/AMSU version 6 products in terms of the regional total column water vapor. These two satellite data sets will also be used as a reference for the assessment of reanalysis data.
Reanalysis data: MERRA, MERRA2, NCEP1, NCEP2, CFSR, ERA-I and JRA55Reanalysis data is widely used in climatology research. It constrains model fields towards observations using a consistent assimilation system. Due to the unaltered model and assimilation system used, reanalysis data can reduce the spurious signals caused by changes in the model or analysis method. At the same time, it shows advantages in many other aspects, such as the time range, spatial coverage and variable types. The details of the seven reanalysis data sets we used are provided in Table 1. The variable TCWV can be obtained from the model output directly. Except for NCEP1 and JRA55, most reanalysis data sets are available from 1979, when the satellite came into service. We used monthly reanalysis data sets during the period of January 2003 to December 2015, the same as for the satellite data sets. Before the assessment, all reanalysis data sets were interpolated into the same grid as the satellite data by using the bilinear interpolation technique. Note that all reanalysis data sets have assimilated AIRS or AMSU radiances during their producing processes, except for the early NCEP1 and NCEP2. The different abilities of the reanalysis data are caused by the different models, assimilation systems or observations.Table 1
Reanalysis data sets used
AIRS or AMSU assimilated or not
GEOS atmospheric model, version 5.2
3D-Var GEOS5-DAS: amended GSI
January 1979 to February 2016
0.5° × 0.667°
Rienecker et al. (2011)
GEOS atmospheric model, version 5.12.4
0.5° × 0.625°
Gelaro et al. (2017)
T62/28 level NCEP global spectral model
Kalnay et al. (1996)
same as NCEP1 but with some physics improvements
3D-Var updated data assimilation system
Kanamitsu et al. (2002)
NCEP Coupled Seasonal Forecast System
Saha et al. (2010)
ECMWF IFS, version Cy31r2
Dee et al. (2011)
JMA GSM as of December 2009
Kobayashi et al. (2015)
2.2 Skill-weighting method
3.1 Evaluation of satellite and reanalysis data
The Tibetan Plateau is one of the most complex terrain areas and is known as the Roof of the World due to its high elevation, especially within the western and southern plateau. Consistent with the topographic features, there are eight defined TP radiosonde stations in total, located in the central and eastern parts of the TP. In the western plateau, where the elevation is above 5 km, there is no radiosonde station as of now due to the tough environment (Fig. 1). Because of the uneven distribution of radiosonde stations, in the following analysis, we first evaluate the satellite and reanalysis data with the radiosonde data as the reference and then choose one satellite data set to evaluate the reanalysis data in terms of the spatial pattern.
To evaluate the reliability of the two sets of satellite data in capturing the features of the annual mean climatology of the TCWV over the TP, we interpolate the gridded satellite data into each radiosonde location using bilinear interpolation and then calculated the difference between them (dots in Fig. 2a, b). Compared to the radiosonde data, both satellite data sets overestimate the total column water vapor around the Tsaidam Basin (0.28–0.83 kg m−2) and underestimate it in other regions (− 0.10 to − 3.74 kg m−2). The underestimation (− 2 to − 38%) is stronger than the overestimation (5–14%). The root-mean-square bias of the satellite data is 2.25 kg m−2 compared with the radiosonde data and the difference between the two satellite data sets is negligible (Bias on the top-right of Fig. 2a, b). We chose AIRS/AMSU as the reference to evaluate the reanalysis data sets with respect to the spatial distribution of the annual mean TCWV over the TP.
To evaluate the reanalysis data sets in reproducing the spatial pattern of the annual mean climatology of the TCWV over the TP, we calculated the spatial correlation coefficients R between the reanalysis data and satellite data (Fig. 2c–j). Compared to the satellite data (AIRS/AMSU), all reanalysis data can reproduce the spatial distribution of the total column water vapor over the TP (R = 0.96 in NCEP1 and R = 0.99 in the others), except for NCEP2 (R = 0.92). Compared with the radiosonde data, the root-mean-square biases of the reanalysis data are smaller than those of the satellite data, with the smallest bias in MERRA2 (1.83 kg m−2) and the largest bias in JRA55 (2.23 kg m−2). Different from the spatially uneven biases in the other data sets, NCEP1, ERA-I, JRA55 and the unweighted ensemble show a spatially consistent underestimation, with that of NCEP1 the largest, and with a mean bias of − 1.78 kg m−2.
To evaluate the reanalysis data in terms of the spatial distribution of the annual cycle features, we calculated the spatial correlation coefficients and RMSE between the reanalysis data and AIRS-only (Fig. 4). The rate of the TCWV in the JJAS to the annual total is above 54% over the TP, with a band of high value extending from the southwestern to the northeastern plateau. There are two high-value centers located over the southwestern plateau and Qilian Mountains. Compared to AIRS-only, most of the reanalysis data sets can reproduce the spatial distribution of the seasonal concentration of the TCWV over the TP (R = 0.84–0.94, RMSE = 0.02–0.04), except for NCEP1 (R = 0.72, RMSE = 0.07) and NCEP2 (R = 0.78, RMSE = 0.04). NCEP1 overestimates the seasonal concentration and produces a spurious concentration center in the middle-northern plateau (Fig. 4d), while NCEP2 cannot reproduce the southwestern plateau center and Qilian Mountains center (Fig. 4e).
In summary, both the satellite and reanalysis data show their ability of reproducing the annual cycle of the total column water vapor over the TP, except for NCEP1 and NCEP2. AIRS-only performs better than AIRS/AMSU, and ERA-I and MERRA2 outperform the other reanalysis data sets.
In Fig. 3, the dispersion bars of each box indicate the interannual variability of the TCWV in the corresponding month. The largest variability is seen in the boreal summer, so the following analysis of the interannual variability will be based on the boreal summer data (JJA mean).
To evaluate the reanalysis data in reproducing the spatial pattern of the interannual variability, the relative interannual variability is calculated, which is defined as the standard deviation of the interannual variability divided by the climatology (Fig. 5b–j). The least squares linear trend of each grid has been removed before the calculation. As shown in Fig. 5b, the relative interannual variability of the total column water vapor over the TP is between 1 and 9%, increasing from the southern plateau to western and northeastern plateau. The maximum of relative interannual variability occurs over the western and northern plateaus. Compared to the satellite data (AIRS/AMSU), the reanalysis data sets overestimate the relative interannual variability due to the underestimation of the climatology (Fig. 5c–j). NCEP1 and NCEP2 lose their advantages in reproducing the spatial pattern of the interannual variability (R = 0.72 and 0.84, respectively), as the satellite data were not assimilated. The ensemble mean shows its advantages of comprehensiveness with respect to the relative interannual variability, with the highest spatial correlation coefficient of 0.93 and lowest RMSE of 0.02. MERRA, MERRA2 and JRA55 also perform well in reproducing the spatial pattern of the relative interannual variability of the summer total column water vapor over the TP (R = 0.92, 0.91, and 0.91, respectively).
In summary, both the satellite data and reanalysis data can reproduce the interannual variability of the summer total column water vapor over the TP. AIRS/AMSU performs better than AIRS-only. In terms of the interannual variability series, NCEP1 or NCEP2 is the best choice, but for the spatial relative interannual variability, the simple unweighted ensemble mean, MERRA, MERRA2 and JRA55 are recommended. Because the difference between AIRS-only and AIRS/AMSU is negligible, we will choose AIRS-only as a reference in the following analysis due to its continual update.
The second mode of the EOF analysis is shown in Fig. 7. Revealed by the satellite data (Fig. 7a), the second EOF mode is the consistent change in the total column water vapor in the main body of the plateau, especially over the northern plateau, which is opposite the southern edge of the Himalayas. The second mode explains approximately 25.10% of the total variance. In general, compared to the AIRS-only (Fig. 7c–j), the performance of the reanalysis data in reproducing the second mode is lower than that in reproducing the first mode, characterized by the lower spatial correlation coefficients (R = 0.63–0.93). Reanalysis data sets of low quality show a common weakness in reproducing the opposite variability along the southern edge of the Himalayas to the main body of the TP, taking MERRA, NCEP1 and NCEP2 as examples (R = 0.63–0.78). For the second mode of the EOF analysis, the best choice is JRA55 (R = 0.93, RMSE = 0.21), followed by MERRA2, the simple unweighted ensemble mean, ERA-I, CFSR, NCEP2, MERRA and NCEP1.
The maximum uncertainty of the reanalysis data sets in terms of the annual mean climatology, annual cycle and interannual variability during the period of 2003 to 2015 is located along the edges of the TP (not shown). These regions of high uncertainty should be given priority when new observation stations are planned.
3.2 Skill-weighted ensemble mean of reanalysis data
To obtain the final weighted ensemble mean of the reanalysis data sets, we calculated the unified skill weight for each reanalysis data set using the information mentioned above (Fig. 8a) and the method introduced in Sect. 2.2. As a result, the skill weights for MERRA, MERRA2, NCEP1, NCEP2, CFSR, ERA-I and JRA55 are 0.12, 0.16, 0.06, 0.10, 0.14, 0.22 and 0.21, respectively, in line with their quality depicted in Fig. 8a. These skill weights are used to construct the skill ensemble mean of the reanalysis data.
The skill-weighted ensemble mean of the reanalysis data is then constructed using the weights above. To evaluate its performance compared with the single reanalysis data sets as well as the simple unweighted ensemble mean, a Taylor diagram is applied, as shown in Fig. 8b. The Taylor diagram can summarize different data performance levels in different cases and enable us to concentrate on the main points. The closer a marker is to the REF point, the better the corresponding data set performance (Taylor 2001). As shown in Fig. 8b, the reanalysis data has the ability to reproduce the total column water vapor over the TP in general, as all spatial correlation coefficients are above 0.6. In detail, the reanalysis data is more reliable in the analysis of the annual mean climatology, as all spatial correlation coefficients are above 0.9 and the standard deviations are almost the same as in the observation (AIRS-only), except for NCEP1 and NCEP2. In terms of the annual cycle, the spatial correlation coefficients are all above 0.7, but the standard deviations are larger in the reanalysis than in the observation (REF > 1.00). ERA-I performs best in reproducing the annual cycle of the total column water vapor over the TP. NCEP1 performs worst, as its standard deviation is twice as large as the observation. In terms of the interannual coherent variation, the reanalysis data reproduce the EOF1 pattern well, as all spatial correlation coefficients are above 0.9, except for NCEP1. JRA55 is the best choice for the analysis related to this west–east coherent variation pattern. Compared with EOF1, the spatial correlation coefficients in terms of EOF2 show a larger dispersion between 0.6 and 0.95. JRA55 and MERRA2 are the best choices for an analysis related to this spatially consistent variation pattern, while NCEP1, NCEP2 and MERRA are not recommended.
Clearly, the single reanalysis data set with the best reliability varies for different time scales considered, so the skill-weighted ensemble mean of the reanalysis data is constructed and evaluated. Compared with the simple unweighted ensemble mean, the skill-weighted ensemble mean of the reanalysis data shows a better performance. To be specific, as a result of the skill weighting, the spatial correlation coefficients between the ensemble mean of the reanalysis data and the observation data increase from 0.991, 0.887, 0.965, and 0.919 to 0.993, 0.899, 0.967, and 0.938 for the climatology, annual cycle, EOF1 and EOF2, respectively. The skill-weighted ensemble mean also performs better than most of the single reanalysis data sets, especially for EOF2 (Fig. 8b). Thus, we recommend the application of the skill-weighted ensemble mean of the reanalysis data in future studies on the water cycle over the TP, as it integrates different time scales and performs better than the simple unweighted ensemble mean.
Based on radiosonde observation data, our work provides a comprehensive assessment of two state-of-the-art satellite data sets (version 6 of AIRS-only and AIRS/AMSU) and seven existing reanalysis data sets (MERRA, MERRA2, NCEP1, NCEP2, CFSR, JRA55, ERA-Interim) in capturing the annual mean climatology, annual cycle and interannual variability features of the TCWV over the TP. A skill-weighted ensemble mean of the reanalysis data sets is also constructed and evaluated. The main results are summarized as follows:
The satellite data sets of both version 6 of AIRS-only and AIRS/AMSU have the ability to reproduce the characteristics of the total column water vapor over the TP, and the difference between them is negligible. The AIRS-only version 6 can be used as a substitute for in situ observation in a related analysis of a longer time period due to its high horizontal resolution and continual update.
The quality of the reanalysis data (MERRA, MERRA2, NCEP1, NCEP2, CFSR, ERA-I, JRA55) varies with the time scale considered. In detail, all of them perform well in reproducing the annual mean climatology of the total column water vapor over the TP, except for NCEP1 and NCEP2. ERA-I is more reliable for the analysis of the annual cycle, while NCEP1 is not recommended. In terms of the interannual variability, JRA55, ERA-I, MERRA2 and CFSR are recommended, whereas NCEP1 is not reliable. Generally, the reanalysis data sets of the third generation (ERA-I and JRA55) perform better due to improvements in the model, assimilation system and observations assimilated.
To integrate the different performances of the reanalysis data on different time scales, we constructed a skill-weighted ensemble mean of the reanalysis data, which performs better than the simple unweighted ensemble mean and most of the single reanalysis data sets. The skill weights for MERRA, MERRA2, NCEP1, NCEP2, CFSR, ERA-I and JRA55 are 0.12, 0.16, 0.06, 0.10, 0.14, 0.22 and 0.21, respectively. The application of the skill-weighted ensemble mean of the reanalysis data is recommended in future studies of the water cycle over the TP, as it takes different time scales into account.
There is a striking contrast between the importance of the Tibetan Plateau to the global climate and the lack of abundant in situ observations. Our analysis provides essential information on both the strengths and weaknesses of the existing substitutes for the observational data, including satellite products and reanalysis data. The work can also supplement the G-VAP results from the perspective of the regional Asian water tower.
This work is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences under Grant no. XDA20060102, the National Natural Science Foundation of China under Grant no. 41775091, and the International Partnership Program of Chinese Academy of Sciences (134111KYSB20160031). We also acknowledge the support from Jiangsu Collaborative Innovation Center for Climate Change. We thank Dr. Aiguo Dai and Junhong Wang at University of Albany, USA for providing the homogenized radiosonde data. Considerable gratitude is owed to the various satellite and reanalysis working teams. The AIRS-only, AIRS/AMSU, MERRA and MERRA2 data were provided by NASA (https://disc.gsfc.nasa.gov). The ERA-Interim data were provided by the European Centre for Medium-Range Weather Forecasts (ECMWF, http://www.ecmwf.int/). The JRA55 data were provided by the Climate Prediction Division of the Global Environment and Marine Department at the Japan Meteorological Agency (http://jra.kishou.go.jp/). The NCEP1 and NCEP2 data were provided by the NOAA Earth System Research Laboratory (ESRL, http://www.esrl.noaa.gov/). The CFSR data can be obtained from the CISL Research Data Archive at https://rda.ucar.edu/pub/cfsr.html. We also appreciate two anonymous reviewers for their insightful comments and detailed instructions, which greatly improved the quality of the work.
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