Water Vapor In Upper Troposphere Research Articles

Response of upper tropospheric water vapor to global warming and ENSO

The upper tropospheric water vapor is a key component of Earth's climate. Understanding variations in upper tropospheric water vapor and identifying its influencing factors is crucial for enhancing our comprehension of global climate change. While many studies have shown the impact of El Niño-Southern Oscillation (ENSO) and global warming on water vapor, how they affect the upper tropospheric water vapor remains unclear. Long-term, high-precision ERA5 specific humidity data from the European Centre for Medium-Range Weather Forecasts (ECMWF) provided the data foundation for this study. On this basis, we successfully obtained the patterns of global warming (Independent Component 1, IC1) and ENSO (Independent Component 2, IC2) by employing the strategy of independent component analysis (ICA) combined with non-parametric optimal dimension selection to investigate the upper tropospheric water vapor variations and responses to ENSO and global warming. The results indicate that global warming and ENSO are the primary factors contributing to water vapor variations in the upper troposphere, achieving the significant correlations of 0.87 and 0.61 with water vapor anomalies respectively. Together, they account for 86% of the global interannual variations in water vapor. Consistent with previous studies, our findings also find positive anomalies in upper tropospheric water vapor during El Niño years and negative anomalies during La Niña years. Moreover, the influence extent of ENSO on upper tropospheric water vapor varies with the changing seasons.

Revisiting the Link Between Thunderstorms and Upper Tropospheric Water Vapor

AbstractAs the Earth's temperatures continue to rise due to increasing greenhouse gases in the atmosphere, a large portion of the warming is due to positive feedbacks from increasing atmospheric water vapor or specific humidity (SH). Some of this water vapor in the boundary layer is transported via deep convection to the upper troposphere (as droplets and ice crystals), moistening the upper troposphere. These small changes in SH in the upper troposphere have a significant impact on the Earth's radiation balance. We compared global daily lightning from the WWLLN data set, and SH data from the ERA5 reanalysis product for 2019, at a spatial resolution of 5°. Our findings show high spatial and temporal correlations between the lightning activity and the SH concentrations in the upper troposphere. The best correlations (r2 = 0.72, p << 0.001) are between lightning activity and UTWV at the 200 mb level (∼12 km altitude), although the correlations with SH at 300 and 400 mb were only slightly lower. Lightning and SH migrate with the seasons north and south of the equator with both parameters showing maxima in the summer hemisphere around 10° latitude. Furthermore, the correlation increases slightly (r2 = 0.73) if a 1‐day lag is considered between the lightning activity and the UTWV. We find that the daily global SH in the upper troposphere can be best estimated using daily global lightning frequency and a power law having an exponent of 0.33.

Convective tropopause over the tropics: Climatology, seasonality, and inter-annual variability inferred from long-term FORMOSAT-3/COSMIC-1 RO data

The present study aims to provide a comprehensive analysis of the convective tropopause height (COTH) and temperature (COT-T) over the tropics using long-term radio occultation (RO) measurements from the Formosa Satellite Mission 3–Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC) from July 2006 to December 2018. The study reveals that the COT-H (COT-T) exhibits a significant longitudinal structure with a maximum (minimum) over the Western Pacific and two secondary maxima (minima) over the American and African regions. The seasonal cycles of COT-H and COT-T are also prominent, with higher and colder values over the Western Pacific in boreal winter and over the Indian region in boreal summer. This seasonality in COT-H and COT-T is consistent with the seasonal process of tropical convective activity and is explained by outgoing longwave radiation (OLR) data. The correlation analysis shows that COT-H (COT-T) is negatively (positively) correlated with OLR, with the highest correlation found in the Indian region. Additionally, the study investigates the relationship between COT-H (COT-T) and upper tropospheric water vapor (UTWV) over the various regions in the tropics. Over the Indian region, the COT-H (COT-T) exhibits a positive (negative) correlation with UTWV, while over the Western Pacific, American, and African regions, it displays a negative (positive) correlation. Finally, the spatial distribution of the convective tropopause in El Niño and La Niña winters was examined. Results show that the COT-H (COT-T) is higher (lower) over the central and eastern Pacific Ocean and lower (higher) over the maritime continent during El Niño compared with La Niña.

Are thunderstorms linked to the rapid Sea ice loss in the Arctic?

Sea ice in the Arctic grows during the winter and retreats in the summer. The highly reflective white surface of sea ice reflects solar energy, cooling the planet. However, when it melts, the darker ocean absorbs more solar radiation, reinforcing the cycle of melting sea ice. Since sea ice plays a critical role in regulating Earth's climate, changes in sea ice may influence global weather patterns and ocean circulations. In addition to the rise in greenhouse gases in the Arctic, upper tropospheric water vapor (UTWV) or the specific humidity (SH) is also increasing, while acting as an additional greenhouse gas trapping in heat released from the Earth's surface. While annual greenhouse gas concentrations in the Arctic are increasing rather smoothly over time, the interannual variability in sea ice is shown here to be linked to the variability of UTWV at 400 hPa in the Arctic. We propose that increasing thunderstorm activity in the Arctic in the last decade is increasing the water vapor in the upper troposphere, partially explaining the interannual variability, and long term decrease, in the Arctic summer sea ice extent. Both the temporal and spatial analysis between UTWV and Sea Ice changes show that the decline of the summer sea ice cover in the Arctic may be linked to the increasing thunderstorm activity in the Arctic in the last decades.

Improving Tropical Cyclogenesis Forecasts of Hurricane Irma (2017) through the Assimilation of All-Sky Infrared Brightness Temperatures

Abstract The assimilation of satellite all-sky infrared (IR) brightness temperatures (BTs) has been shown in previous studies to improve intensity forecasts of tropical cyclones. In this study, we examine whether assimilating all-sky IR BTs can also potentially improve tropical cyclogenesis forecasts by improving the pregenesis cloud and moisture fields. By using an ensemble-based data assimilation system, we show that the assimilation of upper-tropospheric water vapor channel BTs observed by the Meteosat-10 SEVIRI instrument two days before the formation of a tropical depression improves the genesis forecast of Hurricane Irma (2017), a classic Cape Verde storm, by up to 24 h while also capturing its later rapid intensification in deterministic forecasts. In an experiment that withholds the assimilation of all-sky IR BTs, the assimilation of conventional observations from the Global Telecommunications System (GTS) leads to the premature genesis of Hurricane Irma by at least 24 h. This premature genesis is shown to result from an overestimation of the spatial coverage of deep convection within the African easterly wave (AEW) from which Irma eventually forms. The gross overestimation of deep convection without all-sky IR BTs is accompanied by higher column saturation fraction, stronger low-level convergence, and the earlier spinup of a low-level meso-β-scale vortex within the AEW that ultimately becomes Hurricane Irma. Through its adjustment to the initial moisture and cloud conditions, the assimilation of all-sky IR BTs leads to a more realistic convective evolution in forecasts and ultimately a more realistic timing of genesis. Significance Statement Every year hurricanes impact the lives of thousands of people living along the eastern coast of the United States. Many of these storms originate from tropical disturbances that exit the west coast of Africa. To give the public more warning time ahead of these storms, it is important to improve the forecasts of their formation. This study uses a system developed at The Pennsylvania State University to incorporate satellite observations into forecasts of a classic Cape Verde storm, Hurricane Irma (2017), two days before it formed. By using satellite-collected radiances, we improve the timing of its formation by up to 24 h due to a better representation of the mesoscale tropical disturbance from which it originated.

The Potential Benefits of Handling Mixture Statistics via a Bi-Gaussian EnKF: Tests With All-Sky Satellite Infrared Radiances.

The meteorological characteristics of cloudy atmospheric columns can be very different from their clear counterparts. Thus, when a forecast ensemble is uncertain about the presence/absence of clouds at a specific atmospheric column (i.e., some members are clear while others are cloudy), that column's ensemble statistics will contain a mixture of clear and cloudy statistics. Such mixtures are inconsistent with the ensemble data assimilation algorithms currently used in numerical weather prediction. Hence, ensemble data assimilation algorithms that can handle such mixtures can potentially outperform currently used algorithms. In this study, we demonstrate the potential benefits of addressing such mixtures through a bi-Gaussian extension of the ensemble Kalman filter (BGEnKF). The BGEnKF is compared against the commonly used ensemble Kalman filter (EnKF) using perfect model observing system simulated experiments (OSSEs) with a realistic weather model (the Weather Research and Forecast model). Synthetic all-sky infrared radiance observations are assimilated in this study. In these OSSEs, the BGEnKF outperforms the EnKF in terms of the horizontal wind components, temperature, specific humidity, and simulated upper tropospheric water vapor channel infrared brightness temperatures. This study is one of the first to demonstrate the potential of a Gaussian mixture model EnKF with a realistic weather model. Our results thus motivate future research toward improving numerical Earth system predictions though explicitly handling mixture statistics.

Extending the HIRS Data Record with IASI Measurements

The High-Resolution Infrared Radiation Sounder (HIRS) on the NOAA and the MetOp satellite series have provided global sounding measurements since the late 1970s, spanning over 40 years. These measurements have been useful in climate change detection, numerical weather prediction, and development of long-term climate data records of profiles of atmospheric temperature and humidity, cloud climatology, upper tropospheric water vapor, outgoing longwave radiation, etc. However, the HIRS instrument is being replaced by the new generation of sounders such as the hyperspectral Infrared Atmospheric Sounding Interferometer (IASI) on recently launched satellites. In order to continue and extend the HIRS record, we use IASI measurements to simulate and derive HIRS-like data for the 12 HIRS longwave channels. The MetOp satellite operated by EUMETSAT carries both the HIRS and the hyper-spectral IASI instrument with accurate spectral and radiometric calibration, providing a great opportunity to consistently calibrate the measurements. The IASI radiances are convolved with the HIRS spectral response functions to produce IASI-simulated HIRS (IHIRS) for the longwave channels. In the present work, IHIRS data are collocated and compared with HIRS observed radiances on the same satellite to develop a calibration table for each of the ascending/descending orbits and cloudy and clear categories. The resulting inter-instrument calibrated IHIRS data was found to agree with HIRS brightness temperatures within 0.05 K for all longwave channels.

Stratospheric Modulation of the MJO through Cirrus Cloud Feedbacks

Abstract Recent observations have indicated significant modulation of the Madden–Julian oscillation (MJO) by the phase of the stratospheric quasi-biennial oscillation (QBO) during boreal winter. Composites of the MJO show that upper-tropospheric ice cloud fraction and water vapor anomalies are generally collocated, and that an eastward tilt with height in cloud fraction exists. Through radiative transfer calculations, it is shown that ice clouds have a stronger tropospheric radiative forcing than do water vapor anomalies, highlighting the importance of incorporating upper-tropospheric–lower-stratospheric processes into simple models of the MJO. The coupled troposphere–stratosphere linear model previously developed by the authors is extended by including a mean wind in the stratosphere and a prognostic equation for cirrus clouds, which are forced dynamically and allowed to modulate tropospheric radiative cooling, similar to the effect of tropospheric water vapor in previous formulations. Under these modifications, the model still produces a slow, eastward-propagating mode that resembles the MJO. The sign of zonal mean wind in the stratosphere is shown to control both the upward wave propagation and tropospheric vertical structure of the mode. Under varying stratospheric wind and interactive cirrus cloud radiation, the MJO-like mode has weaker growth rates under stratospheric westerlies than easterlies, consistent with the observed MJO–QBO relationship. These results are directly attributable to an enhanced barotropic mode under QBO easterlies. It is also shown that differential zonal advection of cirrus clouds leads to weaker growth rates under stratospheric westerlies than easterlies. Implications and limitations of the linear theory are discussed. Significance Statement Recent observations have shown that the strength of the Madden–Julian oscillation (MJO), a global-scale envelope of wind and rain that slowly moves eastward in the tropics and dominates global-weather variations on time scales of around a month, is strongly influenced by the direction of the winds in the lower stratosphere, the layer of the atmosphere that lies above where weather occurs. So far, modeling studies have been unable to reproduce this connection in global climate models. The purpose of this study is to investigate the mechanisms through which the stratosphere can modulate the MJO, by using simple theoretical models. In particular, we point to the role that ice clouds high in the atmosphere play in influencing the MJO.

Impact of the El Niño–Southern Oscillation on upper-tropospheric water vapor

The majority of present-day and future global warming is related to various positive feedbacks in the climate system that tend to amplify the initial radiative forcing from increasing greenhouse gases in the atmosphere. One essential positive feedback is the rise in upper tropospheric water vapor (UTWV) or the specific humidity (SH) that acts as an intense greenhouse gas trapping in additional heat released from the Earth's surface. In this paper we expand on previous studies showing that the interannual variability of UTWV in the tropical and extratropical climates is strongly linked with the El Nino-Southern Oscillation (ENSO). We have expanded on previous studies by looking at the response of the long-term UTWV concentrations to the ENSO over a 41-year period (1979–2019) using the ERA5 reanalysis data. For the first time we look at the physical mechanism for the El Nino and La Nina connections to UTWV that is shown to be related to changes in deep convection over the central and eastern Pacific Ocean. In agreement with previous studies, we find that El Nino years have positive anomalies in UTWV, while in La Nina years there are negative anomalies in UTWV (±10%).

Intercomparison of upper tropospheric and lower stratospheric water vapor measurements over the Asian Summer Monsoon during the StratoClim campaign

Abstract. In situ measurements in the climatically important upper troposphere–lower stratosphere (UTLS) are critical for understanding controls on cloud formation, the entry of water into the stratosphere, and hydration–dehydration of the tropical tropopause layer. Accurate in situ measurement of water vapor in the UTLS however is difficult because of low water vapor concentrations (<5 ppmv) and a challenging low temperature–pressure environment. The StratoClim campaign out of Kathmandu, Nepal, in July and August 2017, which made the first high-altitude aircraft measurements in the Asian Summer Monsoon (ASM), also provided an opportunity to intercompare three in situ hygrometers mounted on the M-55 Geophysica: ChiWIS (Chicago Water Isotope Spectrometer), FISH (Fast In situ Stratospheric Hygrometer), and FLASH (Fluorescent Lyman-α Stratospheric Hygrometer). Instrument agreement was very good, suggesting no intrinsic technique-dependent biases: ChiWIS measures by mid-infrared laser absorption spectroscopy and FISH and FLASH by Lyman-α induced fluorescence. In clear-sky UTLS conditions (H2O<10 ppmv), mean and standard deviations of differences in paired observations between ChiWIS and FLASH were only (-1.4±5.9) % and those between FISH and FLASH only (-1.5±8.0) %. Agreement between ChiWIS and FLASH for in-cloud conditions is even tighter, at (+0.7±7.6) %. Estimated realized instrumental precision in UTLS conditions was 0.05, 0.2, and 0.1 ppmv for ChiWIS, FLASH, and FISH, respectively. This level of accuracy and precision allows the confident detection of fine-scale spatial structures in UTLS water vapor required for understanding the role of convection and the ASM in the stratospheric water vapor budget.

The Spatiotemporal Patterns of the Upper-Tropospheric Water Vapor over the Tibetan Plateau in Summer Based on EOF Analysis

Abstract Using empirical orthogonal function (EOF) analysis, we investigate the spatial pattern and temporal variation of the upper-tropospheric water vapor (UTWV) over the Tibetan Plateau (TP) in summer based on the fifth-generation ECMWF atmospheric reanalysis (ERA5). The main factors affecting UTWV in different regions over the TP are also discussed. The results show that the spatial distribution of the UTWV over the TP mainly displays the so-called uniform, east–west dipole, and north–south dipole modes. The three modes show significant periods of 5, 8, and 4 years, respectively. The first mode exhibits a small but significant positive trend over the TP, which can be mainly attributed to the intensification of the South Asian summer monsoon and accounts for 31% of the UTWV variance, followed by the South Asian high and the Indian low with 17% and 16%, respectively. The second mode shows opposite variation of water vapor over the eastern and western TP, mainly resulting from the weakened westerly wind over the eastern TP and the enhanced westerly wind over the western TP, which accounts for 57% of the UTWV variance. The third mode exhibits a positive trend of water vapor over the northern TP, mainly attributed to the ascending motion associated with local anticyclonic circulation over the northeastern TP and the decrease of the static stability over the northern TP, which account for 45% and 11% of the UTWV variance, respectively. The tropopause folds also contribute to the positive trend of water vapor shown in the third mode.

Retrieval of Tropospheric Water Vapor From Airborne Far‐Infrared Measurements: A Case Study

AbstractWe describe studies undertaken in support of the Far‐infrared Outgoing Radiation Understanding and Monitoring mission, European Space Agency's ninth Earth Explorer, designed to investigate whether airborne observations of far‐infrared radiances can provide beneficial information on mid and upper tropospheric water vapor concentrations. Initially we perform a joint temperature and water vapor retrieval and show that the water vapor retrieval exploiting far‐infrared measurements from the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) shows improvement over the a‐priori Unified Model global forecast when compared to in situ dropsonde measurements. For this case the improvement is particularly noticeable in the mid‐upper troposphere. Equivalent retrievals using mid‐infrared radiances measured by the Airborne Research Interferometer Evaluation System (ARIES) show much reduced performance, with the degrees of freedom for signal (DFS), reduced by a factor of almost 2. Further sensitivity studies show that this advantage is decreased, but still present when the spectral resolution of the TAFTS measurements is reduced to match that of ARIES. The beneficial role of the far infrared for this case is further confirmed by performing water vapor only retrievals using ARIES and TAFTS individually, and then in combination. We find that the combined retrieval has a DFS value of 6.7 for water vapor, marginally larger than that obtained for the TAFTS retrieval and almost twice as large as that obtained for ARIES. These results provide observational support of theoretical studies highlighting the potential improvement that far‐infrared observations could bring for the retrieval of tropospheric water vapor.

Observed HIRS and Aqua MODIS Thermal Infrared Moisture Determinations in the 2000s

This paper compares the tropospheric moisture data records derived from High-resolution Infrared Radiation Sounder (HIRS) and Moderate Resolution Imaging Spectro-radiometer (MODIS) measurements from the years 2003 through 2013. Total Precipitable Water Vapor (TPW) and Upper Tropospheric Precipitable Water Vapor (UTPW) are derived using the infrared spectral bands in the CO2 and H2O absorption bands as well as in the atmospheric windows. Retrieval of TPW and UTPW uses a statistical regression algorithm performed using clear sky radiances (and Brightness Temperatures) measured over land and ocean for both day and night. The TPW and UTPW seasonal cycles of HIRS and MODIS observations are found to be in synchronization with zonal mean values for one degree latitude bands within 2.0 mm and 0.07 mm, respectively.

Accuracy of Vaisala RS41 and RS92 Upper Tropospheric Humidity Compared to Satellite Hyperspectral Infrared Measurements

Radiosondes are important for calibrating satellite sensors and assessing sounding retrievals. Vaisala RS41 radiosondes have mostly replaced RS92 in the Global Climate Observing System (GCOS) Reference Upper Air Network (GRUAN) and the conventional network. This study assesses RS41 and RS92 upper tropospheric humidity (UTH) accuracy by comparing with Infrared Atmospheric Sounding Interferometer (IASI) upper tropospheric water vapor absorption spectrum measurements. Using single RS41 and RS92 soundings at three GRUAN and DOE Atmospheric Radiation Measurement (ARM) sites and dual RS92/RS41 launches at three additional GRUAN sites, collocated with cloud-free IASI radiances (OBS), we compute Line-by-Line Radiative Transfer Model radiances for radiosonde profiles (CAL). We analyze OBS-CAL differences from 2015 to 2020, for daytime, nighttime, and dusk/dawn separately if data is available, for standard (STD) RS92 and RS41 processing, and RS92 GRUAN Data Processing (GDP; RS41 GDP is in development). We find that daytime RS41 (even without GDP) has ~1% smaller UTH errors than GDP RS92. RS41 may still have a dry bias of 1–1.5% for both daytime and nighttime, and a similar error for nighttime RS92 GDP, while standard RS92 may have a dry bias of 3–4%. These sonde humidity biases are probably upper limits since “cloud-free” scenes could still be cloud contaminated. Radiances computed from European Centre for Medium-Range Weather Forecasts (ECMWF) analyses match better than radiosondes with IASI measurements, perhaps because ECMWF assimilates IASI measurements. Relative differences between RS41 STD and RS92 GDP, or between radiosondes and ECMWF humidity profiles obtained from the radiance analysis, are consistent with their differences obtained directly from the RH measurements.

Assessment of Upper Tropospheric Water Vapor Monthly Variation in Reanalyses With Near‐Global Homogenized 6.5‐μm Radiances From Geostationary Satellites

AbstractThe monthly variation of upper tropospheric water vapor (UTWV) simulated by six reanalysis data sets is evaluated with homogenized water vapor radiance observations from international geostationary (GEO) weather satellites by using a profile‐to‐radiance approach over 45°N to 45°S regions for the period 2015–2017. Results show that reanalysis data sets have an overall good agreement with observations. However, a widespread wet bias is found in all reanalyses and is more dominant in large‐scale subsidence regions. JRA55 has the smallest wet bias while MERRA‐2 exhibits the most humid upper troposphere. The temporal variation of brightness temperatures in response to the warm phase of El Niño‐Southern Oscillation (ENSO) in 2015–2016 indicates that the UTWV field is regulated by both ascending and descending branches of the large‐scale circulation. All six reanalyses roughly capture the temporal variation of UTWV in the developing and decay year of this ENSO event. However, they tend to overestimate the eastward propagation of high UTWV in the developing year, especially MERRA‐2. The UTWV gradient over the tropical Pacific in the decay year is underestimated, with a dry bias over the convective western Pacific and a wet bias over the eastern Pacific in reanalysis data sets. These results may provide a useful tool for the climate modeling community for identifying and solving problems associated with UTWV simulation.

High-Resolution Fengyun-4 Satellite Measurements of Dynamical Tropopause Structure and Variability

The dynamical tropopause is the interface between the stratosphere and the troposphere, whose variation gives indication of weather and climate changes. In the past, the dynamical tropopause height determination mainly depends on analysis and diagnose methods. While, due to the high computational cost, it is difficult to obtain tropopause structures with high spatiotemporal resolution in real time by these methods. To solve this problem, the statistical method is used to establish the dynamical tropopause pressure retrieval model based on Fengyun-4A geostationary meteorological satellite observations. Four regression schemes including random forest (RF) regression are evaluated. By comparison with GEOS-5 (the Goddard Earth Observing System Model of version 5) and ERA-Interim (European Center for Medium-Range Weather Forecasts Reanalysis-Interim) reanalysis, it is found that among the four schemes, the RF-based retrieval model is most accurate and reliable (RMSEs (root mean square errors) are 25.99 hPa and 43.05 hPa, respectively, as compared to GEOS-5 and ERA-Interim reanalysis). A series of sensitivity experiments are performed to investigate the contributions of the predictors in the RF-based model. Results suggest that 6.25 μm channel information representing the distributions of the potential vorticity and water vapor in upper troposphere has the greatest contribution, while 10.8 and 12 μm channels information have relatively weak influences. Therefore, a simplified model without involving a brightness temperature of 10.8 and 12 μm can be adopted to improve the calculation efficiency.

A Study on the Trend of the Upper Tropospheric Water Vapor over the Tibetan Plateau in Summer

Using ERA-Interim and ERA-5 reanalysis datasets combined with measurements from the Microwave Limb Sounder (MLS), we divided the Tibetan Plateau (TP) into three regions to analyse the long-term trends of water vapor in the upper troposphere and its possible mechanisms. The conclusions are as follows. The water vapor at 200 hPa in the summers from 1979 to 2016 over Region 1 (34°N-40°N, 80°E-100°E) shows a positive trend, which is caused by the rising surface temperature and the northward shift of the South Asian High (SAH). The positive trend of water vapor over Region 2 (28°N-33°N, 80°E-90°E) is attributed to the enhanced South Asian summer monsoon, which leads to increases in horizontal transport of water vapor and vertical water vapor flux. The increase in the vertical transport of water vapor is responsible for the increase in water vapor over the Region 2. The negative trend of water vapor over Region 3 (27°N-30°N, 93°E-98°E) is mainly due to the weakening of water vapor transport caused by the weakening of convective activities.

All-sky assimilation of infrared radiances sensitive to mid- and upper-tropospheric moisture and cloud

Abstract. All-sky assimilation of infrared (IR) radiances has not yet become operational at any weather forecasting centre, but it promises to bring new observations in sensitive areas and avoid the need for cloud detection. A new all-sky IR configuration gives results comparable to (and in some areas better than) clear-sky assimilation of the same data, meaning that operational implementation is now feasible. The impact of seven upper-tropospheric water vapour (WV) sounding channels from the Infrared Atmospheric Sounding Interferometer (IASI) is evaluated in both all-sky and clear-sky approaches. All-sky radiative transfer simulations (and the forecast model's cloud fields) are now sufficiently accurate that systematic errors are comparable to those of clear-sky assimilation outside of a few difficult areas such as deep convection. All-sky assimilation brings 65 % more data than clear-sky assimilation globally, with the biggest increases in midlatitude storm tracks and tropical convective areas. However, all-sky gives slightly less weight to any one observation than in the clear-sky approach. In the midlatitudes, all-sky and clear-sky assimilation have similarly beneficial impact on mid- and upper-tropospheric dynamical forecast fields. Here the addition of data in cloudy areas is offset by the slightly lower weight given to the observations. But in the tropics, all-sky assimilation is significantly more beneficial than clear-sky assimilation, with improved dynamical short-range forecasts throughout the troposphere and stratosphere.

Interannual variations of water vapor in the tropical upper troposphere and the lower and middle stratosphere and their connections to ENSO and QBO

Abstract. In this study, we analyze the Aura Microwave Limb Sounder water vapor data in the tropical upper troposphere and the lower and middle stratosphere (UTLMS) (from 215 to 6 hPa) for the period from August 2004 to September 2017 using time-lag regression analysis and composite analysis to explore the interannual variations of tropical UTLMS water vapor and their connections to El Niño–Southern Oscillation (ENSO) and quasi-biennial oscillation (QBO). Our analysis shows that the interannual tropical UTLMS water vapor anomalies are strongly related to ENSO and QBO which together can explain more than half (∼ 50 %–60 %) but not all variance of the interannual tropical water vapor anomalies. We find that ENSO's impact is strong in the upper troposphere (∼ 215–∼ 120 hPa) and near the tropopause (∼ 110–∼ 90 hPa), with a ∼ 3-month lag but weak in the lower and middle stratosphere (∼ 80 to ∼ 6 hPa). In contrast, QBO's role is large in the lower and middle stratosphere, with an upward-propagating signal starting at the tropopause (100 hPa) with a ∼ 2-month lag, peaking in the middle stratosphere near 15 hPa with a ∼ 21-month lag. The phase lag is based on the 50 hPa QBO index used by many previous studies. This observational evidence supports that the QBO's impact on the tropical stratospheric water vapor is from its modulation on the tropical tropopause temperature and then transported upward with the tape recorder as suggested by many previous studies. In the upper troposphere, ENSO is more important than QBO for the interannual tropical water vapor anomalies that are positive during the warm ENSO phases but negative during the cold ENSO phases. Near the tropopause, both ENSO and QBO are important for the interannual tropical water vapor anomalies. Warm ENSO phase and westerly QBO phase tend to cause positive water vapor anomalies, while cold ENSO phase and easterly QBO phase tend to cause negative water vapor anomalies. As a result, the interannual tropical water vapor anomalies near the tropopause are different depending on different ENSO and QBO phase combinations. In the lower and middle stratosphere, QBO is more important than ENSO for the interannual tropical water vapor anomalies. For the westerly QBO phases, interannual tropical water vapor anomalies are positive near the tropopause and in the lower stratosphere but negative in the middle stratosphere and positive again above. Vice versa for the easterly QBO phases.

Correlated observation error models for assimilating all-sky infrared radiances

Abstract. The benefit of hyperspectral infrared sounders to weather forecasting has been improved with the representation of inter-channel correlations in the observation error model. A further step would be to assimilate these observations in all-sky conditions. However, in cloudy skies, observation errors exhibit much stronger inter-channel correlations, as well as much larger variances, compared to clear-sky conditions. An observation error model is developed to represent these effects, building from the symmetric error models developed for all-sky microwave assimilation. The combination of variational quality control with correlated errors is also introduced. The new error model is tested in all-sky assimilation of seven water vapour sounding channels from the Infrared Atmospheric Sounding Interferometer (IASI). However, its initial formulation degrades both tropospheric and stratospheric analyses. To explain this, the eigendeparture and eigenjacobian are introduced as a way of understanding the effect of correlated observation errors in data assimilation. The trailing eigenvalues can be problematic because they strongly amplify high-order harmonic combinations of the water vapour channels, which could have at least three consequences. First, if there are small inter-channel biases, these can be greatly amplified. Second, the trailing eigenjacobians map onto features resembling gravity waves that the data assimilation may not be able to handle. Finally, these harmonic combinations can amplify trace sensitivities, for example, revealing a strong upper stratospheric sensitivity over high cloud in what are usually mid- to upper-tropospheric water vapour channels. A likely explanation is the sensitivity to gravity wave features that are present in the observations but hard for the data assimilation to handle. After reducing the sensitivity to the trailing eigenjacobians, the new error covariance matrix gives good results in all-sky infrared assimilation.