Bias and spread in Coupled Model Intercomparison Project Phase 5 simulated vertical specific humidity (q) structure are examined and related to both precipitation and column water vapor (cwv) near ...Manaus, Brazil, site of the recent Green Ocean Amazon campaign. Simulated seasonal mean q profiles are typically too dry, especially at low levels and during the local dry season, consistent with previously identified surface hydroclimate biases in the Amazon. Multimodel empirical orthogonal function analysis of the models' monthly climatological q profiles indicates two significant modes of ensemble spread in moisture vertical structure, with the leading mode peaked at low levels and the second mode in the lower free troposphere (LFT). While both modes project onto simulated cwv spread, only the first projects on precipitation, suggesting inconsistent sensitivity of simulated rainfall to LFT moisture. Relative to observations, models with high cwv and low‐level moisture errors tend to exhibit high precipitation error.
Key Points
CMIP5 vertical moisture profiles in the Amazon are too dry compared to observations at low levels, especially during the dry season
Two modes capture spread in vertical moisture structure, with one mode peaked at low levels and the other in the lower free troposphere
Comparing these modes to rainfall spread points to inconsistent sensitivity of CMIP5‐simulated rainfall to vertical moisture structure
Frequency of AERs in three groups..
ABSTRACT
Atmospheric rivers (ARs), filamentary patterns of strong water vapour fluxes, play a prominent role in global poleward moisture transport and have ...profound impacts on extreme rainfalls (ERs). Previous AR research has mainly focused on the mid‐latitude regions, whereas the characteristics of ARs in low latitudes and their relationship with local ERs remain largely unknown. This study investigates the spatiotemporal characteristics of ARs over the Bay of Bengal and their relationship with ERs after landing on the northern Indian subcontinent using the ERA‐Interim reanalysis data. During the study period from 1979 to 2011, a total of 149 ARs have been identified, which feature a bimodal temporal pattern with more events observed in May and October. The AR axes generally stretch northeastwards over the bay and land in Bangladesh and Burma. A total of 24% of ARs occurring during tropical cyclones implies a possible connection between them, in addition to the similar intra‐annual distribution. In summer, as the tropical cyclones are weak and the northward water vapour flux decreases due to topographic blocking of the Western Ghats, it is less likely to form intensified water vapour pathway, though the atmospheric humidity is high in the study region. Furthermore, a close correlation between ARs and ERs is manifested. A large proportion of ARs would lead to ERs, with a small fraction of ERs occur after ARs. In addition, although persistent ARs constitute the majority of identified events, rainfall intensity will not be enhanced by the increase in AR duration. This study enriches the knowledge of AR characteristics in low latitudes and provides new pathways to understand the hydrological cycles in the Indian Peninsula and the Bay of Bengal.
In this study, the authors calibrated the performance of the Regional Climate Model, version 3 (RegCM3), with the Massachusetts Institute of Technology (MIT)–Emanuel cumulus parameterization scheme ...over the Coordinated Regional Climate Downscaling Experiment (CORDEX) East Asia domain by tuning seven selected parameters based on the multiple very fast simulated annealing (MVFSA) approach. The seven parameters were selected based on previous studies using RegCM3 with the MIT–Emanuel convection scheme. The results show the simulated spatial pattern of rainfall, and the probability density function distribution of daily rainfall rates is significantly improved in the optimal simulation. Sensitivity analysis suggests that the parameter relative humidity criteria (RHC) has the largest effect on the model results. Followed by an increase of RHC, an increase of total rainfall is found over the northern equatorial western Pacific, mainly contributed by the increase of explicit rainfall. The increases of the convergence of low-level water vapor transport and the associated increases in cloud water favor the increase of explicit rainfall. The identified optimal parameters constrained by total rainfall have positive effects on the low-level circulation and surface air temperature. Furthermore, the optimized parameters based on the chosen extreme case are transferable to a normal case and the model’s new version with a mixed convection scheme.
Water vapor plays a vital role in the Arctic sea ice and climate change. The Atmospheric Infrared Sounder (AIRS) and the Moderate Resolution Imaging Spectroradiometer (MODIS) are useful tools for ...studying the water vapor variations over the entire Arctic. In this paper, we discuss precipitable water vapor (PWV) variations from the AIRS and MODIS Level-3 data products in response to Arctic sea ice change during December 2002-November 2016. The results indicate that both AIRS- and MODIS-derived PWV measurements tend to be underestimated, and this underestimation is generally greater for the latter than the former over both land and ocean. Additionally, both the AIRS and MODIS retrievals suggest that the atmospheres are drier in the solid sea ice pack and, to a lesser extent, in the marginal ice zone, than in the open ocean in each season. Moreover, statistically significant correlations between the PWV and sea ice concentration (SIC) anomalies are generally observed in all seasons under different ice conditions except in the open ocean, where variations in the PWV may be attributed to the poleward moisture transport from lower latitudes. The Arctic PWV and SIC variation patterns are also dominated by the atmospheric and ice conditions in the solid sea ice pack, respectively. The monthly mean PWV and SIC time series analysis further reveals that the Arctic PWV variations tend to be affected by a delayed effect of approximately 1.2-1.3 months from SIC changes from a climatological perspective.
Abstract This study presents a new formulation for the norms and scalar products used in tangent linear or adjoint models to determine forecast errors and sensitivity to observations and to calculate ...singular vectors. The new norm is derived from the concept of moist-air available enthalpy, which is one of the availability functions referred to as exergy in general thermodynamics. It is shown that the sum of the kinetic energy and the moist-air available enthalpy can be used to define a new moist-air squared norm that is quadratic in 1) wind components, 2) temperature, 3) surface pressure, and 4) water vapor content. Preliminary numerical applications are performed to show that the new weighting factors for temperature and water vapor are significantly different from those used in observation impact studies, and are in better agreement with observed analysis increments. These numerical applications confirm that the weighting factors for water vapor and temperature exhibit a large increase with height (by several orders of magnitude) and a minimum in the midtroposphere, respectively.
Using the ERA5 reanalysis dataset during the period 1979–2019, the diurnal variation in summer water vapor budget (Bt) over the Tibetan Plateau (TP) is investigated in this study. It is found that ...the TP Bt shows a distinct diurnal cycle. It tends to increase in the morning, reaches a peak in the afternoon, and falls to a minimum in the early morning. The diurnal variations in four boundary water vapor budgets of the TP contribute to the growth in the TP Bt from the early morning to the afternoon, of which the western and eastern boundaries are more important. To understand the reasons for the diurnal variations in boundary water vapor budgets, the temporal evolutions of water vapor transports and relevant circulations at the four boundaries are examined. The results show that the temporal evolutions of water vapor transports and budgets at the four boundaries are essentially regulated by the changes in the orographic thermodynamic effect. Specifically, rapid and strong warming (cooling) on the TP slopes generates anomalous water vapor inputs (outputs) by anomalous upslope (downslope) flows during the daytime (nighttime). At the southern and western boundaries, apart from the terrain effects, the diurnal variation in the Indian southerly monsoon also has an effect on the changes in water vapor budgets by modulating the water vapor input towards the TP below 700 hPa. At the northern and eastern boundaries, under the orographic thermodynamic effects, low-level water vapor transports towards the TP accompanying by plateau-scale vertical circulations, exist significant diurnal variations and thereby adjust the boundary water vapor budgets. In this study, it is also found that the deviated water vapor flux vectors over the TP present a daily clockwise rotation, which mainly results from the diurnal variation in wind below 450 hPa. In addition, the largest amount of precipitation over the TP occurs 2–3 h after the Bt peak.
Water vapor and cirrus clouds in the tropical tropopause layer (TTL) are important for the climate and are largely controlled by temperature in the TTL. On interannual timescales, both stratospheric ...and tropospheric modes of the large‐scale variability could affect temperatures in the TTL. Here multiple linear regression (MLR) is used to investigate explained variance in the cold point tropopause temperature (CPT), cold point tropopause height (CPZ), 83 hPa water vapor (WV83), 83 hPa ozone (O383), and total cirrus cloud fraction with cloud base (TTLCCF) and top (ALLCF) above 14.5 km, all averaged over 15°S‐15°N. Predictors of the MLR are a set of stratospheric and tropospheric large‐scale modes of variability. The MLR explains significant variance in CPT (76%), CPZ (78%), WV83 (65%), O383 (62%), TTLCCF (52%), and ALLCF (36%). The interannual variability of CPT and WV83 is dominated by stratospheric processes associated with the Quasi‐Biennial Oscillation (QBO) and Brewer‐Dobson Circulation (BDC), whereas the variability of CPZ, O383, TTLCCF and ALLCF is also controlled by 500 hPa temperature (T500). Residual variability in CPT and CPZ not captured by the MLR are further significantly correlated to stratospheric temperature. It is shown that the portion of the BDC's shallow branch missed by the eddy heat flux based BDC index contributes significant amounts of the explained variances.
Plain Language Summary
Between the tropical upper troposphere and lower stratosphere, water can exist as either vapor or ice. The amount of water that enters the stratosphere depends on the portion of vapor that is frozen out by the coldest temperature that air experiences in this region, which on interannual timescales could be modulated by both large‐scale stratospheric and tropospheric modes of variability. Here we show that 76%, 65%, and 52% of the interannual variance in cold point temperature, water vapor at 83 hPa, and ice cloud fraction in this region can be explained using a multiple linear regression (MLR), where the predictors are the modes of the large‐scale variability. Stratospheric processes are much more important in controlling the interannual variance of cold point temperature and water vapor at 83 hPa, but notably, the height of the cold point is controlled by both stratospheric and tropospheric processes. Residual variability of the cold point temperature not captured by the MLR is still connected to temperature variability in the stratosphere.
Key Points
The interannual variability in the cold point tropopause temperature averaged over 15°S‐15°N is driven by stratospheric processes
This cold point tropopause temperature residual after regressing out large‐scale modes is still correlated with stratospheric temperature
The portion of the BDC's shallow branch, which is independent of our eddy heat flux BDC index, is an important source of TTL variability
This article evaluates the performance of Indian National Satellite System (INSAT)-3D sounder temperature (T) and water vapor (WV) by comparing with the collocated radiosonde observations obtained at ...different stations over Indian region. The assessment is carried out in terms of bias in the INSAT-3D measurements by assuming the radiosonde observations as a standard measurement technique. The study is carried out to find the performance of INSAT-3D sounder data on different surface types (land, coast, and ocean), seasons, and latitude zones. The main findings are the large bias in INSAT-3D-derived T and WV profiles over land compared to the coastal and oceanic stations, which may be attributed to the large contrasting surface emissivities over land. The WV measurements of INSAT-3D showed a relative dry bias of 15%-25% in the lower troposphere. Near the surface, a warm bias (~2 K) is observed over the land, and a cold bias (~1 K) is observed over the coastal and oceanic stations. Cold biases of ~1 K (land) and ~0.5 K (coastal and oceanic) are observed in the middle troposphere. The variability of the T bias is relatively large over land, particularly during winter and premonsoon. The variability of T bias is large in the 25°-35°N latitudinal zone mainly in winter followed by premonsoon, postmonsoon, and monsoon. A consistent warm bias of ~2-3 K is observed in the upper troposphere particularly at around 100 hPa on all the surface types in all seasons over all latitude zones. Thus, bias correction is advised in using these data for upper tropospheric and lower stratospheric (UTLS) studies.
Water vapour convectively injected into the mid-latitude lowermost stratosphere could affect stratospheric ozone. The associated potential ozone loss process requires low temperatures together with ...elevated water vapour mixing ratios. Since this ozone loss is initiated by heterogeneous chlorine activation on liquid aerosols, an increase in sulfate aerosol surface area due to a volcanic eruption or geoengineering could increase the likelihood of its occurrence. However, the chemical mechanism of this ozone loss process has not yet been analysed in sufficient detail and its sensitivity to various conditions is not yet clear. Under conditions of climate change associated with an increase in greenhouse gases, both a stratospheric cooling and an increase in water vapour convectively injected into the stratosphere are expected. Understanding the influence of low temperatures, elevated water vapour and enhanced sulfate particles on this ozone loss mechanism is a key step in estimating the impact of climate change and potential sulfate geoengineering on mid-latitude ozone.
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