This study introduces a multivariable covariance index (MVCI) to illustrate the synoptic features of mei-yu in the Yangtze River valley (YRV) region, which contains information of three indicators of ...mei-yu including precipitation, surface relative humidity, and tropospheric vertical motion. The interdecadal variation in the synoptic features of mei-yu during 1961–2016 is investigated using the MVCI. The date of mei-yu peak and the intensity of mei-yu underwent noticeable interdecadal variations over past decades, which are characterized by a delayed (relatively early) mei-yu peak and a relatively large (small) mei-yu intensity during 1985–97 (1961–80 and 2006–16). The mechanisms of these interdecadal variations are further discussed. The interdecadal variation in the date of mei-yu peak is mainly modulated by the meridional water vapor transport over eastern China during June, which may be partially attributed to an influence of the Pacific decadal oscillation (PDO) on the clockwise gyre over the North Pacific during boreal summer. The interdecadal variation in mei-yu intensity is associated with the interdecadal variation of tropospheric vertical motion over the YRV region during boreal summer, which may be partially attributed to an interaction between the PDO and the large-scale tropical east–west circulation during boreal summer. In addition, the interdecadal variation in the water vapor flux budget and relative humidity over the YRV region also exerted an impact on the interdecadal variation of mei-yu intensity in the YRV region.
Atmospheric rivers (ARs) play vital roles in the western United States and related regions globally, not only producing heavy precipitation and flooding, but also providing beneficial water supply. ...This paper introduces a scale for the intensity and impacts of ARs. Its utility may be greatest where ARs are the most impactful storm type and hurricanes, nor’easters, and tornadoes are nearly nonexistent. Two parameters dominate the hydrologic outcomes and impacts of ARs: vertically integrated water vapor transport (IVT) and AR duration i.e., the duration of at least minimal AR conditions (IVT ≥ 250 kg m−1 s−1). The scale uses an observed or predicted time series of IVT at a given geographic location and is based on the maximum IVT and AR duration at that point during an AR event. AR categories 1–5 are defined by thresholds for maximum IVT (3-h average) of 250, 500, 750, 1,000, and 1,250 kg m−1 s−1, and by IVT exceeding 250 kg m−1 s−1 continuously for 24–48 h. If the AR event duration is less than 24 h, it is downgraded by one category. If it is longer than 48 h, it is upgraded one category. The scale recognizes that weak ARs are often mostly beneficial because they can enhance water supply and snowpack, while stronger ARs can become mostly hazardous, for example, if they strike an area with antecedent conditions that enhance vulnerability, such as burn scars or wet conditions. Extended durations can enhance impacts. Short durations can mitigate impacts.
Narrow corridors of water vapor transport known as atmospheric rivers (ARs) contribute to extreme precipitation and flooding along the West Coast of the United States, but knowledge of their ...influence over the interior is limited. Here, the authors use Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data, Climate Prediction Center (CPC) precipitation analyses, and Snowpack Telemetry (SNOTEL) observations to describe the characteristics of cool-season (November-April) ARs over the western United States. It is shown that AR frequency and duration exhibit a maximum along the Oregon-Washington coast, a strong transition zone upwind (west) of and over the Cascade-Sierra ranges, and a broad minimum that extends from the "high" Sierra south of Lake Tahoe eastward across the central Great Basin and into the deep interior. East of the Cascade-Sierra ranges, AR frequency and duration are largest over the interior northwest, while AR duration is large compared to AR frequency over the interior southwest. The fractions of cool-season precipitation and top-decile 24-h precipitation events attributable to ARs are largest over and west of the Cascade-Sierra ranges. Farther east, these fractions are largest over the northwest and southwest interior, with distinctly different large-scale patterns and AR orientations enabling AR penetration into each of these regions. In contrast, AR-related precipitation over the Great Basin east of the high Sierra is rare. These results indicate that water vapor depletion over major topographic barriers is a key contributor to AR decay, with ARs playing a more prominent role in the inland precipitation climatology where lower or less continuous topography facilitates the inland penetration of ARs.
A new method for automatic detection of atmospheric rivers (ARs) is developed and applied to an atmospheric reanalysis, yielding an extensive catalog of ARs land‐falling along the west coast of North ...America during 1948–2017. This catalog provides a large array of variables that can be used to examine AR cases and their climate‐scale variability in exceptional detail. The new record of AR activity, as presented, validated and examined here, provides a perspective on the seasonal cycle and the interannual‐interdecadal variability of AR activity affecting the hydroclimate of western North America. Importantly, AR intensity does not exactly follow the climatological pattern of AR frequency. Strong links to hydroclimate are demonstrated using a high‐resolution precipitation data set. We describe the seasonal progression of AR activity and diagnose linkages with climate variability expressed in Pacific sea surface temperatures, revealing links to Pacific decadal variability, recent regional anomalies, as well as a generally rising trend in land‐falling AR activity. The latter trend is consistent with a long‐term increase in vapor transport from the warming North Pacific onto the North American continent. The new catalog provides unprecedented opportunities to study the climate‐scale behavior and predictability of ARs affecting western North America.
Key Points
Using a new detection scheme, a 69 yearlong catalog of atmospheric rivers land‐falling upon western North America is created and validated
AR landfalls show a marked seasonal progression from the Gulf of Alaska in the early fall to northern California in early winter
The seasonal intensity of AR landfalls varies from year to year and from decade to decade in relation to Pacific SST variability
Plain Language Summary
We have created a new seven‐decade‐long catalog of atmospheric river behavior land‐falling upon the west coast of North America. The catalog has been validated against independent precipitation observations to ensure that the atmospheric rivers represented therein are associated with extreme orographic precipitation. Our results clearly delineate a prominent role for atmospheric rivers in California's hydroclimate. Atmospheric river variability has been particularly important in the recent California drought as well as its most recent lapse. We also detect a long‐term increasing trend in water vapor transport impinging on the west coast of North America associated with atmospheric rivers and overall wintertime water vapor transport associated with climate warming. Our results, moreover, suggest that potential predictability of seasonal behavior of atmospheric rivers may hinge on sources of climatic variability somewhat different from that of total water vapor transport.
It is generally believed that water vapour from the North Atlantic transported by the mid‐latitude westerlies dominates precipitation variations in arid Central Asia, where monsoonal water vapour ...source is beyond the reach. This study reexamines water vapour sources for arid Central Asia based on daily reanalysis data. The results reveal two water vapour sources for arid Central Asia: the westerly water vapour transport from the North Atlantic and the monsoonal water vapour transport from the tropical Indian Ocean and South China Sea. The monsoonal transport includes two pathways: lower tropospheric water vapour along the eastern and northern periphery of the Tibetan Plateau from the tropical Indian Ocean and South China Sea, and middle‐upper tropospheric water vapour via the Tibetan Plateau from the tropical Indian Ocean. Precipitation variability during the monsoonal transport events explains 81% of the total variance. The monsoonal transport is related to a weakened East Asian summer monsoon (EASM) that is accompanied by persistent westward shift of the western Pacific subtropical high and increasing frequency of Mongolian anticyclone activities. The EASM has experienced a persistent weakening trend since 1958, causing an increasing contribution of the monsoonal water vapour transport and thereby enhancing summer precipitation in arid Central Asia. In particular, the monsoonal easterly water vapour has become the dominant source for arid Central Asia in the lower troposphere during the last decade. This study highlights the important role of the EASM in the wetting trend of summers in arid Central Asia during the past six decades.
This study explored the physical mechanism for the recent wetting trend of summers in arid Central Asia. Enhanced water vapour transport from the tropical Indian Ocean and South China Sea is responsible for the wetting of arid Central Asia. This anomalous water vapour transport is caused by a persistent weakening of the East Asian summer monsoon.
Water vapor in the stratosphere is primarily controlled by temperatures in the tropical upper troposphere and lower stratosphere. However, the direct impact of deep convection on the global lower ...stratospheric water vapor budget is still an actively debated issue. Two complementary modeling approaches are used to investigate the convective impact in boreal winter and summer. Convective influence is diagnosed by tracing trajectories through convective cloud top altitude fields derived from global rainfall and brightness temperature data. Backward trajectory model simulations coupled with a detailed treatment of cloud microphysical processes indicate that convection moistens the global lower stratosphere by approximately 0.3 ppmv in boreal winter and summer 2010. The diurnal peak in convection is responsible for about half of the total convective moistening during winter and nearly all of the convective moistening during summer. Deep convective clouds overshooting the tropopause have relatively minor effect on global lower stratospheric water vapor. A forward trajectory model coupled with a simplified cloud module is used to estimate the relative magnitude of the interannual variability of the convective impact. Combining the results from the two models, we find that the convective impact on the global lower stratospheric water vapor during 2006–2016 is approximately 0.3 ppmv with year‐to‐year variations of up to 0.1 ppmv. An important mechanism of convective hydration of the lower stratosphere is via the detrainment of saturated air and ice into the tropical uppermost troposphere and the subsequent upward transport of some of these moist air parcels across relatively warm and subsaturated tropopause.
Plain Language Summary
Stratosphere is extremely dry, but small changes in the humidity of the stratosphere can have a big impact on Earth's climate. Water vapor in the stratosphere is primarily determined by temperatures in the tropical upper atmosphere (between the tropospheric and stratospheric layers), but deep convective clouds that rapidly transport humid air up to this region could potentially influence stratospheric water vapor as well. This study uses two complementary modeling approaches to estimate the overall impact of deep convection on global stratospheric humidity. We find that convection moistens the lower stratosphere by about 10% in boreal winters and summers with smaller (by about a third) year‐to‐year variations during the 2006–2016 period. The daytime peak in convection is responsible for about half of the total convective moistening during boreal winter and nearly all of the convective moistening during boreal summer. Deep convective cloud tops that penetrate into the lower stratosphere have a relatively small effect on stratospheric water vapor. Convection moistens the lower stratosphere by transporting humid air laden with numerous ice crystals to the tropical uppermost troposphere, just below the stratosphere. Some of this humid air subsequently ascends into the stratosphere and ultimately increases the humidity of the lower stratosphere.
Key Points
Convection moistens the global lower stratosphere by 0.3 ppmv with interannual variations of up to 0.1 ppmv in winter and summer 2006–2016
Convection moistens the lower stratosphere via the detrainment of saturated air and ice into the tropical uppermost troposphere
In the absence of cloud‐top mixing, overshooting convection has a relatively small effect on global lower stratospheric water vapor
Water vapour is an important atmospheric gas with a direct impact on climate and weather. Given the significant variations of water vapour distribution in the tridimensional and temporal space, it is ...of great significance to use remote sensing data to monitor it on a large scale. One of the main missions of the Medium Resolution Spectral Imager (MERSI) onboard FengYun (FY)-3A satellite is to monitor the distribution of precipitable water vapour (PWV). However, the official MERSI PWV data is not widely used due to the limitation of its accuracy. The primary objective of this study is to develop FY-3A/MERSI PWV data with higher accuracy. The study area of this paper is the continental United States (125°W-65°W, 25°-50°N), and the selected period is the year 2010. Three PWV retrieval models for three near-infrared (NIR) channels (0.905, 0.940 and 0.980 μm) of MERSI are constructed based on the matching results between MERSI data and SuomiNET PWV data. Compared with the existing semi-empirical PWV retrieval algorithms, the algorithm developed in this work has two main changes: (1) MERSI PWV retrieval models are constructed using data after removing outliers rather than all data. (2) The algorithm can identify abnormal retrieval results. The validation results based on PWV data derived from Aerosol Robotic NETwork (AERONET) indicate that these two changes both contribute positively to enhancing the accuracy of MERSI PWV data. The root mean square errors (RMSE) of MERSI PWV data developed using the three channels at 0.905, 0.940 and 0.980 μm are 0.28 cm, 0.22 cm and 0.24 cm, respectively. The relative errors (RE) of these PWV data are 0.14, 0.10 and 0.12, respectively. In comparison to the official MERSI PWV data, the RMSE of MERSI PWV data retrieved in this study is reduced by at least 44%, and RE is reduced by at least 42%.
Recent work on atmospheric rivers (ARs) has led to a characterization of these impactful features as primarily cold-season phenomena. Here, an all-season analysis of AR incidence in the North Pacific ...basin is performed for the period spanning 1979–2014 using the NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis dataset. An occurrence-based detection algorithm is developed and employed to identify and characterize ARs in instantaneous fields of anomalous vertically integrated water vapor transport. The all-season climatology and variability of AR frequencies due to the seasonal cycle, the El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation (MJO), and their interactions are presented based on composites of the detected features. The results highlight that ARs exist throughout the year over the North Pacific, although their preferred locations shift substantially throughout the year. This seasonal cycle manifests itself as northward and westward displacement of ARs during the Northern Hemisphere warm seasons, rather than an absolute change in the number of ARs within the domain. It is also shown that changes to the North Pacific mean-state due to ENSO and the MJO may enhance or completely offset the seasonal cycle of AR activity, but that such influences on AR frequencies vary greatly based on location.
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.
Univariate bias correction algorithms, such as quantile mapping, are used to address systematic biases in climate model output. Intervariable dependence structure (e.g., between different quantities ...like temperature and precipitation or between sites) is typically ignored, which can have an impact on subsequent calculations that depend on multiple climate variables. A novel multivariate bias correction (MBC) algorithm is introduced as a multidimensional analog of univariate quantile mapping. Two variants are presented. MBCp and MBCr respectively correct Pearson correlation and Spearman rank correlation dependence structure, with marginal distributions in both constrained to match observed distributions via quantile mapping. MBC is demonstrated on two case studies: 1) bivariate bias correction of monthly temperature and precipitation output from a large ensemble of climate models and 2) multivariate correction of vertical humidity and wind profiles, including subsequent calculation of vertically integrated water vapor transport and detection of atmospheric rivers. The energy distance is recommended as an omnibus measure of performance for model selection. As expected, substantial improvements in performance relative to quantile mapping are found in each case. For reference, characteristics of the MBC algorithm are compared against existing bivariate and multivariate bias correction techniques. MBC performs competitively and fills a role as a flexible, general purpose multivariate bias correction algorithm.
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