Vapor Anomalies Research Articles

Long‐Term Temperature Impacts of the Hunga Volcanic Eruption in the Stratosphere and Above

AbstractGlobal average upper atmosphere temperature changes linked with the Hunga volcanic eruption (January 2022) are analyzed based on satellite measurements and compared with chemistry‐climate model simulations. Results show stratospheric cooling of −0.5 to −1.0 K in the middle and upper stratosphere during 2022 through middle 2023, followed by stronger cooling (−1.0 to −2.0 K) in the mesosphere after middle 2023. The cooling patterns follow the upward propagating water vapor (H2O) anomalies from Hunga, and similar behavior is found between observations and model simulations. While the stratospheric cooling is mainly due to radiative cooling from enhanced H2O, the mesospheric temperature changes result from ozone losses in the mesosphere, which are in‐turn driven by HOx radicals from Hunga H2O. Comparisons with the multi‐decade climate record show that Hunga impacts on stratospheric temperatures have similar magnitude, but opposite sign, to temperature effects from the large El Chichón (1982) and Pinatubo (1991) volcanic eruptions.

Long-Term Climate Impacts of Large Stratospheric Water Vapor Perturbations

Abstract The amount of water vapor injected into the stratosphere after the eruption of Hunga Tonga–Hunga Ha’apai (HTHH) was unprecedented, and it is therefore unclear what it might mean for surface climate. We use chemistry–climate model simulations to assess the long-term surface impacts of stratospheric water vapor (SWV) anomalies similar to those caused by HTHH but neglect the relatively minor aerosol loading from the eruption. The simulations show that the SWV anomalies lead to strong and persistent warming of Northern Hemisphere landmasses in boreal winter, and austral winter cooling over Australia, years after eruption, demonstrating that large SWV forcing can have surface impacts on a decadal time scale. We also emphasize that the surface response to SWV anomalies is more complex than simple warming due to greenhouse forcing and is influenced by factors such as regional circulation patterns and cloud feedbacks. Further research is needed to fully understand the multiyear effects of SWV anomalies and their relationship with climate phenomena like El Niño–Southern Oscillation. Significance Statement Volcanic eruptions typically cool Earth’s surface by releasing sulfur dioxide, which then converts into aerosols, which reflect sunlight. However, a recent eruption released a significant amount of water vapor—a strong greenhouse gas—into the stratosphere with unknown consequences. This study neglects the aerosol effect and examines the consequences of large stratospheric water vapor anomalies and reveals that surface temperatures across large regions of the world increase by over 1.5°C for several years, although some areas experience cooling close to 1°C. Additionally, the research suggests a potential connection between the eruption and sea surface temperatures in the tropical Pacific, which warrants further investigation.

Interdecadal variability of summer precipitation in the Three River Source Region: Influences of SST and zonal shifts of the East Asian subtropical westerly jet

Summer precipitation in the Three Rivers Source Region (TRSR) of China is vital for the headwaters of the Yellow, Yangtze, and Lancang rivers and exhibits significant interdecadal variability. This study investigates the influence of the East Asian westerly jet (EAWJ) on TRSR rainfall. A strong correlation is found between TRSR summer precipitation and the Jet Zonal Position Index (JZPI) of the EAWJ from 1961 to 2019 (R = 0.619, p < 0.01). During periods when a positive JZPI indicates a westward shift in the EAWJ, enhanced water vapor anomalies, warmer air, and low-level convergence anomalies contribute to increased TRSR summer precipitation. Using empirical orthogonal function and regression analyses, this research identifies the influence of large-scale circulation anomalies associated with the Atlantic–Eurasian teleconnection (AEA) from the North Atlantic (NA). The interdecadal variability between the NA and central tropical Pacific (CTP) significantly affects TRSR precipitation. This influence is mediated through the AEA via a Rossby wave train extending eastward along the EAWJ, and another south of 45°N. Moreover, the NA–CTP Opposite Phase Index (OPI), which quantifies the difference between the summer mean sea surface temperatures of the NA and the CTP, is identified as a critical factor in modulating the strength of this teleconnection and influencing the zonal position of the EAWJ.摘要本研究探讨了年代际东亚西风急流的纬向移动对黄河, 长江和澜沧江源头—三江源区夏季降水的影响. 研究发现, 在1961–2019年间, 三江源区夏季降水量与急流纬向位置指数 (JZPI) 在年代际尺度上存在强正相关 (R=0.619, p<0.01) . 当JZPI呈正值, 也就是东亚西风急流向西移动时, 增强的水汽, 温暖的气流和低层辐合异常有助于三江源区夏季降水增加. 北大西洋与中太平洋之间的年代际变化对三江源区的降水有着显著的影响, 主要以北大西洋–欧亚遥相关 (AEA) 方式, 通过沿东亚西风急流向东延伸的罗斯贝波列, 以及45°N以南的另一波列来影响三江源区的降水. 进一步分析发现, 北大西洋与中太平洋反向相位指数 (OPI) 作为一个关键因素, 可量化北大西洋与中太平洋之间夏季平均海表温度的差异, 调控该遥相关的强度并影响急流的纬向位置.

How heating tracers drive self-lofting long-lived stratospheric anticyclones: simple dynamical models

Abstract. Long-lived “bubbles” of wildfire smoke or volcanic aerosol have recently been observed in the stratosphere, co-located with ozone, carbon monoxide, and water vapour anomalies. These bubbles often survive for several weeks, during which time they ascend through vertical distances of 15 km or more. Meteorological analysis data suggest that this aerosol is contained within strong, persistent anticyclonic vortices. Absorption of solar radiation by the aerosol is hypothesised to drive the ascent of the bubbles, but the dynamics of how this heating gives rise to a single-sign anticyclonic vorticity anomaly have thus far been unclear. We present a description of heating-driven stratospheric vortices, based on an axisymmetric balanced model. The simplest version of this model includes a specified localised heating moving upwards at fixed velocity and produces a steadily translating solution with a single-signed anticyclonic vortex co-located with the heating, with corresponding temperature anomalies forming a vertical dipole, matching observations. A more complex version includes the two-way interaction between a heating tracer, representing the aerosol, and the dynamics. An evolving tracer provides heating which drives a secondary circulation, and this in turn transports the tracer. Through this two-way interaction an initial distribution of tracer drives a circulation and forms a self-lofting tracer-filled anticyclonic vortex. Scaling arguments show that upward velocity is proportional to heating magnitude, but the magnitude of peak quasigeostrophic vorticity is O(f) (f is the Coriolis parameter) and independent of the heating magnitude. Estimates of vorticity from observations match our theoretical predictions. We discuss 3-D effects such as vortex stripping and dispersion of tracer outside the vortex by the large-scale flow, which cannot be captured explicitly by the axisymmetric model and are likely to be important in the real atmosphere. The large O(f) vorticity of the fully developed anticyclones explains their observed persistence and their effective confinement of tracers. To further investigate the early stages of formation of tracer-filled vortices, we consider an idealised configuration of a homogeneous tracer layer. A linearised calculation reveals that the upper part of the layer is destabilised due to the decrease in tracer concentrations with height there, which sets up a self-reinforcing effect where upward lofting of tracer results in stronger heating and hence stronger lofting. Small amplitude disturbances form isolated tracer plumes that ascend out of the initial layer, indicative of a self-organisation of the flow. The relevance of these idealised models to formation and persistence of tracer-filled vortices in the real atmosphere is discussed, and it is suggested that a key factor in their formation is the time taken to reach the fully developed stage, which is shorter for strong heating rates.

Synoptic and planetary-scale dynamics modulate antarctic atmospheric river precipitation intensity

Although rare, atmospheric rivers substantially influence the interannual variability of Antarctic surface mass balance. Here we use MERRA-2 reanalysis to identify characteristics unique to atmospheric river environments by comparing (1) Analog (environments that feature high-low pressure couplets, similar to Atmospheric River environments, but no Atmospheric River), (2) Atmospheric River, and (3) Top Atmospheric River (highest precipitation) timesteps during 1980–2019 around Antarctica. We find significant differences between Atmospheric River and Analog environments including more intense and poleward-shifted mid-tropospheric geopotential height couplets as well as larger atmospheric moisture anomalies. We find similar significant enhancement in synoptic-scale dynamic drivers of Top Atmospheric Rivers compared to all Atmospheric River environments, but no significant difference in local integrated water vapor anomalies. Instead, our results highlight the importance of large-scale dynamic drivers during Top Atmospheric River timesteps, including amplified Rossby waves excited by tropical convection.

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.

The Estimated Climate Impact of the Hunga Tonga‐Hunga Ha'apai Eruption Plume

AbstractOn 15 January 2022, the Hunga Tonga‐Hunga Ha'apai (HT) eruption injected SO2 and water into the middle stratosphere. The SO2 is rapidly converted to sulfate aerosols. The aerosol and water vapor anomalies have persisted in the Southern Hemisphere throughout 2022. The water vapor anomaly increases the net downward IR radiative flux whereas the aerosol layer reduces the direct solar forcing. The direct solar flux reduction is larger than the increased IR flux. Thus, the net tropospheric forcing will be negative. The changes in radiative forcing peak in July and August and diminish thereafter. Scaling to the observed cooling after the 1991 Pinatubo eruption, HT would cool the 2022 Southern Hemisphere's average surface temperatures by less than 0.037°C.

Significant Stratospheric Moistening Following Extreme El Niño Events

The moistening impact of El Niño on the tropical lower stratosphere has been extensively studied, yet a long-standing challenge is its potential nonlinearities regarding the strength of El Niño. Extreme El Niño’s hydration in 2015/2016 was unprecedented in the satellite era, providing a great opportunity to distinguish the differential response of water vapor to extreme and moderate El Niño. Using ERA5 and MERRA-2 reanalysis data from 1979–2019, we compare the composite tropical lower stratospheric water vapor anomalies throughout all extreme and moderate El Niño episodes since the satellite era. We validate the variations in the lower stratospheric water vapor during the two distinct El Niño episodes using a three-dimensional chemistry transport model simulating the same period. The model reproduces the observed pattern in lower stratospheric water vapor. Both demonstrate that robust moistening during extreme El Niño events occurs throughout the tropical lower stratosphere. However, moderate El Niño events seem to have a weak effect on lower stratospheric water vapor. In comparison to moderate El Niño, the strong convective activities induced by extreme El Niño release large amounts of latent heat, causing extensive and intense warming in the tropical upper troposphere and lower stratosphere, thus greatly increasing the water vapor content in the tropical lower stratosphere. Additionally, moderate El Niño events have strong seasonality in their hydration effect in the tropics, whereas the intense moistening effect of extreme El Niño events prevails in all seasons during their episodes.

Reducing the Spring Barrier in Predicting Summer Arctic Sea Ice Concentration

AbstractThe predictive skill of summer sea ice concentration (SIC) in the Arctic presents a steep decline when initialized before June, which is the so‐called spring predictability barrier for Arctic sea ice. This study explores the potential influence of surface heat flux, cloud and water vapor anomalies on monthly to seasonal predictions of Arctic SIC anomalies. The results show an enhancement in skill predicting Arctic September SIC in the models that use surface fluxes, clouds, or water vapor in combination with SIC and surface sea temperature as predictors when initialized in boreal spring. This result shows the potential to reduce the spring barrier for Arctic SIC predictions by including the surface heat budget. The enhanced predictive skill can be very likely linked to the improved representation of the thermodynamics associated with water vapor and cloudiness anomalies in spring.

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.

Convective Couplings with Equatorial Rossby Waves and Equatorial Kelvin Waves. Part II: Coupled Precipitation Characteristics

Abstract Detailed precipitation characteristics coupled with equatorial Rossby and Kelvin waves are investigated. We prepare a rainfall event dataset using the Tropical Rainfall Measurement Mission (TRMM) Precipitation Radar (PR) level 2 data. Utilizing three indices, area size, maximum echo-top height, and stratiform precipitation ratio, rainfall events are classified into mesoscale convective system (MCS), deep, congestus, and shallow convective events, and “other” type. We perform composite analyses based on the wave phase defined in Part I. Precipitation amount in Rossby waves is in phase with column water vapor (CWV) anomalies and is mainly contributed by MCSs, which are simultaneously activated with deep convection. The large CWV can support deep development and organization of convection. Shallow and congestus convective events indicate their peaks just before the active phase on 10°N or in the later part of the convectively suppressed phase on the equator. A five-step evolution is shown in Kelvin waves. In the first stage, shallow convective events are triggered by high SST, followed by a dominance of congestus convective events. Then, in the developing stage, deep convective events become dominant. In the mature stage, heavy precipitation is contributed by MCSs, and mostly stratiform rain is maximized at later stages. Kelvin waves indicate relatively weak connection to CWV fluctuation. Although contrasted evolutions are indicated, large contributions by MCSs to precipitation amount are common among the two coupled waves. This is considered to result in the commonality of the equivalent depths with their top-heavy heating. Significance Statement A coupling mechanism between equatorial waves and convective activity is a key issue in the tropical meteorology. While many previous idealized studies suggested some instability mechanisms, detailed precipitation characteristics is not enough investigated. We prepare rainfall-event dataset observed from a spaceborne Precipitation Radar on board the TRMM satellite to quantify detailed precipitation characteristics statistically and compare equatorial Rossby waves and equatorial Kelvin waves. We found that organized convective systems and deep convection are simultaneously activated in the Rossby waves and a clear transition of convective activity is shown in the Kelvin waves. These characteristics highly correspond to waves’ synoptic-scale structures. Our observational results of detailed evolutions of rainfall events will improve understanding of coupling processes.

The Ice-and-Snow Tourism in Harbin Met Its Waterloo: Analysis of the Causes of the Warm Winter with Reduced Snowfall in 2018/2019

Harbin, located in northeast China (NEC), has obvious monsoon climate characteristics due to the influence of its geographical environment. Under the control of the polar continental air mass, winter in Harbin is exceedingly cold and long, with the frequent invasion of the cold and dry air from the north. Because of its intensely cold climate in winter, Harbin has created a local form of tourism with its own characteristics: the snow and ice landscape attracts a large number of tourists. Therefore, the anomalies of air temperature and precipitation in winter have an important impact on the livelihood of the local people and economy. In the winter of 2018/2019, the ice and snow tourism in Harbin was harshly affected by the extreme weather, and the direct cause is the anomalies of atmospheric circulation. There is a center of strong positive geopotential height anomalies over east China, which favors the movement of warm air northwards to the NEC, resulting in warmer-than-normal air temperature. Anomalous precipitation is largely controlled by the anomalies of local water vapor and air temperature. The aim of this study was to determine whether the warmer-than-normal temperature, which made the atmosphere more resistant to saturation, was the primary cause of the reduced snowfall. The relative importance of water vapor and air temperature anomalies to the anomalous precipitation was compared. The results suggest that the warmer-than-normal temperature affected all levels, but its impact on the near-surface level was greater. At the middle and upper levels (above 850 hPa), in addition to the warmer-than-normal temperature, the amount of water vapor was less than normal. These conditions both reduced the amount of snow; however, by comparison, the dryness of the air contributed more significantly.

Using InSAR Data to Improve the Water Vapor Distribution Downstream of the Core of the South American Low‐Level Jet

AbstractTwo and half years of Interferometric Synthetic Aperture Radar (InSAR) images obtained by Sentinel‐1 near Santa Cruz de la Sierra, Bolivia, before August 2019 are here analyzed and assimilated in the Weather Research and Forecast model (WRF) to assess the quality of the water vapor field at the core of the South American Low‐level Jet, and the downstream propagation of the implied water vapor anomalies into a large sector of South America. Due to the topographic locking of the Jet near the edge of the Andes cordillera at Santa Cruz, this experiment allows an assessment of the extension of the spatial impact of InSAR data assimilation in varying circulation conditions. That data improves the model skill at varying locations thousands of kilometers downstream, in both the distribution of water vapor assessed at a large number of well‐distributed GNSS observations and precipitation assessed against the Global Precipitation dataset and ERA5 reanalysis. Gains in the precipitation forecast skill are found to have a larger impact on the forecast of light rain, and lead to an almost cancellation of forecasts of no rain in cases with moderate to heavy rain. Because the distribution of water vapor is a main driver of weather, and InSAR is one of the few sources of data that can look down to mesoscale resolution regardless of daytime or weather, it is suggested that it may have a significant positive impact on short‐range weather forecasting and, maybe more importantly, on the quality of the climate of the forecast model.

The Critical Role of Euro‐Atlantic Blocking in Promoting Snowfall in Central Greenland

AbstractThe Greenland Ice Sheet (GrIS) is losing mass at an increasing rate yet mass gain from snowfall still exceeds the loss attributed to surface melt processes on an annual basis. This work assesses the relationship between persistent atmospheric blocking across the Euro‐Atlantic region and enhanced precipitation processes over the central GrIS during June–August and September–November. Results show that the vast majority of snowfall events in the central GrIS coincide with Euro‐Atlantic blocking. During June–August, snowfall events are produced primarily by mixed‐phase clouds (88%) and are linked to a persistent blocking anticyclone over southern Greenland (84%). The blocking anticyclone slowly advects warm, moist air masses into western and southern Greenland, with positive temperature and water vapor anomalies that intensify over the central GrIS. A zonal integrated water vapor transport pattern south of Greenland indicates a southern shift of the North Atlantic storm track associated with the high‐latitude blocking. In contrast, snowfall events during September–November are largely produced by ice‐phase clouds (85%) and are associated with a blocking anticyclone over the Nordic Seas and blocked flow over northern Europe (78%). The blocking anticyclone deflects the westerly North Atlantic storm track poleward and enables the rapid transport of warm, moist air masses up the steep southeastern edge of the GrIS, with positive temperature and water vapor anomalies to the east and southeast of Greenland. These results emphasize the critical role of Euro‐Atlantic blocking in promoting snowfall processes over the central GrIS and the importance of accurate representation of blocking in climate model projections.

Convective Couplings with Equatorial Rossby Waves and Equatorial Kelvin Waves. Part I: Coupled Wave Structures

Abstract This study investigates precipitation amounts and apparent heat sources, which are coupled with equatorial Kelvin waves and equatorial Rossby waves, using TRMM PR level 2 data products. The synoptic structures of wave disturbances are also studied using the ERA5 dataset. We define the wave phase of equatorial waves based on FFT-filtered brightness temperature and conduct composite analyses. Rossby waves show a vertically upright structure and their upright vortices induce large-amplitude column water vapor (CWV) anomalies. Precipitation activity is almost in phase with CWV, and thus is consistent with a moisture mode. Kelvin waves, on the other hand, indicate a nearly quadrature phase relationship between temperature and vertical velocity, like gravity wave structure. Specific humidity develops from near the surface to the middle troposphere as the Kelvin wave progresses. A clear negative CWV anomaly also does not exist despite the existence of negative precipitation anomalies. Convective activity corresponds well with its tilting structure of moisture and modulates the phase relationship between temperature and vertical motion. For both wave cases, apparent heat sources can amplify available potential energy despite the difference of coupling mechanisms of these two waves; precipitation is driven by CWV fluctuation for the Rossby wave case, and by buoyancy-based fluctuations for the Kelvin wave case. These can be observational evidence of actual coupling processes that is comparable to previous idealized studies. Significance Statement A coupling mechanism between equatorial waves and convective activity is a significant issue in tropical meteorology. While many previous idealized studies suggested some instability mechanisms, their true roles are not yet clear because detailed precipitation characteristics are not well investigated. We aim to quantify precipitation and synoptic-scale wave disturbances, and compare equatorial Rossby waves and equatorial Kelvin waves, which should have different instability coupling modes between each other, in order to shed light on a convectively coupling mechanism. We found that precipitation is actually driven by column moisture in Rossby waves and by dynamical fluctuation in Kelvin waves. Despite these competing mechanisms, similar top-heavy heating can maintain convectively coupled disturbances. Our observational results will support and improve theoretical studies.

Precursors and Formation Mechanisms of Event-Based Extreme Precipitation during Springtime in Central-Eastern China

AbstractThis study investigates the precursors and formation mechanisms of spring (April–May) event-based extreme precipitation (EEP) during 1961–2014 in central-eastern China. The EEP events during springtime are primarily characterized by extreme precipitation that occurs at the first half of an event. During early stages of spring EEP events, a Rossby wave grows over western Europe and the North Atlantic Ocean. The wave propagates eastward toward East Asia, exhibiting a circumglobal teleconnection (CGT) pattern. A strong anticyclone related to the CGT pattern is formed over the islands of Japan in the upper troposphere, enhancing the divergence anomalies and bringing more water vapor anomalies from the Sea of Japan into central-eastern China. Meanwhile, the westerly jet jumps northward and anomalous southwesterly water vapor flux is significantly prevalent, both associated with the onset of the Bay of Bengal summer monsoon (BOBSM). When the anomalous southwesterly and northeasterly moisture fluxes into central-eastern China combine, strong convergence is formed, providing abundant water vapor for extreme precipitation. The moisture budget analysis further suggests that the dynamic processes associated with horizontal wind anomalies play a crucial role in the moisture convergence for the spring EEP events. The advection of zonal and meridional moisture is strongly related to the anomalous winds of the CGT waves and BOBSM, respectively; the horizontal thermodynamic processes related to specific humidity and vertical advection contribute much less. The results indicate the preceding signals in the midlatitudes and subtropics for the spring EEP events, enabling extreme precipitation forecasting and hydrological prediction.

Heavy versus extreme rainfall events in southeast Australia

AbstractFocussing on the major cities of Brisbane, Sydney, and Melbourne in southeast Australia, this study seeks to determine the environmental factors that distinguish between heavy rainfall events (HREs) and extreme rainfall events (EREs). Using daily rain gauge observations, HREs and EREs are defined for each domain based, respectively, on the 95th and 99th percentiles of wet‐day rainfall for the period 1979–2018. K‐means clustering is applied to mean sea‐level pressure, data from ERA5 to obtain a set of representative large‐scale circulation patterns associated with these events. Composite synoptic maps, mean vertical profiles, and a series of column‐integrated diagnostics are then examined for each cluster and used to compare the environmental characteristics of HREs and EREs. For all three cities, HREs are associated with an upper‐level trough to the west, with large‐scale ascent, positive column water vapour (CWV) anomalies, and strong moisture transport over the analysis domain. For Brisbane and Sydney, the clusters are characterised by a coastal trough/low with moist onshore flow from the Tasman and Coral Seas. For Melbourne, circulation patterns are more distinct, with clusters characterised by a front, a cut‐off low, and an inland trough. Compared with HREs, EREs show a more amplified upper‐level trough, with stronger vertical motion and larger CWV anomalies over the analysis domain. In Brisbane and Sydney, EREs also feature stronger and deeper onshore flow, promoting enhanced moisture transport. A diagnostic termed upward vapour transport, which combines the key ingredients of high CWV and large‐scale ascent, is shown to discriminate well between HREs and EREs in all three domains. In contrast, surface temperature, which is frequently linked to rainfall extremes via Clausius–Clapeyron scaling, shows significant overlap between the different event categories, particularly for Brisbane and Sydney.

Drought stress and hurricane defoliation influence mountain clouds and moisture recycling in a tropical forest

Mountain ranges generate clouds, precipitation, and perennial streamflow for water supplies, but the role of forest cover in mountain hydrometeorology and cloud formation is not well understood. In the Luquillo Experimental Forest of Puerto Rico, mountains are immersed in clouds nightly, providing a steady precipitation source to support the tropical forest ecosystems and human uses. A severe drought in 2015 and the removal of forest canopy (defoliation) by Hurricane Maria in 2017 created natural experiments to examine interactions between the living forest and hydroclimatic processes. These unprecedented land-based observations over 4.5 y revealed that the orographic cloud system was highly responsive to local land-surface moisture and energy balances moderated by the forest. Cloud layer thickness and immersion frequency on the mountain slope correlated with antecedent rainfall, linking recycled terrestrial moisture to the formation of mountain clouds; and cloud-base altitude rose during drought stress and posthurricane defoliation. Changes in diurnal cycles of temperature and vapor-pressure deficit and an increase in sensible versus latent heat flux quantified local meteorological response to forest disturbances. Temperature and water vapor anomalies along the mountain slope persisted for at least 12 mo posthurricane, showing that understory recovery did not replace intact forest canopy function. In many similar settings around the world, prolonged drought, increasing temperatures, and deforestation could affect orographic cloud precipitation and the humans and ecosystems that depend on it.

Examination of Global Midlatitude Atmospheric River Lifecycles Using an Object‐Oriented Methodology

AbstractTracking atmospheric rivers (ARs) across their lifecycles is a field of recent interest with a multitude of emerging methodologies. The CONNected‐objECT (CONNECT) algorithm is adapted for the tracking of global midlatitude AR lifecycles and associated precipitation by implementing a seeded region growing segmentation algorithm, creating the AR‐CONNECT algorithm. To facilitate the permissiveness of the methodology, AR‐CONNECT is without hard‐coded geometric criteria yet is still shown to extract synoptic‐scale elongated objects &gt;99.99% of the time. One of the consequences of the methodology is the ability to occasionally track atmospheric water vapor anomalies before evolving into AR geometries, effectively tracking AR genesis further back than other studies. With the aid of subdaily satellite‐derived rain data, we investigate the climatology, trends, and patterns of AR lifecycles from 1983–2016 and compare with other AR tracking studies. We find that AR frequency, genesis, and terminus locations are in generally good agreement with other AR tracking methodologies, though with key differences, and that AR frequencies in each hemisphere are determined by the number of AR hotspots. Furthermore, we uncover evidence that certain AR characteristics, such as frequency, areal extent, and duration, show evidence of increasing trends. Midlatitude precipitation uncovered by AR‐CONNECT shows contributions up to 50% over land and 65% over the ocean. Trend analysis of AR precipitation shows an increase in precipitation associated with ARs propagated by the Southern Jet Stream and ARs that traverse over the Sahara Desert, among others, but is determined not to be a driver of changes in global precipitation.

The influence of water vapor anomalies on clouds and their radiative effect at Ny-Ålesund

Abstract. The occurrence of events with increased and decreased integrated water vapor (IWV) at the Arctic site Ny-Ålesund, their relation to cloud properties, and the surface cloud radiative effect (CRE) is investigated. For this study, we used almost 2.5 years (from June 2016 to October 2018) of ground-based cloud observations processed with the Cloudnet algorithm, IWV from a microwave radiometer (MWR), long-term radiosonde observations, and backward trajectories FLEXTRA. Moist and dry anomalies were found to be associated with North Atlantic flows and air transport within the Arctic region, respectively. The amount of water vapor is often correlated to cloud occurrence, presence of cloud liquid water, and liquid water path (LWP) and ice water path (IWP). In turn, changes in the cloud properties cause differences in surface CRE. During dry anomalies, in autumn, winter, and spring, the mean net surface CRE was lower by 2–37 W m−2 with respect to normal conditions, while in summer the cloud-related surface cooling was reduced by 49 W m−2. In contrast, under moist conditions in summer the mean net surface CRE becomes more negative by 25 W m−2, while in other seasons the mean net surface CRE was increased by 5–37 W m−2. Trends in the occurrence of dry and moist anomalies were analyzed based on a 25-year radiosonde database. Dry anomalies have become less frequent, with rates for different seasons ranging from −12.8 % per decade to −4 % per decade, while the occurrence of moist events has increased at rates from 2.8 % per decade to 6.4 % per decade.