Horizontal Moisture Transport Research Articles

Interannual Fluctuations and Their Low-Frequency Modulation of Summertime Heavy Daily Rainfall Potential in Western Japan

Heavy rainfall under the conditions of the changing climate has recently garnered considerable attention. The statistics on heavy daily rainfall offer vital information for assessing present and future extreme events and for clarifying the impacts of global climate variability and change, working to form a favorable background. By analyzing a set of large-ensemble simulations using a global atmospheric model, this study demonstrated that two different physical processes in global climate variability control the interannual fluctuations in the 99th- and 90th-percentile values of summertime daily rainfall (i.e., the potential amounts) on Kyushu Island in western Japan. The 90th-percentile values were closely related to large-scale horizontal moisture transport anomalies due to changes in the subtropical high in the northwestern Pacific, which was usually accompanied by basin-scale warming in the Indian Ocean subsequent to the wintertime El Niño events. The contributions of the sea surface temperatures over the northern Indian Ocean and the eastern tropical Pacific Ocean showed low-frequency modulations, mainly due to the influences of the global warming tendency and the interdecadal variability in the climate system, respectively. In contrast, tropical cyclone activity played a major role in changing the 99th-percentile value. The potentials of both the tropical cyclone intensity and the existence density fluctuated, largely owing to the summertime sea surface temperature over the tropical Pacific, which can be modulated by the El Niño diversity on interdecadal timescales.

Interannual Fluctuations and Their Low-Frequency Modulation of Summertime Heavy Daily Rainfall Potential in Western Japan

Heavy rainfall under the conditions of the changing climate has recently garnered considerable attention. The statistics on heavy daily rainfall offer vital information for assessing present and future extreme events and for clarifying the impacts of global climate variability and change, working to form a favorable background. By analyzing a set of large-ensemble simulations using a global atmospheric model, this study demonstrated that two different physical processes in global climate variability control the interannual fluctuations in the 99th- and 90th-percentile values of summertime daily rainfall (i.e., the potential amounts) on Kyushu Island in western Japan. The 90th-percentile values were closely related to large-scale horizontal moisture transport anomalies due to changes in the subtropical high in the northwestern Pacific, which was usually accompanied by basin-scale warming in the Indian Ocean subsequent to the wintertime El Niño events. The contributions of the sea surface temperatures over the northern Indian Ocean and the eastern tropical Pacific Ocean showed low-frequency modulations, mainly due to the influences of the global warming tendency and the interdecadal variability in the climate system, respectively. In contrast, tropical cyclone activity played a major role in changing the 99th-percentile value. The potentials of both the tropical cyclone intensity and the existence density fluctuated, largely owing to the summertime sea surface temperature over the tropical Pacific, which can be modulated by the El Niño diversity on interdecadal timescales.

Evaluating the Representations of Atmospheric Rivers and Their Associated Precipitation in Reanalyses With Satellite Observations

AbstractAtmospheric rivers (ARs) are filaments of enhanced horizontal moisture transport in the atmosphere. Due to their prominent role in the meridional moisture transport and regional weather extremes, ARs have been studied extensively in recent years. Yet, the representations of ARs and their associated precipitation on a global scale remains largely unknown. In this study, we developed an AR detection algorithm specifically for satellite observations using moisture and the geostrophic winds derived from 3D geopotential height field from the combined retrievals of the Atmospheric Infrared Sounder and the Advanced Microwave Sounding Unit on NASA Aqua satellite. This algorithm enables us to develop the first global AR catalog based solely on satellite observations. The satellite‐based AR catalog is then combined with the satellite‐based precipitation (Integrated Muti‐SatellitE Retrievals for GPM) to evaluate the representations of ARs and AR‐induced precipitation in reanalysis products. Our results show that the spreads in AR frequency and AR length distribution are generally small across data sets, while the spread in AR width is relatively larger. Reanalysis products are found to consistently underestimate both mean and extreme AR‐related precipitation. However, all reanalyses tend to precipitate too often under AR conditions, especially over low latitude regions. This finding is consistent with the “drizzling” bias which has plagued generations of climate models. Overall, the findings of this study can help to improve the representations of ARs and associated precipitation in reanalyses and climate models.

Atmospheric water vapour transport in ACCESS‐S2 and the potential for enhancing skill of subseasonal forecasts of precipitation

AbstractExtended warning of above‐average and extreme precipitation is valuable to a wide range of stakeholders. However, the sporadic nature of precipitation makes it difficult to forecast skilfully beyond one week. Subseasonal forecasting is a growing area of science that aims to predict average weather conditions multiple weeks in advance using dynamical models. Building on recent work in this area, we test the hypothesis that using large‐scale horizontal moisture transport as a predictor for precipitation may increase the forecast skill of the above‐median and high‐precipitation weeks on subseasonal time‐scales. We analysed retrospective forecast (hindcast) sets from the Australian Bureau of Meteorology's latest operational subseasonal‐to‐seasonal forecasting model, ACCESS‐S2, to compare the forecast skill of precipitation using integrated water vapour transport (IVT) as a proxy, compared to using precipitation forecasts directly. We show that ACCESS‐S2 precipitation generally produces more skilful forecasts, except over some regions where IVT could be a useful additional diagnostic for warning of heavy precipitation events.

Adverse impact of terrain steepness on thermally driven initiation of orographic convection

Abstract. Diurnal mountain winds precondition the environment for deep moist convection through horizontal and vertical transport of heat and moisture. They also play a key role in convection initiation, especially in strongly inhibited environments, by lifting air parcels above the level of free convection. Despite its relevance, the impact of these thermally driven circulations on convection initiation has yet to be examined systematically. Using idealized large-eddy simulations (Δx=50 m) with the Weather Research and Forecasting (WRF) model, we study the effect of cross-valley circulations on convection initiation under synoptically undisturbed and convectively inhibited conditions, considering quasi-2D mountain ranges of different heights and widths. In particular, we contrast convection initiation over relatively steep mountains (20 % average slope) and less steep ones (10 %). One distinctive finding is that, under identical environmental conditions, relatively steep mountain ranges lead to a delayed onset and lower intensity of deep moist convection, although they cause stronger thermal updrafts at ridge tops. The temporal evolution of convective indices, such as convective inhibition and convective available potential energy, shows that destabilization over the steeper mountains is slower, presumably due to lower low-level moisture. Analysis of the ridgetop moisture budget reveals the competing effects of moisture advection by the mean thermally driven circulation and turbulent moisture transport. In general, at mountaintops, the divergence of the turbulent moisture flux offsets the convergence of the advective moisture flux almost entirely. Due to the stronger ridgetop updraft, the mean advective moistening over the steeper mountains is higher; nevertheless, the total moistening is lower and the width of the updraft zone is narrower on average. Thus, buoyant updrafts over the steeper mountains are more strongly affected by the turbulent entrainment of environmental air, which depletes their moisture and cloud water content and makes them less effective at initiating deep convection. Saturated updrafts over less steep mountains, on the other hand, gain more moisture from the vapor flux at cloud base, leading to significantly higher moisture accumulation. The lower entrainment rates in these simulations are revealed by the fact that equivalent potential temperature in the cloud decreases less strongly with height than over steeper terrain. The precipitation efficiency, a measure of how much of the condensed water eventually precipitates, is considerably larger over the less steep mountains, also due to lower total condensation compared with the steeper simulations. The relationship between mountain size and precipitation amount depends on the thermodynamic profile. It is nearly linear in cases with low initial convective inhibition but more complex otherwise. The weaker convection over steeper mountains is a robust finding, valid over a range of background environmental stability and mountain sizes.

Combinations of drivers that most favor the occurrence of daily precipitation extremes

Previous studies indicate atmospheric instability, total column water vapor, and horizontal moisture transport as major drivers of precipitation extremes, however little is known about how the combination of these drivers affects precipitation extremes across the world. Here, using daily data from the ERA-5 reanalysis spanning the period 1981–2020, we identified the combinations of extreme values for these three major drivers that enhance the probability of daily precipitation extremes on a global scale. Our findings show that extreme daily precipitation is practically impossible without any of these drivers being extreme. Atmospheric instability is the primary driver of precipitation extremes, meaning that, among the three cases of the drivers being extreme in isolation, extreme atmospheric instability is associated with the highest average probability of extreme precipitation over landmasses (29% during December–February, 32% during June–August). When considering the combination of two drivers being simultaneously extreme, joint extremes of atmospheric instability and total column water vapor (and non-extreme horizontal moisture transport) lead to the highest probability of extreme precipitation (69% during December–February, 70% during June–August), which is similar to the probability under three drivers in extreme conditions (67% and 72%). Our results point to a latitudinal variation of the combination that leads to the highest probability of extreme precipitation. In subtropics, the case of the three extreme drivers dominates, whereas in extratropical regions, the dominant combination is that of the joint extremes of atmospheric instability and total column water vapor (and non-extreme horizontal moisture transport). By providing information on the most important drivers of precipitation extremes worldwide, these results can serve as a basis for evaluating precipitation extremes in climate models and understanding projected changes, which is vital for developing robust risk assessments.

A Dimensionless Parameter for Predicting Convective Self‐Aggregation Onset in a Stochastic Reaction‐Diffusion Model of Tropical Radiative‐Convective Equilibrium

AbstractWe introduce a minimal stochastic lattice model for the column relative humidity (R) in the tropics, which incorporates convective moistening, horizontal transport and subsidence drying. The probability of convection occurring in a location increases with R, based on Tropical Rainfall Measuring Mission observations, providing a positive feedback that could lead to aggregation. We show that the simple model reproduces many aspects of full‐physics cloud resolving model experiments. Depending on model parameter settings and domain size and resolution choices, it can produce both random and aggregated equilibrium states. Clustering occurs more readily with larger domains and coarser resolutions, in agreement with full‐physics models. Using dimensional arguments and fits from empirical data, we derive a dimensionless parameter which we call the aggregation number, Nag, that predicts whether a specific model and experiment setup will result in an aggregated or random state. The parameter includes the moistening feedback strength, the horizontal moisture transport efficiency, the subsidence timescale, the domain size and spatial resolution. Using large ensembles of experiments, we show that the transition between random and aggregated states occurs at a critical value of Nag. We argue that Nag could help to understand the differences in aggregation states between full‐physics, cloud resolving models, which show little consensus about the robustness of self‐organized patterns, whose emergence appears to be sensitive to the model setup, physics and parameterizations.

Low-Level Circulation and Its Coupling with Free-Tropospheric Variability as a Mechanism of Spontaneous Aggregation of Moist Convection

Abstract The organization of clouds has been widely studied by numerical modeling as an essential problem in climate science. Convective self-aggregation (CSA) occurs in radiative–convective equilibrium when the model domain size is sufficiently large. However, we have not yet reached a comprehensive understanding of the mechanism of CSA onset. This study argues that low-level circulation is responsible for horizontal moisture transport and that its coupling with variabilities of diabatic heating and moisture in the free troposphere is essential. We simulated scattered and aggregated convection by varying the domain size as a control parameter constraining the horizontal scale associated with the CSA onset. Based on a new analysis method quantifying the circulation spanning dry and moist regions, we found that 1) the upgradient moisture transport in the aggregated cases is associated with low-level circulation development, amplifying the horizontal moisture contrast; 2) the horizontal buoyancy gradient due to strong radiative cooling in the dry region intensifies the low-level circulation; 3) the free-tropospheric subsidence intrudes into the boundary layer in the dry region preceding the intensification of low-level circulation; and 4) the subsidence intrusion is due to a weakening of convective heating in the free troposphere associated with the moisture variability at a larger horizontal scale. This study provides new insights into the organization mechanism of clouds unifying the different mechanisms impacting CSA: the free-tropospheric moisture, radiation, convection, and low-level circulation.

Assessing the impact of the recent warming in the East China Sea on a torrential rain event in northern Kyushu (Japan) in early July 2017

Sea surface temperature (SST) in the East China Sea (ECS) has undergone a rapid rise in recent decades, but the associated impact on extreme weather remains under debate. Here, using a cloud-permitting model, we assess the impact of the ECS warming observed since the 1980s on a torrential rain event that caused devastating floods and landslides in the Kyushu Island, western Japan, in July 2017. Without the increasing trends of SST and air temperature, the model cannot reproduce the observed extremely high amount of precipitation during the event, i.e., >700 mm/12-h. The SST increase is found more influential in determining the precipitation amount. Without the ocean warming, increases in precipitable water and horizontal moisture transport due to the atmospheric warming would not lead to precipitation increase during this event. The change in the amount of precipitation can be largely explained by the change in the updraft intensity of the convective system. Higher SST suppresses downward surface sensible heat flux and enhances upward latent heat flux along the paths of air parcels flowing into the convective system in this case. This increases the equivalent potential temperature in the lower troposphere, which enhances the convective available potential energy in the lower troposphere, leading to intensification of the convective system and thereby the increase of precipitation. The findings of this case study suggest an important role of the warming ECS in the intensification of torrential rain events around Japan and the necessity of further assessment of the role of the ocean warming in the torrential rains.

The contrasting effects of thermodynamic and dynamic processes on East Asian summer monsoon precipitation during the Last Glacial Maximum: a data-model comparison

The Last Glacial Maximum (LGM; 21 ka BP) was the most recent glacial period when the global ice sheet volume was at a maximum. Therefore, the LGM can be used to investigate atmospheric dynamics under a climate that differed significantly from the present. This study quantitatively compares pollen records of boreal summer (June–July–August) precipitation with the PMIP3 LGM simulations. The data-model comparison shows an overall agreement on a drier than pre-industrial East Asian summer monsoon (EASM) climate. Nevertheless, 17 out of 55 records show a regional precipitation increase that is also simulated over the additional land mass area due to sea level drop. The thermodynamic and dynamic responses are analyzed to explain a drier LGM EASM as a combination of these two antagonistic mechanisms. Relatively low atmospheric moisture content was the main thermodynamic control on the lower LGM (relative to pre-industrial levels) EASM precipitation amounts in both the reconstructions and the models. In contrast, two dynamic processes in relation to stationary eddy activity act to increase EASM precipitation regionally in records and simulations, respectively. Precipitation increase in records is explained by dynamic enhancement of the horizontal moisture transport, while dynamic enhancement of the vertical moisture transport leads to simulated precipitation increase over the specific region where landmass was exposed during LGM along continental coastlines of China due to significant drop in sea level (relative to pre-industrial levels). Overall, the opposing effects of thermodynamic and dynamic processes on precipitation during the LGM provide a means to reconcile the spatial heterogeneity of recorded precipitation changes in sign, although data-model comparison suggests that the simulated dynamic wetting mechanism is too weak relative to the thermodynamic drying mechanism over data-model disagreement regions.

Horizontal Moisture Transport Dominates the Regional Moistening Patterns in the Arctic

AbstractAlong with the amplified warming and dramatic sea ice decline, the Arctic has experienced regionally and seasonally variable moistening of the atmosphere. Based on reanalysis data, this study demonstrates that the regional moistening patterns during the last four decades, 1979–2018, were predominantly shaped by the strong trends in horizontal moisture transport. Our results suggest that the trends in moisture transport were largely driven by changes in atmospheric circulation. Trends in evaporation in the Arctic had a smaller role in shaping the moistening patterns. Both horizontal moisture transport and local evaporation have been affected by the diminishing sea ice cover during the cold seasons from autumn to spring. Increases in evaporation have been restricted to the vicinity of the sea ice margin over a limited period during the local sea ice decline. For the first time we demonstrate that, after the sea ice has disappeared from a region, evaporation over the open sea has had negative trends due to the effect of horizontal moisture transport to suppress evaporation. Near the sea ice margin, the trends in moisture transport and evaporation and the cloud response to those have been circulation dependent. The future moisture and cloud distributions in the Arctic are expected to respond to changes in atmospheric pressure patterns; circulation and moisture transport will also control where and when efficient surface evaporation can occur.

Strong Dependence of Wintertime Arctic Moisture and Cloud Distributions on Atmospheric Large-Scale Circulation

AbstractThis study gives a comprehensive picture of how atmospheric large-scale circulation is related to moisture transport and to distributions of moisture, clouds, and surface downward longwave radiation in the Arctic in winter. Anomaly distributions of the abovementioned variables are compared in 30 characteristic wintertime atmospheric circulation regimes, which are allocated from 15 years (2003–17) of mean sea level pressure data of ERA-Interim reanalysis applying the self-organizing map method. The characteristic circulation regimes are further related to known climate indices—the North Atlantic Oscillation (NAO), the Arctic Oscillation (AO), and Greenland blocking index—as well as to a frequent high pressure pattern across the Arctic Ocean from Siberia to North America, herein called the Arctic bridge. Effects of large-scale circulation on moisture, cloud, and longwave radiation are to a large extent occurring through the impact of horizontal moisture transport. Evaporation is typically not efficient enough to shape those distributions, and much of the moisture evaporated in the Arctic is transported southward. The positive phase of the NAO and AO increases moisture and clouds in northern Europe and the eastern North Atlantic Ocean, and a strong Greenland blocking typically increases those in the southwest of Greenland. When the Arctic bridge is lacking, the amount of moisture, clouds, and downward longwave radiation is anomalously high near the North Pole. Our results reveal a strong dependence of moisture, clouds, and longwave radiation on atmospheric pressure fields, which also appears to be important from a climate change perspective.

The Mechanism and Predictability of an Elevated Convection Initiation Event in a Weak-Lifting Environment in Central-Eastern China

Abstract An elevated convection initiation (CI) of a quasi-linear mesoscale convective system (MCS) that occurred in a weak-lifting environment in the early morning on 23 June 2016 in central-eastern China was investigated using observational analysis and convection-permitting numerical simulations. This MCS gradually developed into a surface-based MCS and eventually produced a strong supercell that spawned an EF4 tornado in Yancheng City of Jiangsu Province and killed 98 people. This elevated MCS was initiated ahead of a surface front without identifiable boundaries at the surface. An elevated moist absolutely unstable layer (MAUL) was found to be conducive to the CI. The MAUL provided negligible convective inhibition and contributed to CI without strong-lifting mechanisms. Numerical simulation results showed that the formation of the elevated MAUL was mainly attributed to adiabatic cooling by weak vertical ascent and sufficient horizontal moisture transport near the terminus of a low-level jet. The weak vertical ascent before the CI was sloping and was likely to be relevant to the layer-lifting process associated with the realization of potential instability. The results showed that the MAUL in this weak-lifting environment was characterized by a shallower depth, a weaker lapse rate, and a longer sustaining period than the conditions in a strong-lifting environment. The predictability of this elevated CI case was examined using a 10-member ensemble forecast. A total of 80% of the ensemble members captured the CI. Rather than a difference in lifting, whether having an elevated MAUL or not was the major difference between CI and non-CI members in the present case.

Atmospheric moisture transport between mid‐latitudes and the Arctic: Regional, seasonal and vertical distributions

Horizontal moisture transport has a manifold role in the Arctic climate system as it distributes atmospheric water vapour and thereby shapes the radiative and hydrological conditions. Moisture transport between the Arctic and the mid‐latitudes was examined based on ERA‐Interim reanalysis. The meridional net transport is only a small part of the water vapour exchange between the Arctic and mid‐latitudes and does not give a complete view of temporal and spatial variations in the transport. Especially near the surface, most of the northwards moisture transport is balanced by the southwards transport, and therefore the meridional net moisture transport at 60°–70°N peaks approximately at 100 hPa higher altitude than the northwards and southwards moisture transports. The total moisture transport (sum of absolute northwards and southwards moisture transports) has a much larger seasonal variation than the net transport (mean meridional transport), and the strength of the total transport is related to atmospheric humidity rather than the wind field. Strong individual moisture transport events contribute to a large part of the northwards moisture transport. This is consistent with the result that the net moisture transport is essentially generated by temporal variations of moisture fluxes. The moisture transport due to stationary zonal variation in the mass flux mostly defines the spatial distribution of the meridional moisture transport. The seasonal cycle of the net moisture transport is related to the seasonal cycle of transient eddy moisture transport but inter‐annual variations of the net moisture transport are largely influenced by the stationary eddy moisture transport.

Characteristics of the Marine Boundary Layer Jet over the South China Sea during the Early Summer Rainy Season of Taiwan

Abstract The marine boundary layer jets (MBLJs) over the northern South China Sea during the early summer rainy season over Taiwan are analyzed using 5-yr (2008–12) National Centers for Environmental Prediction Climate Forecast System Reanalysis data with a 6-h interval. The MBLJ is distinctly different from the low-level jets associated with the subsynoptic frontal systems. During this period, the MBLJ events over the northern South China Sea mainly occur during the second half of the monsoon rainy season over Taiwan (after 1 June) and have a wind speed maximum around the 925-hPa level. The MBLJs are mainly related to the subsynoptic-scale pressure gradients related to a relatively deep mei-yu trough over southeastern China and a stronger-than-normal west Pacific subtropical high. Within the MBL, there is a three-way balance among pressure gradients, Coriolis force, and surface friction, with cross-isobar ageostrophic winds pointing toward the mei-yu trough throughout the diurnal cycle. At the jet core, the vertical wind profile resembles an Ekman spiral with supergeostrophic winds >12 m s−1 near the top of the MBL. The MBLJs are strongest at night and close to geostrophic flow in the late afternoon/early evening. This is because the friction velocity and ageostrophic wind decrease during daytime in response to mixing in the lowest levels. The MBLJs play an important role in horizontal moisture transport from the northern South China Sea to the Taiwan area. In the frontal zone, the moisture tongue extends vertically upward. The rainfall production is related to vertical motions in the frontal zone or localized lifting due to orographic effects.

Influence of Environmental Conditions on Forecasting of an Advection-Radiation Fog: A Case Study from the Casablanca Region, Morocco

In this study, a dense advection-radiation fog that occurred over the Grand Casablanca region, Morocco, during the night of 23–24 December 2013, is investigated. The adverse visibility induced by the fog patch led to a series of collisions and loss of life on a highway of the region. This fog event is simulated by the Meso-NH research model. Conventional observations from two synoptic stations, satellite imagery, and the European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis are used to analyze the physical processes during the whole life cycle of the event. Some hypotheses on the influence of environmental conditions (topography, land-sea heterogeneity, urbanization) on the numerical fog prediction are presented. Observational analysis together with numerical results show that the horizontal moisture transport, linked to a northerly wind during the afternoon, drove the onset of this fog event. The formation stage was governed by weak turbulence and nocturnal radiative cooling at both stations (costal station, GMMC, and inland station, GMMN). Horizontal moisture flux convergence was observed at the top of the fog layer, over the coastal station, during the mature phase, in addition to the radiative-turbulent interactions. The latter was the main mechanism leading to fog thickening at the inland station. The dissipation phase was governed by a zone of horizontal moisture flux divergence linked to a dry wind from the south. Sensitivity experiments show that numerical coastal fog prediction is influenced by local topography, urbanization and aerosol types, but less by land cover.

Quasi-biweekly oscillations of the South Asian monsoon and its co-evolution in the upper and lower troposphere

The Upper Tropospheric Quasi-Biweekly Oscillation (UQBW) of the South Asian monsoon is studied using the potential vorticity field on the 370 K isentrope. The UQBW is shown to be a common occurrence in the upper troposphere during the monsoon, and its typical evolution is described. We suggest that the UQBW is a phenomenon of both the middle and tropical latitudes, owing its existence to the presence of the planetary-scale upper-tropospheric monsoon anticyclone. The UQBW is first identified as Rossby waves originating in the northern flank of the monsoon anticyclone. These Rossby waves break when reaching the Pacific Ocean, and their associated cyclonic PV anomalies move southward to the east of Asia and then westward across the Indian Ocean and Africa advected by the monsoon anticyclone. A strong correlation, or co-evolution, between the UQBW and quasi-biweekly oscillations in the lower troposphere (QBW) is also found. In particular, analysis of vertically-integrated horizontal moisture transport, 850 hPa geopotential, and outgoing long-wave radiation show that the UQBW is usually observed at the same time as, and co-evolves with, the lower tropospheric QBW over South Asia. We discuss the nature of the UQBW, and its possible physical link with the QBW.

A case‐study of a monsoon low that formed over the sea and intensified over land as seen in ECMWF analyses

A case‐study is presented of a tropical low that formed near Darwin, Australia, during the monsoon and subsequently intensified over land. The study is based on European Centre for Medium‐range Weather Forecast (ECMWF) analyses. Interpretations of the formation over the sea are given in terms of vorticity dynamics. The thermodynamic support for the intensification and maintenance of the low over land is investigated also. The analyses indicate that the intensification of the low depends on repeated bursts of deep convection occurring near the centre of the circulation that promote the further concentration of vorticity near the centre. This concentration of vorticity increases the local circulation about the centre, which amounts to increasing the local tangential wind speed and, through approximate gradient wind balance above the boundary layer, to a lowering of the central pressure. It is found that the horizontal transport of moisture into a mesoscale column centred on the low is approximately equal to the moisture lost by precipitation, so that total precipitable water levels are not rapidly depleted over land. While the contribution to the overall moisture budget by surface fluxes is comparatively small, these fluxes are necessary to maintain conditionally unstable conditions near the vortex centre so that deep convective bursts can continue to occur there, even when the system is located far inland.

An Objective Detection Method for Convective Cold Pool Events and Its Application to Northern Africa

Abstract Convective cold pool events in (semi) arid areas have significant impacts on their environment. They reach horizontal extents of up to several hundred kilometers and the associated turbulence and shear can cause dust emissions and threaten aviation safety. Furthermore, cold pools play a major role in the organization of deep convection and in horizontal moisture transport. They have even been proposed to have impacts on larger-scale monsoon dynamics. Cold pools are not well represented in models using a convective parameterization. To test and improve these models, it is necessary to reliably detect cold pool occurrence from standard observational data. Former studies, however, focused on single cases or short time periods. Here, an objective and automated method for the generation of multiyear climatologies of cold-pool events is presented. The algorithm combines standard surface observations with satellite microwave data. Representativeness of stations and influence of their spatial density are addressed by comparison to a satellite-only climatology. Applying this algorithm to data from automatic weather stations and manned synoptic stations in and south of the Atlas Mountains in Morocco and Algeria reveals the frequent occurrence of cold pool events in this region. On the order of six cold-pool events per month are detected from May to September when the Saharan heat low is in its northernmost position. The events tend to cluster into several-days-long convectively active periods, often with strong events on consecutive days. The algorithm is flexible enough to be applied in comparable regions around the world.

Diagnosing links between atmospheric moisture and extreme daily precipitation over the UK

ABSTRACTAtmospheric moisture characteristics associated with the heaviest 1% of daily rainfall events affecting regions of the British Isles are analysed over the period 1997–2008. A blended satellite/rain‐gauge data set (GPCP‐1DD) and regionally averaged daily rain‐gauge observations (HadUKP) are combined with the ERA Interim reanalysis. These are compared with simulations from the HadGEM2‐A climate model which applied observed sea surface temperature and realistic radiative forcings. Median extreme daily rainfall across the identified events and locations is larger for GPCP (32 mm day−1) than HadUKP and the simulations (∼25 mm day−1). The heaviest observed and simulated daily rainfall events are associated with increased specific humidity and horizontal transport of moisture (median 850 hPa specific humidity of ∼6 g kg−1 and vapour transport of ∼150 g kg− 1m s− 1 for both observed and simulated events). Extreme daily rainfall events are less common during spring and summer across much of the British Isles, but in the south east region, they contribute up to 60% of the total number of distinct extreme daily rainfall events during these months. Compared to winter events, the summer events over south east Britain are associated with a greater magnitude and more southerly location of moisture maxima and less spatially extensive regions of enhanced moisture transport. This contrasting dependence of extreme daily rainfall on moisture characteristics implies a range of driving mechanisms that depend upon location and season. Higher spatial and temporal resolution data are required to explore these processes further, which is vital in assessing future projected changes in rainfall and associated flooding.