Cold Pool Outflows Research Articles

Observation Analysis and Numerical Simulation of the Urban Barrier Effect on Thunderstorm Organization

The urban underlying surface may affect the thunderstorm process. However, current research on this phenomenon is still in its infancy. This paper aimed to analyze the influence of the urban underlying surface on the evolution of thunderstorm organization through ground observation and numerical simulation. The results indicated that when the thunderstorm system with strong synoptic conditions passed through the built-up area of Beijing, it exhibited obvious bifurcation and detour. The dynamic field of near-surface cold pools could serve as diagnostic indicators for understanding how the urban underlying surface affects the thunderstorm process. The large-scale compact-rise clusters in the city center could alter the movement direction and path of the cold pool outflow, thereby influencing the thunderstorm organization process. In addition to the spatial configuration of the building complex, the city size might also be an important factor influencing the thunderstorm process. This study might provide a fundamental foundation and technical support for predicting and assessing urban thunderstorm disasters.

The Development of Atmospheric Bores in Non‐Uniform Baroclinic Environments and Their Roles in the Maintenance, Structure, and Evolution of an MCS

AbstractThis study used radar observations and a high‐resolution numerical simulation to explore the interactions between an mesoscale convective system (MCS), cold pool outflows, and atmospheric bores in a non‐uniform baroclinic environment. The bores were generated by a nocturnal MCS that occurred on 2–3 June 2017 over the southern North China Plain. The goal of this investigation is to determine how the structure of bores varied within this non‐uniform environment and whether and how the bores would maintain the MCS and alter its structure. To the southwest of the MCS, where there was large CAPE and a well‐mixed boundary layer, discrete convection initiation occurred behind a single radar fine line (RFL) maintaining the propagation of the MCS. To the southeast of the MCS, multiple RFLs were found suggesting the generation of an undular bore in an environment containing an intense nocturnal stable boundary layer with dry upper layers and little CAPE. Hydraulic and nonlinear theory were applied to the simulation of the MCS revealing that the differences in the bore evolution depended on both the characteristics of the cold pool and the variations in the ambient environment. Thus, the characteristics of the ambient environment and the associated differences in bore structure impacted the maintenance and organization of the MCS. This study implies the importance of an accurate representation of the low‐level ambient environment and the microphysics and kinematics within the MCS to accurately simulate and forecast cold pools, the generation and evolution of bores, and their impact on nocturnal MCSs.

Formation Mechanisms of the “5·31” Record-Breaking Extreme Heavy Rainfall Process in South China in 2021

Based on the fifth-generation European Center for Medium-Range Weather Forecasts reanalysis data (ERA5), the real-time observation data from weather stations, and the radar products in Guangdong Province, we analyze the precipitation properties and formation mechanisms of the “5·31” extreme heavy rainfall process with record-breaking 3-h accumulated rainfall in South China during 2021. The results show that the extreme heavy rainfall process is caused by the joint actions of weather systems such as a weak upper-level short-wave trough, a surface stationary front, and a low-level southwesterly jet. Before the heavy precipitation process, there is large precipitable water content and deep warm clouds, which provides a potential for the occurrence and development of the heavy rainfall process in Longhua Town of Longmen County and its surrounding areas. Simultaneously, the low-level southwesterly jet provides abundant warm-wet water vapor for the heavy rainfall area. The vertical atmospheric environmental conditions, such as strong horizontal temperature gradient, high convective available potential energy, high-temperature difference between 850 hPa and 500 hPa, and low convective inhibition, maintain for a long duration in the heavy rainfall area, which are favorable for the occurrence and development of high-efficiency convective precipitation caused by water vapor condensation due to the uplift of low-level warm-wet airflows. The combined effects of the enhanced low-level southwesterly airflow, the stationary front, the mesoscale surface convergence line generated by cold pool outflows, the terrain influence, and the train effect of the precipitation echoes make heavy precipitation near Longhua last longer and stronger than other areas, leading to the extreme heavy rainfall with the record-breaking 3-h accumulated rainfall in Longhua.

Analysis of the Triggering and Maintenance Mechanisms of a Record-Breaking Warm-Sector Extreme-Rainfall Process in Front of an Upper-Level Trough in Tianjin, China

A short-time rainstorm exceeding the extreme historical rainfall occurred in the Jinnan District of Tianjin, China, on 3 July 2022. Due to the concentrated time period of precipitation, it caused serious water accumulation in the Jinnan District. The purpose of this paper is to study the weather mechanism of this extreme rainstorm in the Jinan District of Tianjin. By analyzing the fine observation facts, we can obtain the mesoscale weather characteristics and environmental conditions of the process. The results provide a reference for similar weather forecasting and warning in the future. Based on the 1 min interval precipitation observation data, the ERA5 reanalysis data, the CINRAD-SA radar reflectivity data of Tanggu, the cloud-top brightness temperature data of the Fengyun-4A satellite, and the Variational Doppler Radar Analysis System data, we comprehensively analyzed a record-breaking extreme rainfall process in Tianjin on 3 July 2022. The results show that the extreme rainfall process presents prominent mesoscale weather characteristics, with high precipitation intensity in a short-term period. This process is influenced by multi-scale weather systems, including the 500 hPa upper-level trough and the long-distance water vapor transport by Typhoon Chaba. The rainstorm event is caused by the combined actions of cold pool outflow produced by the upstream precipitation, the easterly disturbance in the boundary layer, the mesoscale temperature front, and the ground convergence line. Specifically, the ground convergence line is formed by the northerly wind of the cold pool outflow and the warm and moist southerly airflow from the ocean, and the temperature front is caused by the horizontal thermal difference of the underlying surface. Both the ground convergence line and temperature front contribute considerably to the triggering of mesoscale convection. The mesoscale secondary circulation is formed in the meridional direction by the meso-γ-scale convergence and its interaction with strong velocity in front of the trough, contributing to the development and maintenance of vertical motion in the Jinnan region of Tianjin and thereby leading to the occurrence and development of this extreme heavy rainfall process.

The Extreme North African Haboob in October 2008: High‐Resolution Simulation of Organized Moist Convection in the Lee of the Atlas, Dust Recirculation and Poleward Transport

AbstractThis study investigates the mesoscale dynamics involved in the 8–11 October 2008 unseasonably strong African dust episode, during which dust was transported to the Iberian Peninsula (IP). We employ observational datasets and a high‐resolution Weather Research and Forecasting model coupled with Chemistry simulations. The analysis shows that during 0900–1200 UTC 9 October, a mesoscale convective system developed over the Atlas Mountains and resulted in a southwestward propagating convective cold pool outflow on the southern foothills of the Anti‐Atlas, which lifted dust from the source region. Between 1200 and 1800 UTC 9 October, new moist convection was enhanced over the Atlas Mountains due to intensifying confluence among a heat low, moist southwesterly Atlantic sea‐breeze front, and northeasterly flow associated with the convective cold pool near western Algeria. This new moist convection intensified the strength of the convective cold pool outflow and haboob, both of which continued propagating southwestward. At 1200 UTC 10 October, the low‐pressure system migrated poleward on the southern slopes of the Anti‐Atlas Mountains in association with a mountain‐plains solenoidal circulation due to the daytime differential heating between the southern slopes of the Anti‐Atlas and nearby atmosphere. The deepening low‐pressure and strengthening Atlantic sea‐breeze redirected an equatorward advancing dust plume into the poleward direction. The dust plume ultimately crossed the Saharan Atlas Mountains on 11 October and finally impacted the IP.

Quantifying Mechanisms of Aeolian Dust Emission: Field Measurements at Etosha Pan, Namibia

AbstractDetermining the controls on aeolian dust emissions from major sources is necessary for reliable quantification of atmospheric aerosol concentrations and fluxes. However, ground‐based measurements of dust emissions at‐source are rare and of generally short duration, failing to capture the annual cycle. Here, we provide new insights into dust dynamics by measuring aerosol concentrations and meteorological conditions for a full year (July 2015–June 2016) at Etosha Pan, Namibia, a globally significant dust source. Surface deployed field instrumentation provided 10‐min averaged data on meteorological conditions, aerosol concentration (mg/m3), and horizontal dust flux (g/m2/min10). A Doppler lidar provided additional data for some of the period. 51 significant dust events were identified in response to strong E‐ENE winds. We demonstrate that these events occurred throughout the year and were not restricted to the austral winter, as previously indicated by satellite observations. Peak horizontal flux occurred in the spring (November) due to strengthening erosive winds and highly desiccating conditions increasing surface erodibility. We identify a strong seasonal differentiation in the meteorological mechanisms controlling dust uplift; low‐level jets on dry winter mornings (61% of all events), and cold pool outflows in humid summer evenings (39% of events). Significantly, we demonstrate a very strong bias toward the contribution of low frequency and high magnitude events, with nearly 31% of annual horizontal dust flux generated by only 6 individual events. Our study demonstrates how longer‐term (≈1 year), ground‐based, and at‐source field measurements can radically improve interpretations of dust event dynamics and controls at major source locations.

Roles of Terrain, Surface Roughness, and Cold Pool Outflows in an Extreme Rainfall Event Over the Coastal Region of South China

AbstractAn extreme rainfall event with maximum 12 hr accumulated rainfall of 464.8 mm associated with a quasi‐stationary mesoscale convective system occurred over the western coastal region of South China in June 2017. An observational analysis shows that early convective storms were initiated near the mountains and moved northeastwards. Moreover, a convergence line between cold northerly winds and warm southerly winds in the lower levels was formed, which favored the development of the quasi‐stationary system. Cold northerly winds were associated with land breeze, downslope winds as well as previous rainfall during earlier period, and cold pool outflows during later period. Cloud‐resolving simulations were performed to examine the roles of terrain, land‐sea surface roughness contrast, and cold pool outflows in the formation of heavy rainfall. Results demonstrate that land‐sea surface roughness contrast and mountains in the middle of Yangjiang and Jiangmen facilitated the formation of the convergence line during earlier period, and cold pool outflows sustained it during later period. Besides orographic lifting, coastal hills helped reduce the stability. Mountains in the middle of Yangjiang hindered the movement of the convective system, without which the associated heavy rainfall would shift farther north. Mt. Tianlu with westward concave morphology played a vital role in the formation of local convergence and the concentration of heavy rainfall. Without it, the total rainfall in the region of interest was reduced. This study suggests the importance of representing processes associated with the complex underlying surface in models for the prediction of heavy rainfall in coastal regions.

Radiative Effects of Increased Water Vapor in the Upper Saharan Air Layer Associated With Enhanced Dustiness

AbstractThe Saharan Air Layer (SAL) is known as an elevated, well‐mixed, warm, dry, frequently dusty layer and plays an important role in regional climate and dust transport. A new analysis of aircraft observations of haboob‐driven dust events shows that although increased dustiness in the SAL is associated with drier conditions in the lower‐SAL as expected, dustiness is also associated with increased moisture in the upper‐SAL, likely originating from cold pool outflows driving the dust uplift. We assess the radiative effects of the observed dust and increased water vapor (WV). The observed WV in the upper‐SAL affects the top‐of‐atmosphere (TOA) direct radiative effect (DRE), while lower‐SAL WV affects the surface DRE and column atmospheric heating. TOA DRE is negative for dust only, while including both the observed dust and WV reduces the magnitude of the negative TOA DRE by 17% (3.0 ± 0.8 Wm−2) when AOD > 0.6. The observed WV structure increases the magnitude of the negative surface DRE from dust by 8% (5.1 ± 0.8 Wm−2) and increases atmospheric heating by 17% (8.0 ± 0.6 Wm−2). These effects are driven by longwave (LW) radiation, whereby WV changes increase the positive TOA LW DRE, decrease the surface LW DRE and change the sign of LW atmospheric heating from negative to positive. WV leads to enhanced cooling in the moist upper‐SAL and heating in the dry lower‐SAL under dustier conditions. Increased WV in the SAL is consistent with other studies demonstrating increasing Saharan WV. This study demonstrates the importance of upper‐SAL WV in determining the radiative effect of dust.

Convective Cold Pool Associated with Offshore Propagation of Convection System over the East Coast of Southern Sumatra, Indonesia

The cold pool outflow has been previously shown to be generated by decaying Mesoscale Convective Complexes (MCCs) in the Maritime Continent. The cold pool also has a main role in the development processes of oceanic convective systems inducing heavy rainfall. This study investigated a cold pool event (January 1-2, 2021) related to a heavy rainfall system over the coastal region of Lampung, Southern Sumatra, within a high-resolution model simulation using a regional numerical weather prediction of the Weather Research and Forecasting (WRF) with convection permitting of 1 km spatial resolution, which was validated by satellite and radar data observations. It is important to note that the intensity, duration, timing, and structure of heavy rainfall simulated were in good agreement with satellite-observed rainfall. The results also showed that a cold pool (CP) plays an important role in inducing Mesoscale Convective Complex (MCC) and was responsible for the development of an offshore propagation of land-based convective systems due to the late afternoon rainfall over inland. This study also suggests that the propagation speed of the CP 8.8 m·s−1 occurring over the seaside of the coastal region, the so-called CP-coastal, is a plausible mechanism for the speed of the offshore-propagating convection, which is dependent on both the background prevailing wind and outflow. These conditions help to maintain the near-surface low temperatures and inhibit cold pool dissipation, which has implications for the development of consecutive convection.

Capturing convection essential for projections of climate change in African dust emission

The summertime Sahara and Sahel are the world’s largest source of airborne mineral dust. Cold-pool outflows from moist convection (‘haboobs’) are a dominant source of summertime uplift but are essentially missing in global models, raising major questions on the reliability of climate projections of dust and dust impacts. Here we use convection-permitting simulations of pan-African climate change, which explicitly capture haboobs, to investigate whether this key limitation of global models affects projections. We show that explicit convection is key to capturing the observed summertime maximum of dust-generating winds, which is missed with parameterised convection. Despite this, future climate changes in dust-generating winds are more sensitive to the effects of explicit convection on the wider meteorology than they are to the haboobs themselves, with model differences in the change in dust-generating winds reaching 60% of current values. The results therefore show the importance of improving convection in climate models for dust projections.

Satellite‐Derived Characteristics of Saharan Cold Pool Outflows During Boreal Summer

AbstractCold pool outflows (CPOs) are thought to be the most significant meteorological mechanism of mineral dust emission from the world's largest source in the central and western Sahara in boreal summer. An absence of CPOs from numerical models and reanalyses used to simulate Saharan dust emission leads to considerable error in modeling of dust fluxes from the Sahara. As such, the role of CPOs in the observed variability of dust through the monsoon season remains unclear. To remedy these issues, an improved observational benchmark is needed. In this research, an automated approach to identify and track CPOs in dust imagery from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) is derived. The approach is found to flag 74.2% of events identified manually (26/35). 1,559 events are tracked for June, July and August of 2004–2017. CPOs follow a clear diurnal cycle, peaking at 1700–1900 Universal Time Coordinated. Propagation speeds decay exponentially through their lifetime, but on average speeds are 1.5 ms−1 higher at night. About 22.5% of the observed events exceed a total traveled distance of 300 km, with an overwhelming preference for northwestwards propagation. Common across the southern central and western Sahara, CPO activity shifts north through summer in line with observed dust emission. The exception to this is the development of an intense hotspot of CPO activity in southern Algeria in August, which does not parallel any known late season outbreaks of dust. The results underline the importance of the southernmost Saharan dust sources, activated by frequent CPO occurrence in early summer.

On a Flood-Producing Coastal Mesoscale Convective Storm Associated with the Kor’easterlies: Multi-Data Analyses Using Remotely-Sensed and In-Situ Observations and Storm-Scale Model Simulations

A flood-producing heavy rainfall event occurred at the mountainous coastal region in the northeast of South Korea on 5–6 August 2018, subsequent to extreme heat waves, through a quasi-stationary mesoscale convective system (MCS). We analyzed the storm environment via a multi-data approach using high-resolution (1-km) simulations from the Weather Research and Forecasting (WRF) and in situ/satellite/radar observations. The brightness temperature, from the Advanced Himawari Imager water vapor band, and the composite radar reflectivity were used to identify characteristics of the MCS and associated precipitations. The following factors affected this back-building MCS: low-level convergence by the Korea easterlies (Kor’easterlies), carrying moist air into the coast; strong vertical wind shear, making the updraft tilted and sustained; coastal fronts and back-building convection bands, formed through interactions among the Kor’easterlies, cold pool outflows, and orography; mid-level advection of cold air and positive relative vorticity, enhancing vertical convection and potential instability; and vigorous updraft releasing potential instability. The pre-storm synoptic environment provided favorable conditions for storm development such as high moisture and temperature over the coastal area and adjacent sea, and enhancement of the Kor’easterlies by expansion of a surface high pressure system. Upper-level north-northwesterly winds prompted the MCS to propagate south-southeastward along the coastline.

Tracking the Gust Fronts of Convective Cold Pools

AbstractIt is increasingly acknowledged that cold pools can influence the initiation of new convective cells. Yet the full complexity of convective organization through cold pool interaction is poorly understood. This lack of understanding may partially be due to the intricacy of the dynamical pattern formed by precipitation cells and their cold pools. Additionally, how exactly cold pools interact is insufficiently known. To better understand this dynamics, we develop a tracking algorithm for cold pool gust fronts. Rather than tracking thermodynamic anomalies, which do not generally coincide with the gust front boundaries, our approach tracks the dynamical cold pool outflow. Our algorithm first determines the locus of the precipitation event. Second, relative to this origin and for each azimuthal bin, the steepest gradient in the near‐surface horizontal radial velocity vr is employed to determine the respective locus of the cold pool gust front edge. Steepest vr gradients imply largest updraft velocities, hence strongest dynamical triggering. Results are compared to a previous algorithm based on the steepest gradient in temperature—highlighting the benefit of the method described here in determining dynamically active gust front regions. Applying the method to a range of numerical experiments, the algorithm successfully tracks an ensemble of cold pools. A linear relation emerges between the peak rain intensity of a given event and maximal vr for its associated cold pool gust front—a relation found to be nearly independent of the specific sensitivity experiment.

A 14‐Year Climatology of Saharan Dust Emission Mechanisms Inferred From Automatically Tracked Plumes

AbstractThe central and western Sahara is the largest source of mineral aerosols during boreal summer, but observed ground‐based data are extremely scarce and typically distant from key source regions. Knowledge of dust emission mechanisms has therefore been mostly limited to short‐term observations from a point or model approximations. To address this deficiency, dust plumes from the central and western Sahara are classified according to emission mechanism for June, July, and August of 2004–2017 using an automated inference method, which accurately tracks the timing, convective association, and geometry of plumes observed with the Spinning Enhanced Visible and Infrared Imager aboard Meteosat Second Generation satellites. From these characteristics, plumes are classified as either low‐level jet or cold pool outflow events. The extensive data set is used to generate the largest available climatology of dust emission sources and Saharan emission mechanisms. Automated inference compares well with ground‐based measurements from the Fennec Campaign (76% accuracy) as well as with an entirely manual approach (88% accuracy). Cold pool activity accounts for 82% of total observed dust and 88% at the point of emission. Dust from cold pools evolves seasonally from hot spots around the Mali‐Niger‐Algeria border triple point toward the central Sahara to the northwest, while dust from low‐level jets is organized along the axis of the northeasterly Harmattan, and dominates emission within the Tidihelt Depression of central Algeria. The widespread importance of cold pool outflows in this research supports the findings of the Fennec Campaign, but low‐level jets remain highly significant in certain isolated hot spots.

On the Creation and Evolution of Small-Scale Low-Level Vorticity Anomalies during Tropical Cyclogenesis

Abstract Recent numerical modeling and observational studies indicate the importance of vortical hot towers (VHTs) in the transformation of a tropical disturbance to a tropical depression. It has recently been recognized that convective-scale downdraft outflows that form within VHTs also preferentially develop positive vertical vorticity around their edges, which is considerably larger in magnitude than ambient values. During a numerical simulation of tropical cyclogenesis it is found that particularly strong low-level convectively induced vorticity anomalies (LCVAs) occasionally form as convection acts on the enhanced vorticity at the edges of cold pools. These features cycle about the larger-scale circulation and are associated with a coincident pressure depression and low-level wind intensification. The LCVAs studied are considerably deeper than the vorticity produced at the edges of VHT cold pool outflows, and their evolution is associated with persistent convection and vortex merger events that act to sustain them. Herein, we highlight the formation and evolution of two representative LCVAs and discuss the environmental parameters that eventually become favorable for one LCVA to reach the center of a larger-scale circulation as tropical cyclogenesis occurs.

Convection‐Permitting Simulation of a Heavy Rainfall Event in Armenia Using the WRF Model

AbstractOn 2 July 2016 afternoon a heavy rainfall (52.8 mm) was observed over southwestern part of Armenia, at Talin station. High‐frequency radar observations and Weather Research and Forecasting (WRF) model output show that initiation of earliest convection occurred over Aragats mountain massif around noon due to low‐level convergence of thermally induced upslope winds. Further convective development was affected by generation of secondary convection as a result of interaction between cold pool outflows from developed convective cells and upslope winds. The high‐resolution WRF run (3 km) using NSSL two‐moment cloud microphysics parametrization and the European Centre for Medium‐Range Weather Forecasts operational model forcing data best reproduces the location, timing, magnitude, and microphysical structure of the observed convective rainfall among six WRF microphysical schemes tested in this study. Radar observations show that cold cloud process typical for continental‐type deep convection was observed in Armenia. The NSSL includes the double‐moment microphysics schemes for both warm and cold cloud processes, which might be a reason for improved simulation of observed heavy rainfall event in Armenia. Using the coarser resolution ERA5 analysis forcing data in the WRF model leads to simulation of earlier rainfall peaks at Talin station and spurious convective rainfall areas. The WRF model forced by the Global Data Assimilation System Final analysis from the National Centers for Environmental Prediction, even run at 3‐km resolution, is not able to reproduce the accurate location of convection and rainfall over the study area.

Can explicit convection improve modelled dust in summertime West Africa?

Abstract. Global and regional models have large systematic errors in their modelled dust fields over West Africa. It is well established that cold-pool outflows from moist convection (haboobs) can raise over 50 % of the dust over parts of the Sahara and Sahel in summer, but parameterised moist convection tends to give a very poor representation of this in models. Here, we test the hypothesis that an explicit representation of convection in the Met Office Unified Model (UM) improves haboob winds and so may reduce errors in modelled dust fields. The results show that despite varying both grid spacing and the representation of convection there are only minor changes in dust aerosol optical depth (AOD) and dust mass loading fields between simulations. In all simulations there is an AOD deficit over the observed central Saharan dust maximum and a high bias in AOD along the west coast: both features are consistent with many climate (CMIP5) models. Cold-pool outflows are present in the explicit simulations and do raise dust. Consistent with this, there is an improved diurnal cycle in dust-generating winds with a seasonal peak in evening winds at locations with moist convection that is absent in simulations with parameterised convection. However, the explicit convection does not change the AOD field in the UM significantly for several reasons. Firstly, the increased windiness in the evening from haboobs is approximately balanced by a reduction in morning winds associated with the breakdown of the nocturnal low-level jet (LLJ). Secondly, although explicit convection increases the frequency of the strongest winds, they are still weaker than observed, especially close to the observed summertime Saharan dust maximum: this results from the fact that, although large mesoscale convective systems (and resultant cold pools) are generated, they have a lower frequency than observed and haboob winds are too weak. Finally, major impacts of the haboobs on winds occur over the Sahel, where, although dust uplift is known to occur in reality, uplift in the simulations is limited by a seasonally constant bare-soil fraction in the model, together with soil moisture and clay fractions which are too restrictive of dust emission in seasonally varying vegetated regions. For future studies, the results demonstrate (1) the improvements in behaviour produced by the explicit representation of convection, (2) the value of simultaneously evaluating both dust and winds and (3) the need to develop parameterisations of the land surface alongside those of dust-generating winds.

Understanding the summertime diurnal cycle of precipitation over sub-Saharan West Africa: regions with daytime rainfall peaks in the absence of significant topographic features

Convection-permitting regional model output is analyzed to better understand the diurnal cycle of rainfall during the height of the West African summer monsoon. This investigation focuses on two regions, the western Bodele of Chad and eastern Burkina Faso, that have a propensity for daytime rainfall, but why this occurs does not conform to our conventional understanding of being directly associated with the presence of significant topographic feature(s). The August diurnal cycle of rainfall for the Bodele is characterized by an afternoon peak, with mesoscale convective systems (MCSs) originating within 500 km accounting for 80% of the afternoon precipitation. These MCSs are associated with a deepening of the monsoon trough over the western Sahara related to northern storm track African easterly wave (AEW) disturbance activity, combined with anomalous ridging over eastern Chad associated with cold pool outflow from convection that originates over the Marra Mountains the previous afternoon. These circulation features enhance the moist low-level southwesterly flow and increase instability over the Bodele. Over Burkina Faso rainfall has a primary afternoon peak, and a secondary morning peak. MCSs account for 95% of the total rainfall. Morning rainfall is primarily due to MCSs forming over the Damergou Gap of Niger, while the afternoon rainfall is associated with MCSs that originate over the Damergou Gap as well as locally. While both types of MCSs are associated with an approaching southern storm track AEW disturbance, it is differences in northern storm track activity that helps explain why some MCSs originate over the Damergou Gap.

Large-eddy simulation of dust-uplift by a haboob density current

Cold pool outflows have been shown from both observations and convection-permitting models to be a dominant source of dust emissions (“haboobs”) in the summertime Sahel and Sahara, and to cause dust uplift over deserts across the world. In this paper Met Office Large Eddy Model (LEM) simulations, which resolve the turbulence within the cold-pools much better than previous studies of haboobs with convection-permitting models, are used to investigate the winds that uplift dust in cold pools, and the resultant dust transport. In order to simulate the cold pool outflow, an idealized cooling is added in the model during the first 2 h of 5.7 h run time. Given the short duration of the runs, dust is treated as a passive tracer. Dust uplift largely occurs in the “head” of the density current, consistent with the few existing observations. In the modeled density current dust is largely restricted to the lowest, coldest and well mixed layers of the cold pool outflow (below around 400 m), except above the “head” of the cold pool where some dust reaches 2.5 km. This rapid transport to above 2 km will contribute to long atmospheric lifetimes of large dust particles from haboobs. Decreasing the model horizontal grid-spacing from 1.0 km to 100 m resolves more turbulence, locally increasing winds, increasing mixing and reducing the propagation speed of the density current. Total accumulated dust uplift is approximately twice as large in 1.0 km runs compared with 100 m runs, suggesting that for studying haboobs in convection-permitting runs the representation of turbulence and mixing is significant. Simulations with surface sensible heat fluxes representative of those from a desert region during daytime show that increasing surface fluxes slows the density current due to increased mixing, but increase dust uplift rates, due to increased downward transport of momentum to the surface.

The early summertime Saharan heat low: sensitivity of the radiation budget and atmospheric heating to water vapour and dust aerosol

Abstract. The Saharan heat low (SHL) is a key component of the west African climate system and an important driver of the west African monsoon across a range of timescales of variability. The physical mechanisms driving the variability in the SHL remain uncertain, although water vapour has been implicated as of primary importance. Here, we quantify the independent effects of variability in dust and water vapour on the radiation budget and atmospheric heating of the region using a radiative transfer model configured with observational input data from the Fennec field campaign at the location of Bordj Badji Mokhtar (BBM) in southern Algeria (21.4∘ N, 0.9∘ E), close to the SHL core for June 2011. Overall, we find dust aerosol and water vapour to be of similar importance in driving variability in the top-of-atmosphere (TOA) radiation budget and therefore the column-integrated heating over the SHL (∼ 7 W m−2 per standard deviation of dust aerosol optical depth – AOD). As such, we infer that SHL intensity is likely to be similarly enhanced by the effects of dust and water vapour surge events. However, the details of the processes differ. Dust generates substantial radiative cooling at the surface (∼ 11 W m−2 per standard deviation of dust AOD), presumably leading to reduced sensible heat flux in the boundary layer, which is more than compensated by direct radiative heating from shortwave (SW) absorption by dust in the dusty boundary layer. In contrast, water vapour invokes a radiative warming at the surface of ∼ 6 W m−2 per standard deviation of column-integrated water vapour in kg m−2. Net effects involve a pronounced net atmospheric radiative convergence with heating rates on average of 0.5 K day−1 and up to 6 K day−1 during synoptic/mesoscale dust events from monsoon surges and convective cold-pool outflows (“haboobs”). On this basis, we make inferences on the processes driving variability in the SHL associated with radiative and advective heating/cooling. Depending on the synoptic context over the region, processes driving variability involve both independent effects of water vapour and dust and compensating events in which dust and water vapour are co-varying. Forecast models typically have biases of up to 2 kg m−2 in column-integrated water vapour (equivalent to a change in 2.6 W m−2 TOA net flux) and typically lack variability in dust and thus are expected to poorly represent these couplings. An improved representation of dust and water vapour and quantification of associated radiative impact in models is thus imperative to further understand the SHL and related climate processes.