Cross‐Saharan transport of water vapor via recycled cold pool outflows from moist convection

Trzeciak, Tomasz M.; Garcia-Carreras, Luis; Marsham, John H.

doi:10.1002/2016gl072108

Cited by 28 publications

(35 citation statements)

References 37 publications

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“…Substantial radiative heating from dust occurs in the PBL up to 6 K day −1 through dust shortwave absorption. This will act to stabilise the PBL with im-plications for dry and moist convection, although Trzeciak et al (2017) suggest that moistening may often counter this. Such events typically involve an additional advective cooling which can be substantial up to 2-5 K day −1 for monsoon surges (Couvreux et al, 2010) but is restricted to the lowest layers (∼ 1 km from the surface).…”

Section: Discussionmentioning

confidence: 99%

“…In the subsequent absence of such observational data, we expect reanalysis errors to be greater given the known problems of reanalysis model representation of mesoscale convective processes in the region Roberts et al, 2015;Todd et al, 2013). Such mesoscale convective "cold-pool" outflow haboobs are known to make a significant contribution to moisture advection in addition to being the dominant dust emission process Trzeciak et al, 2017). Red arrows in Fig.…”

Section: Atmospheric Profile and Surface Characteristicsmentioning

confidence: 99%

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

et al. 2018

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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.

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Section: Discussionmentioning

confidence: 99%

Section: Atmospheric Profile and Surface Characteristicsmentioning

confidence: 99%

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

et al. 2018

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“…Additional uncertainties stem from the assumption that dust beneath 4 km is influenced by the underlying surface for SA and SD regions. Dust may be inhomogeneously distributed in the vertical and is likely to be subject to dry or moist convection and the daily redevelopment of a deep boundary layer (Trzeciak et al, 2017). The air mass may have received the dust deposited from upper layers from previous days as a result of sedimentation.…”

Section: The Hematite Content Of Dust Aerosolmentioning

confidence: 99%

Aircraft and ground measurements of dust aerosols over the west African coast in summer 2015 during ICE-D and AER-D

et al. 2018

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Abstract. During the summertime, dust from the Sahara can be efficiently transported westwards within the Saharan air layer (SAL). This can lead to high aerosol loadings being observed above a relatively clean marine boundary layer (MBL) in the tropical Atlantic Ocean. These dust layers can impart significant radiative effects through strong visible and IR light absorption and scattering, and can also have indirect impacts by altering cloud properties. The processing of the dust aerosol can result in changes in both direct and indirect radiative effects, leading to significant uncertainty in climate prediction in this region. During August 2015, measurements of aerosol and cloud properties were conducted off the coast of west Africa as part of the Ice in Cloud ExperimentDust (ICE-D) and AERosol properties -Dust (AER-D) campaigns. Observations were obtained over a 4-week period using the UK Facility for Atmospheric Airborne Measurements (FAAM) BAe 146 aircraft based on Santiago Island, Cabo Verde. Ground-based observations were collected from Praia (14 • 57 N, 23 • 29 W; 100 m a.s.l.), also located on Santiago Island. The dust in the SAL was mostly sampled in situ at altitudes of 2-4 km, and the potential dust age was estimated by backward trajectory analysis. The particle mass concentration (at diameter d = 0.1-20 µm) decreased with transport time. Mean effective diameter (D eff ) for supermicron SAL dust (d = 1-20 µm) was found to be 5-6 µm regardless of dust age, whereas submicron D eff (d = 0.1-1 µm) showed a decreasing trend with longer transport.For the first time, an airborne laser-induced incandescence instrument (the single particle soot photometer -SP2) was deployed to measure the hematite content of dust. For the Sahel-influenced dust in the SAL, the observed hematite mass fraction of dust (F Hm ) was found to be anti-correlated with the single scattering albedo (SSA, λ = 550 nm, for particles d < 2.5 µm); as potential dust age increased from 2 to 7 days, F Hm increased from 2.5 to 4.5 %, SSA decreased from 0.97 to 0.93 and the derived imaginary part (k) of the refractive index at 550 nm increased from 0.0015 to 0.0035. However, the optical properties of Sahara-influenced plumes (not influenced by the Sahel) were independent of dust age and hematite content with SSA ∼ 0.95 and k ∼ 0.0028. This indicates that the absorbing component of dust may be source dependent, or that gravitational settling of larger particles may lead to a higher fraction of more absorbing clay-iron aggregates at smaller sizes. Mie calculation using the measured size distribution and size-resolved refractive indices of the absorbing components (black carbon and hematite) reproduces the measured SSA to within ±0.02 for SAL dust by assuming a goethite / hematite mass ratio of 2. Overall, hematite and goethite constituted 40-80 % of the absorption for particles d < 2.5 µm, and black carbon (BC) contributed 10-37 %. This highlights the importance of size-dependent composition in determining the optical properties of dust and also the ...

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“…These occur as convective downdrafts reach the surface and fan out radially as density currents. Observations collected during the Fennec Campaign indicate that CPOs play the dominant role in emission of dust during the Saharan dust maximum in summer (Allen et al, ), and manual tracking of these events by satellite shows that they can encourage reinitiation of deep convection in the central Sahara where convective inhibition is high (Trzeciak et al, ). Although the bulk of outflow events are associated with the West African Monsoon (WAM), CPOs are also observed south of the Atlas mountain range where orography enhances convection (Emmel et al, ; Redl et al, ).…”

Section: Introductionmentioning

confidence: 99%

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

2019

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The 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.

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Cross‐Saharan transport of water vapor via recycled cold pool outflows from moist convection

Cited by 28 publications

References 37 publications

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

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

Aircraft and ground measurements of dust aerosols over the west African coast in summer 2015 during ICE-D and AER-D

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

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