Displays

We investigate the impact of an eastward moving cut-off low (CoL) on the formation of a river of smoke as well as on the vertical distribution of biomass burning aerosols (BBAs) in the troposphere during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) campaign in September 2017. The CoL developed in the westerlies over the Southeast Atlantic Ocean and advected over southern Africa between 1 and 6 September 2017. Northern Namibia, were most of the AEROCLO-sA related operations took place, was under the influence of a well formed, stationary, isolated CoL on 3 and 4 September. Subsequently, the fast evolving CoL travelled south-eastward over South Africa between 5 and 6 September before merging back with the main westerly flow. Based on the use of a tailored complementary suite of global and mesoscale numerical simulations as well as ground-based, airborne and space-borne observations of the atmospheric dynamics, thermodynamics and composition, the picture emerges that the characteristics of the river of smoke (timing, vertical extent of the BBA layer) are very much tied to the later (fast evolving) stage of the evolution of the CoL than the earlier (stationary) stage.  The mechanisms by which the CoL observed over southern Africa influences the vertical structure of the BBA layer is essentially through the ascending (descending) motion above the BBA layer to the northeast (southwest) of the CoL center. In the presence of the CoL, the top of the BBA layer over northern Namibia was found to reach attitudes in excess of 8 km AMSL. This is much higher that the height of the top of the BBA layer over the regions where the smoke originated from (Angola, Zambia, Zimbabwe, Mozambic) which was between 4 and 5 km AMSL. Also, the CoL favored the formation of mid-level clouds near the top of the BBA layer. Mid-level clouds were embedded in the river of smoke that were related to the circulation and ascending motions in the lee of the CoL.

How to cite: Flamant, C., Gaetani, M., Chaboureau, J.-P., Chazette, P., and Formenti, P.: The impact of an eastward traveling cut-off low on tropospheric composition and the formation of a river of smoke: a case from AEROCLO-sA, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2823, https://doi.org/10.5194/egusphere-egu2020-2823, 2020.

We investigate the transport of dust and biomass burning aerosols in South Atlantic during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) campaign in September 2017. A regional Meso-NH simulation has been run using a 12-km horizontal grid-spacing without deep convection parameterization, an on-line dust emission scheme, a passive tracer of biomass burning aerosol (BBA) emitted using the daily Global Fire Emissions Database and online-computed backward trajectories. The simulation captures both the aerosol optical depth and the vertical distribution of aerosols as observed from airborne and spaceborne lidars. It also reproduces the occurrence of deep convection over Congo and stratocumulus over South Atlantic well. A Lagrangian analysis reveals the origin of aerosols in the South Atlantic. Dust aerosols found just above the stratocumulus were emitted from the coasts and the Ethosha Pan a few days earlier. The BBAs located between 1 and 5 km come mainly from Angola in about 3.5 days. The 8-12 km layer is fed by up to 12 % of the air masses that experienced convection over the Congo Basin in the last 5 days. This amount is much reduced in the sensitivity simulation with a deep convection parameterization.

How to cite: Chaboureau, J.-P., Labbouz, L., Flamant, C., and Hodzic, A.: Dust and biomass burning aerosol transport in South Atlantic during AEROCLO-sA, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3723, https://doi.org/10.5194/egusphere-egu2020-3723, 2020.

The western coast of southern Africa off Namibia is characterized by a semi-permanent and widespread stratocumulus (Sc) cloud deck, very frequent coastal fog, and the oceanic northern Benguela upwelling system (nBUS). It is also the crossroad of large quantities of natural and anthropogenic aerosols of distant and local origins (biogenic, anthropogenic, biomass burning, sea salt and mineral dust) from continental and marine sources, with significant differences in terms of physico-chemical and optical properties, water affinity, scale and height of transport, which are not well represented in climate models.

In this presentation we will illustrate the results of the first extensive chemical and microphysical characterisation of aerosol particles in the area that has been conducted since 2016 at  the coastal Henties Bay experimental site (22°6’ S, 14°17’ E) in the framework of the AErosol, RadiatiOn and CLOuds in southern Africa (AEROCLO-sA) and the Atmospheric Research in the Southern Africa and Indian Ocean (ARSAIO) projects.

Synergetic filter sampling and online measurements provide examples of the numerous new particle formation in link with marine biogenic emissions and the apportionment of maritime sulfate aerosols, including their biogenic component.

How to cite: Formenti, P., Klopper, D., Chevaillier, S., D’Anna, B., Desboeufs, K., Doussin, J.-F., Feron, A., Giorio, C., Mallet, M. D., Mirande-Brét, C., Monod, A., Namwoonde, A., Triquet, S., and Piketh, S.: Chemical composition and characterization of aerosols off Namibia: results from the AEROCLO-sA project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3844, https://doi.org/10.5194/egusphere-egu2020-3844, 2020.

Atmospheric dynamics over southern Africa and South Atlantic is dominated by complex aerosol-radiation-cloud interactions, and the characterisation of the tropospheric distribution of aerosols is essential for the full understanding of these interactions.

During austral winter, a compact low cloud deck over South Atlantic contrasts clear sky over southern Africa, where forest fires triggered by dry conditions emit large amount of biomass burning aerosols in the free troposphere. Most of the aerosol burden crosses the Tropical Atlantic embedded in the tropical easterly flow. However, mid-latitude synoptic disturbances can deflect part of the aerosols from the main transport path towards southern extra-tropics.

In this study, a characterisation of the synoptic variability controlling biomass burning aerosols in southern Africa and South Atlantic during austral winter is presented. By analysing ECMWF reanalysis data, a weather regime classification of the region is constructed and used to characterise the aerosol distribution in the period 2003-2017. Results show three southward transport paths, each associated with a specific circulation regime.

How to cite: Gaetani, M., Alvarez Castro, M. C., Flamant, C., Pohl, B., and Formenti, P.: A weather regime characterisation of winter biomass aerosol transport in southern Africa, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5508, https://doi.org/10.5194/egusphere-egu2020-5508, 2020.

Instances of contact and separation between biomass-burning aerosols and marine stratocumulus have been observed over the South-East Atlantic Ocean. In-situ measurements of aerosol and cloud properties were made onboard the NASA P-3 aircraft during the ObseRvations of Aerosols above Clouds and their intEractionS ORACLES field campaign in Sep. 2016, Aug. 2017 and Oct. 2018. Variations in vertical profiles of droplet concentration N_c, effective radius R_e, and precipitation susceptibility S_o (change in rain rate R with N_c) as a function of cloud thickness H, were determined for varying above- and below-cloud aerosol concentration N_a. The data were sorted into 4 regimes according to whether a prominent aerosol layer (N_a > 500 cm^-3) was in contact or separated from cloud tops, and whether the clouds occurred within a clean or dirty boundary layer (BL).

The N_c and R_e were calculated using the number distribution function n(D) for 3 < D < 1280 μm from the Cloud and Aerosol Spectrometer and the 2D‐Stereo probe. N_a was calculated using n(D) for 0.1 < D < 3 μm from the Passive Cavity Aerosol Spectrometer Probe, and rain rate R was derived using droplet mass m(D), fall speed u(D) and n(D) for 50 < D < 1280 μm. Across the 3 campaigns, a total of 359 vertical profiles were flown through clouds and for 181 contact, or C-cases (N_a > 500 cm^-3 within 100 m above cloud), the mean N_c was 77 cm^-3 higher and mean R_e 1.78 μm lower, compared to 178 separated, or S-cases (N_a < 500 cm^-3 within 100 m above cloud). Within clean BLs (N_a < 400 cm^-3 within 100 m below cloud), mean N_c for C-cases was 42% higher than S-cases. Within dirty BLs (N_a > 400 cm^-3 within 100 m below cloud), mean N_c for C-cases was 53% higher, and the N_c change increased from 20% near cloud base to 75% near cloud top. Although cloud-top entrainment increased N_c throughout the cloud layer, its effect was weaker (stronger) near cloud base (top) within dirty BLs.

For all data combined, the average S_o was positive (0.94) implying R decreased with an increase in N_c. For C- and S-cases, the average S_o was 0.60 and 1.01, respectively, with the difference arising from low S_o (-0.06) for thin (H < 131 m) C-case clouds. When the thin, C-case clouds were further classified into clean & dirty BL cases, S_o was 0.67 and -0.57, respectively. Condensational growth is limited by low H in thin clouds and increasing N_c in clean conditions decreased R_e, hindering collision-coalescence. However, in dirty conditions with low R_e, increasing N_c only increased the number and collision efficiency of small droplets. The mean N_c (average S­_o) in the cleanest and dirtiest conditions (S-case & clean BL and C-case & dirty BL) was 84 cm^-3 and 206 cm^-3 (1.17 and 0.43), respectively. When condensational growth is no longer limited by H, R_e decreases with N_c and clean clouds with the highest R_e are the most susceptible to precipitation suppression.

How to cite: Gupta, S., McFarquhar, G., O'Brien, J., Poellot, M., and Dzambo, A.: Dependence of Cloud and Precipitation Properties over the South-East Atlantic on Aerosol Concentrations Above and Below Clouds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5748, https://doi.org/10.5194/egusphere-egu2020-5748, 2020.

This contribution presents new findings on synoptic-scale mechanisms that control the day-to-day variability of fog and low clouds (FLCs) in the Namib region.
FLCs are a defining element of the Namib-region climate and a crucial source of water for many species and ecosystems. Still, little is known on the processes driving Namib-region FLCs, in large part due to the very sparse observational records. Specifically, there is an ongoing debate in the scientific literature concerning the relevance of different mechanisms responsible for fog formation in the region. In this contribution, a new long-term satellite-based data set of FLC occurrence is used in conjunction with reanalysis data and backtrajectories to systematically analyze dynamical and thermodynamical differences between days with and without FLCs in the central Namib during two different seasons. The main findings are:

The findings lead to a better understanding of Namib-region FLCs and help broaden the understanding of low clouds along the southwestern African coastline and the southeast Atlantic. 

How to cite: Andersen, H., Cermak, J., Fuchs, J., Knippertz, P., Gaetani, M., Quinting, J., Sippel, S., and Vogt, R.: NaFoLiCA: Synoptic-scale controls of fog and low cloud variability in the Namib Desert, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7938, https://doi.org/10.5194/egusphere-egu2020-7938, 2020.

The Namibian coast line experiences fog when moist air from the southeast Atlantic is advected
over the desert landscape. We run the WRF model with the Thompson (2008) microphysics scheme,
with a default CCN number concentration of 100 cm-1, to forecast next day fog over the Namib
desert. Model output of liquid water content at the lowest level in the atmosphere is used to
represent fog and is evaluated against in situ observations of visibility and satellite products of
fog/low stratus. Preliminary results indicate that the model captures the spatial pattern of fog
excellently, however, the model over predicts fog occurrence. These results serve as the control run
for a future model sensitivity study.

How to cite: Weston, M., Piketh, S., Formenti, P., Brocardo, S., Andersen, H., and Voogt, R.: Evaluation of next day fog forecast over Namibia using the WRF model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19860, https://doi.org/10.5194/egusphere-egu2020-19860, 2020.

The southeast Atlantic serves as a natural laboratory for studying aerosol-cloud-radiation interactions due to the abundant presence of quasi-permanent stratocumulus and overlying biomass burning smoke aerosols during austral winters. Aerosol and cloud properties from the Spectrometers for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) and Solar Spectral Flux Radiometer (SSFR) on board NASA P-3 and High Spectral Resolution Lidar (HSRL) on board NASA ER-2 during the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign are used to compare with satellite retrievals. Aerosol and cloud properties from regional climate models such as WRF-Chem, WRF-Chem (with CAM5), ALADIN, GEOS-CHEM, EAM-E3SM, MERRA-2, and GEOS-5 with aerosol schemes are also compared against airborne measurements and satellite retrievals to evaluate and address the current model deficiencies in the southeast Atlantic. A preliminary estimate of the direct aerosol radiative effects over the southeast Atlantic will be presented.

How to cite: Chang, I. and the NASA ORACLES Team: Radiative Properties of Aerosols and Clouds from Observations and Models over the Southeast Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13086, https://doi.org/10.5194/egusphere-egu2020-13086, 2020.

Climate models have large uncertainty to represent the low clouds in the transitional zone from stratocumulus to cumulus clouds. This talk presents an observational study of a mesoscale cloud system near Ascension Island during the CLouds and Aerosol Radiative Impacts and Forcing (CLARIFY) field campaign. Extensive aircraft measurements were made to investigate the cloud microphysics when convection developed in a stratiform cloud system. The aircraft penetrated the clouds at levels below, within, and above the main layer. In-cloud penetrations show that the development of convections increased the drop number concentration, the effective radius of drops, the size of drizzle drops, and liquid water content. In the process of convection development, the morphology of the cloud changed from an overcast stratocumulus system to organised convective clouds. The wind shear and the compensating subsidence associated with the convections seemed to be responsible for the appearance of the system. The results indicate that the convective clouds significantly affected the stratocumulus cloud properties in the transition. Our study of the microphysical processes in the transitional zone helps to improve the representation and evolution of low clouds in models. 

How to cite: Cui, Z., Blyth, A., Abel, S., Barrett, P., and Gordon, H.: Development of convection in a mesoscale cloud system and its effect on the microphysics over the tropical Atlantic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3001, https://doi.org/10.5194/egusphere-egu2020-3001, 2020.

Due to the wide spread nature of marine stratocumulus cloud they have a significant impact on the Earth’s radiation budget. Such clouds are sensitive to the presence of aerosol, which can promote the break-up of a cloud deck into pockets of open cell convection (POC). The transition from a cloud deck to pockets of open cells changes the overall cloud albedo thus affecting the Earth’s radiation budget. The representation of stratocumulus cloud and the transition to POCs is poorly represented in current climate and weather models. This study aims to improve understanding of this process using extensive in-situ measurements made during the CLARIFY campaign of stratocumulus cloud decks, transition areas between overcast and open cell cloud structures as well as areas of POCs, to inform and compare to large-eddy simulations.

A variety of different aerosol situations occurred during CLARIFY, combinations of polluted/clean boundary layer and polluted/clean conditions above the cloud layer. Large-eddy simulations are conducted to investigate the sensitivity of clouds to changes in the observed aerosol conditions with a particular focus on whether or not the change in aerosol initiates cloud breakup.

The MetOffice NERC Cloud model (MONC) is used to preform the large-eddy simulations and employs the CASIM cloud microphysics scheme which includes activation of aerosol particles to cloud drops. Such a model set-up allows direct interaction between aerosols and clouds. Observations from CLARIFY are used to initialise and evaluate model simulations.

How to cite: Simpson, E. and Choularton, T.: Modelling study on the formation of pockets of open cells in marine stratocumulus cloud during CLARIFY, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16043, https://doi.org/10.5194/egusphere-egu2020-16043, 2020.