scholarly journals Impact of the Variability in Vertical Separation between Biomass-Burning Aerosols and Marine Stratocumulus on Cloud Microphysical Properties over the Southeast Atlantic

2020 ◽  
Author(s):  
Siddhant Gupta ◽  
Greg M. McFarquhar ◽  
Joseph R. O'Brien ◽  
David J. Delene ◽  
Michael R. Poellot ◽  
...  
Keyword(s):  
Biomass Burning ◽  
Cloud Droplet ◽  
Liquid Water Content ◽  
Droplet Evaporation ◽  
Cloud Layer ◽  
Aerosol Layer ◽  
Marine Stratocumulus ◽  
Microphysical Properties ◽  
Cloud Tops ◽  
The Impact

Abstract. Marine stratocumulus cloud properties over the southeast Atlantic Ocean are impacted by contact between above-cloud biomass-burning aerosols and cloud tops. Different vertical separations (0 to 2000 m) between the aerosol layer and cloud tops were observed on six research flights in September 2016 during the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign. There were 30 contact profiles where the aerosol layer with aerosol concentration (Na) > 500 cm−3 was within 100 m of cloud tops, and 41 separated profiles where the aerosol layer with Na > 500 cm−3 was located more than 100 m above cloud tops. For contact profiles, the average cloud droplet concentration (Nc) in the cloud layer was up to 68 cm−3 higher, the effective radius (Re) up to 1.3 µm lower and the liquid water content (LWC) within 0.01 g m−3 compared to separated profiles. Free tropospheric humidity was higher in the presence of biomass-burning aerosols and contact profiles had a smaller decrease in humidity (and positive buoyancy) across cloud tops due to higher median above-cloud Na (895 cm−3) compared to separated profiles (30 cm−3). Due to droplet evaporation from entrainment mixing of warm, dry free tropospheric air into the clouds, the median Nc and LWC for contact profiles decreased with height by 21 % and 9 % in the top 20 % of the cloud layer. The impact of droplet evaporation was stronger during separated profiles as a greater decrease in humidity (and negative buoyancy) across cloud tops led to greater decreases in median Nc (30 %) and LWC (16 %) near cloud tops. Below-cloud Na was sampled during 61 profiles, and most contact profiles (20 out of 28) were within high-Na (> 350 cm−3) boundary layers while most separated profiles (22 out of 33) were within low-Na (

scholarly journals Impact of the variability in vertical separation between biomass burning aerosols and marine stratocumulus on cloud microphysical properties over the Southeast Atlantic

2021 ◽  
Vol 21 (6) ◽  
pp. 4615-4635
Author(s):  
Siddhant Gupta ◽  
Greg M. McFarquhar ◽  
Joseph R. O'Brien ◽  
David J. Delene ◽  
Michael R. Poellot ◽  
...  
Keyword(s):  
Boundary Layers ◽  
Biomass Burning ◽  
Liquid Water ◽  
Droplet Evaporation ◽  
Cloud Layer ◽  
Cloud Base ◽  
Aerosol Layer ◽  
Marine Stratocumulus ◽  
Cloud Tops ◽  
The Impact

Abstract. Marine stratocumulus cloud properties over the Southeast Atlantic Ocean are impacted by contact between above-cloud biomass burning aerosols and cloud tops. Different vertical separations (0 to 2000 m) between the aerosol layer and cloud tops were observed on six research flights in September 2016 during the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign. There were 30 contact profiles, where an aerosol layer with aerosol concentration (Na) > 500 cm−3 was within 100 m of cloud tops, and 41 separated profiles, where the aerosol layer with Na > 500 cm−3 was located more than 100 m above cloud tops. For contact profiles, the average cloud droplet concentration (Nc) in the cloud layer was up to 68 cm−3 higher, the effective radius (Re) up to 1.3 µm lower, and the liquid water content (LWC) within 0.01 g m−3 compared to separated profiles. Free-tropospheric humidity was higher in the presence of biomass burning aerosols, and contact profiles had a smaller decrease in humidity (and positive buoyancy) across cloud tops with higher median above-cloud Na (895 cm−3) compared to separated profiles (30 cm−3). Due to droplet evaporation from entrainment mixing of warm, dry free-tropospheric air into the clouds, the median Nc and LWC for contact profiles decreased with height by 21 and 9 % in the top 20 % of the cloud layer. The impact of droplet evaporation was stronger during separated profiles as a greater decrease in humidity (and negative buoyancy) across cloud tops led to greater decreases in median Nc (30 %) and LWC (16 %) near cloud tops. Below-cloud Na was sampled during 61 profiles, and most contact profiles (20 out of 28) were within high-Na (> 350 cm−3) boundary layers, while most separated profiles (22 out of 33) were within low-Na (< 350 cm−3) boundary layers. Although the differences in below-cloud Na were statistically insignificant, contact profiles within low-Na boundary layers had up to 34.9 cm−3 higher Nc compared to separated profiles. This is consistent with a weaker impact of droplet evaporation in the presence of biomass burning aerosols within 100 m above cloud tops. For contact profiles within high-Na boundary layers, the presence of biomass burning aerosols led to higher below-cloud Na (up to 70.5 cm−3) and additional droplet nucleation above the cloud base along with weaker droplet evaporation. Consequently, the contact profiles in high-Na boundary layers had up to 88.4 cm−3 higher Nc compared to separated profiles. These results motivate investigations of aerosol–cloud–precipitation interactions over the Southeast Atlantic since the changes in Nc and Re induced by the presence of above-cloud biomass burning aerosols are likely to impact precipitation rates, liquid water path, and cloud fraction, and modulate closed-to-open-cell transitions.

scholarly journals Vertical structure of biomass burning aerosol transported over the southeast Atlantic Ocean

2021 ◽  
Author(s):  
Harshvardhan Harshvardhan ◽  
Richard Ferrare ◽  
Sharon Burton ◽  
Johnathan Hair ◽  
Chris Hostetler ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
Biomass Burning ◽  
Radiative Forcing ◽  
High Spectral Resolution ◽  
Aerosol Layer ◽  
Aerosol Extinction ◽  
Microphysical Properties ◽  
Stratocumulus Clouds ◽  
Fine Mode ◽  
Cloud Tops

Abstract. Biomass burning in southwestern Africa produces smoke plumes that are transported over the Atlantic Ocean and overlie vast regions of stratocumulus clouds. This aerosol layer contributes to direct and indirect radiative forcing of the atmosphere in this region, particularly during the months of August, September and October. There was a multi-year international campaign to study this aerosol and its interactions with clouds. Here we report on the evolution of aerosol distributions and properties as measured by the airborne high spectral resolution lidar (HSRL) during the ORACLES (Observations of Aerosols above Clouds and their intEractionS) campaign in September 2016. The NASA Langley HSRL-2 instrument was flown on the NASA ER-2 aircraft for several days in September 2016. Data were aggregated at two pairs of 2° × 2° grid boxes to examine the evolution of the vertical profile of aerosol properties during transport over the ocean. Results showed that the structure of the profile of aerosol extinction and microphysical properties is maintained over a one to two-day time scale. The fraction of aerosol in the fine mode between 50 and 500 nm remained above 0.95 and the effective radius of this fine mode was 0.16 μm from 3 to 5 km in altitude. This indicates that there is essentially no scavenging or dry deposition at these altitudes. Moreover, there is very little day to day variation in these properties, such that time sampling as happens in such campaigns, may be representative of longer periods such as monthly means. Below 3 km there is considerable mixing with larger aerosol, most likely continental source near land. Furthermore, these measurements indicated that there was a distinct gap between the bottom of the aerosol layer and cloud tops at the selected locations as evidenced by a layer of several hundred meters that contained relatively low aerosol extinction values above the clouds.

scholarly journals Monte Carlo-based subgrid parameterization of vertical velocity and stratiform cloud microphysics in ECHAM5.5-HAM2

2013 ◽  
Vol 13 (2) ◽  
pp. 5477-5507
Author(s):  
J. Tonttila ◽  
P. Räisänen ◽  
H. Järvinen
Keyword(s):  
Monte Carlo ◽  
Vertical Velocity ◽  
Cloud Droplet ◽  
Cloud Microphysics ◽  
Radiative Effects ◽  
Cloud Droplets ◽  
Microphysical Properties ◽  
Cloud Radiative Effects ◽  
Cloud Activation ◽  
The Impact

Abstract. A new method for parameterizing the subgrid variations of vertical velocity and cloud droplet number concentration (CDNC) is presented for GCMs. These parameterizations build on top of existing parameterizations that create stochastic subgrid cloud columns inside the GCM grid-cells, which can be employed by the Monte Carlo independent column approximation approach for radiative transfer. The new model version adds a description for vertical velocity in individual subgrid columns, which can be used to compute cloud activation and the subgrid distribution of the number of cloud droplets explicitly. This provides a consistent way for simulating the cloud radiative effects with two-moment cloud microphysical properties defined in subgrid-scale. The primary impact of the new parameterizations is to decrease the CDNC over polluted continents, while over the oceans the impact is smaller. This promotes changes in the global distribution of the cloud radiative effects and might thus have implications on model estimation of the indirect radiative effect of aerosols.

scholarly journals The influence of cloud structure and droplet concentration on the reflectance of shortwave radiation

1996 ◽  
Vol 14 (8) ◽  
pp. 845-852 ◽  
Author(s):  
P. F. Coley ◽  
P. R. Jonas
Keyword(s):  
Solar Radiation ◽  
Monte Carlo Model ◽  
Liquid Water Content ◽  
Cloud Fraction ◽  
Cloud Layer ◽  
Shortwave Radiation ◽  
Transmission Properties ◽  
Droplet Concentration ◽  
The Impact ◽  
Side Illumination

Abstract. The effects of cloud shadowing, channelling, cloud side illumination and droplet concentration are investigated with regard to the reflection of shortwave solar radiation. Using simple geometric clouds, coupled with a Monte Carlo model the transmission properties of idealized cloud layers are found. The clouds are illuminated with direct solar radiation from above. The main conclusion reached is that the distribution of the cloud has a very large influence on the reflectivity of a cloud layer. In particular, if the cloud contains vertical gaps through the cloud layer in which the liquid water content is zero, then, smaller more numerous gaps are more influential on the radiation than fewer, larger gaps with equal cloud fraction. At very low solar zenith angles channelling of the radiation reduces the reflection expected on the basis of the percentage cloud cover. At high solar zenith angles the illumination of the cloud edges significantly increases the reflection despite the shadowing of one cloud by another when the width of the gaps is small. The impact of droplet concentration upon the reflection of cloud layers is also investigated. It is found that at low solar zenith angles where channelling is important, the lower concentrations increase the transmission. Conversely, when cloud edge illumination is dominant the cloud distribution is found to be more important for the higher concentrations.

scholarly journals Mid-level clouds are frequent above the southeast Atlantic stratocumulus clouds

2020 ◽  
Vol 20 (18) ◽  
pp. 11025-11043
Author(s):  
Adeyemi A. Adebiyi ◽  
Paquita Zuidema ◽  
Ian Chang ◽  
Sharon P. Burton ◽  
Brian Cairns
Keyword(s):  
Cloud Droplet ◽  
Droplet Size Distribution ◽  
Aerosol Layer ◽  
Cloud Optical Depth ◽  
Low Level ◽  
Cloud Properties ◽  
Cloud Droplet Size Distribution ◽  
Stratocumulus Clouds ◽  
Absorbing Aerosols ◽  
Cloud Tops

Abstract. Shortwave-absorbing aerosols seasonally overlay extensive low-level stratocumulus clouds over the southeast Atlantic. While much attention has focused on the interactions between the low-level clouds and the overlying aerosols, few studies have focused on the mid-level clouds that also occur over the region. The presence of mid-level clouds over the region complicates the space-based remote-sensing retrievals of cloud properties and the evaluation of cloud radiation budgets. Here we characterize the mid-level clouds over the southeast Atlantic using lidar- and radar-based satellite cloud retrievals and observations collected in September 2016 during the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) field campaign. We find that mid-level clouds over the southeast Atlantic are relatively common, with the majority of the clouds occurring between altitudes of 5 and 7 km and at temperatures between 0 and −20 ∘C. The mid-level clouds occur at the top of a moist mid-tropospheric smoke-aerosol layer, most frequently between August and October, and closer to the southern African coast than farther offshore. They occur more frequently during the night than during the day. Between July and October, approximately 64 % of the mid-level clouds had a geometric cloud thickness less than 1 km, corresponding to a cloud optical depth of less than 4. A lidar-based depolarization–backscatter relationship for September 2016 indicates that the mid-level clouds are liquid-only clouds with no evidence of the existence of ice. In addition, a polarimeter-derived cloud droplet size distribution indicates that approximately 85 % of the September 2016 mid-level clouds had an effective radius less than 7 µm, which could further discourage the ability of the clouds to glaciate. These clouds are mostly associated with synoptically modulated mid-tropospheric moisture outflow that can be linked to the detrainment from the continental-based clouds. Overall, the supercooled mid-level clouds reduce the radiative cooling rates of the underlying low-altitude cloud tops by approximately 10 K d−1, thus influencing the regional cloud radiative budget.

scholarly journals Vertical redistribution of moisture and aerosol in orographic mixed-phase clouds

2020 ◽  
Vol 20 (13) ◽  
pp. 7979-8001
Author(s):  
Annette K. Miltenberger ◽  
Paul R. Field ◽  
Adrian H. Hill ◽  
Andrew J. Heymsfield
Keyword(s):  
Cloud Droplet ◽  
Wave Structure ◽  
Specific Humidity ◽  
Dust Particles ◽  
Microphysical Properties ◽  
Microphysical Processes ◽  
The Right ◽  
A Minor ◽  
Mixed Phase Clouds ◽  
The Impact

Abstract. Orographic wave clouds offer a natural laboratory to investigate cloud microphysical processes and their representation in atmospheric models. Wave clouds impact the larger-scale flow by the vertical redistribution of moisture and aerosol. Here we use detailed cloud microphysical observations from the Ice in Clouds Experiment – Layer Clouds (ICE-L) campaign to evaluate the recently developed Cloud Aerosol Interacting Microphysics (CASIM) module in the Met Office Unified Model (UM) with a particular focus on different parameterizations for heterogeneous freezing. Modelled and observed thermodynamic and microphysical properties agree very well (deviation of air temperature <1 K; specific humidity <0.2 g kg−1; vertical velocity <1 m s−1; cloud droplet number concentration <40 cm−3), with the exception of an overestimated total condensate content and too long a sedimentation tail. The accurate reproduction of the environmental thermodynamic and dynamical wave structure enables the model to reproduce the right cloud in the right place and at the right time. All heterogeneous freezing parameterizations except Atkinson et al. (2013) perform reasonably well, with the best agreement in terms of the temperature dependency of ice crystal number concentrations for the parameterizations of DeMott et al. (2010) and Tobo et al. (2013). The novel capabilities of CASIM allowed testing of the impact of assuming different soluble fractions of dust particles on immersion freezing, but this is found to only have a minor impact on hydrometeor mass and number concentrations. The simulations were further used to quantify the modification of moisture and aerosol profiles by the wave cloud. The changes in both variables are on order of 15 % of their upstream values, but the modifications have very different vertical structures for the two variables. Using a large number of idealized simulations we investigate how the induced changes depend on the wave period (100–1800 s), cloud top temperature (−15 to −50 ∘C), and cloud thickness (1–5 km) and propose a conceptual model to describe these dependencies.

scholarly journals Aerosol Impacts on Thermally Driven Orographic Convection

2016 ◽  
Vol 73 (8) ◽  
pp. 3115-3132 ◽  
Author(s):  
Alison D. Nugent ◽  
Campbell D. Watson ◽  
Gregory Thompson ◽  
Ronald B. Smith
Keyword(s):  
Cloud Layer ◽  
Background Wind ◽  
High Concentration ◽  
Microphysics Scheme ◽  
Microphysical Properties ◽  
Thermally Driven ◽  
Orographic Clouds ◽  
The Impact ◽  
Aerosol Source ◽  
Precipitation Formation

Abstract Observations from the Dominica Experiment (DOMEX) field campaign clearly show aerosols having an impact on cloud microphysical properties in thermally driven orographic clouds. It is hypothesized that when convection is forced by island surface heating, aerosols from the mostly forested island surface are lofted into the clouds, resulting in the observed high concentration of aerosols and the high concentration of small cloud droplets. When trying to understand the impact of these surface-based aerosols on precipitation, however, observed differences in cloud-layer moisture add to the complexity. The WRF Model with the aerosol-aware Thompson microphysics scheme is used to study six idealized scenarios of thermally driven island convection: with and without a surface aerosol source, with a relatively dry cloud layer and with a moist cloud layer, and with no wind and with a weak background wind. It is found that at least a weak background wind is needed to ensure Dominica-relevant results and that the effect of cloud-layer moisture on cloud and precipitation formation dominates over the effect of aerosol. The aerosol impact is limited by the dominance of precipitation formation through accretion. Nevertheless, in order to match observed cloud microphysical properties and precipitation, both a relatively dry cloud layer and a surface aerosol source are needed. The impact of a surface aerosol source on precipitation is strongest when the environment is not conducive to cloud growth.

scholarly journals Radar-radiometer retrievals of cloud number concentration and dispersion parameter in nondrizzling marine stratocumulus

2013 ◽  
Vol 6 (7) ◽  
pp. 1817-1828 ◽  
Author(s):  
J. Rémillard ◽  
P. Kollias ◽  
W. Szyrmer
Keyword(s):  
Liquid Water Content ◽  
Added Value ◽  
The Novel ◽  
Cloud Particle ◽  
Marine Stratocumulus ◽  
Cloud Optical Depth ◽  
Remote Sensors ◽  
Microphysical Properties ◽  
The One ◽  
Additional Constraints

Abstract. The retrieval of cloud microphysical properties from remote sensors is challenging. In the past, ground-based radar-radiometer measurements have been successfully used to retrieve the liquid water content profile in nondrizzling clouds but offer little constraint in retrieving other moments of the cloud particle size distribution (PSD). Here, a microphysical condensational model under steady-state supersaturation conditions is utilized to provide additional constraints to the well-established radar-radiometer retrieval techniques. The coupling of the model with the observations allows the retrieval of the three parameters of a lognormal PSD, with two of them being height dependent. Two periods of stratocumulus from the Azores are used to evaluate the novel technique. The results appear reasonable in two nondrizzling periods: continental-like number concentrations are retrieved, in agreement with the drizzle-free cloud conditions. The cloud optical depth derived from the retrieved distributions compares well in magnitude and variability with the one derived independently from a narrow field of view zenith radiometer. Uncertainties coming from the measurements are propagated to the retrieved quantities to estimate their errors. In general, errors smaller than 20% should be attainable for most parameters, demonstrating the added value of the new technique.

scholarly journals Retrieval of aerosol microphysical and optical properties above liquid clouds from POLDER/PARASOL polarization measurements

2012 ◽  
Vol 5 (4) ◽  
pp. 6083-6145 ◽  
Author(s):  
F. Waquet ◽  
C. Cornet ◽  
J.-L. Deuzé ◽  
O. Dubovik ◽  
F. Ducos ◽  
...  
Keyword(s):  
Size Distribution ◽  
Biomass Burning ◽  
Optical Thickness ◽  
Radiative Forcing ◽  
Mineral Dust ◽  
Cloud Layer ◽  
Dust Particles ◽  
Aerosol Layer ◽  
Polarization Measurements ◽  
Time Period

Abstract. Most of the current aerosol retrievals from passive sensors are restricted to cloud-free scenes, which strongly reduces our ability to monitor the aerosol properties at a global scale. The presence of Aerosols Above Clouds (AAC) affects the polarized light reflected by the cloud layer, as shown by the spaceborne measurements provided by the POlarization and Directionality of Earth Reflectances (POLDER) instrument. We present new developments that allow retrieving the properties of mineral dust particles when they are present above clouds. These particles do not much polarize light but strongly attenuate the polarized cloud bow generated by the beneath liquid cloud layer. The spectral attenuation can be used to qualitatively identify the nature of the particles (i.e. mineral dust particles or biomass burning aerosols) whereas the magnitude of the attenuation is related to the optical thickness of the aerosol layer. We provide accurate polarized radiance calculations for AAC scenes and evaluate the contribution of the POLDER polarization measurements for the simultaneous retrieval of the aerosol and clouds properties. We investigate various scenes with mineral dust particles and biomass burning aerosols above clouds. We found that the magnitude of the primary cloud bow cannot be accurately estimated with a plane parallel transfer radiative code. The errors for the modelling of the polarized cloud bow are between 5 and 8% for homogenous cloudy scenes, as shown by a 3-D radiative transfer code. For clouds, our results confirm that the droplets size distribution is narrow in high latitude ocean regions and that the droplets effective radii retrieved from polarization measurements and from total radiance measurements are generally close for AAC scenes (departures smaller than 2 μm). For the aerosols, the POLDER polarization measurements are primarily sensitive to the particles load, size distribution, shape and real refractive index. An algorithm was developed to retrieve the Aerosol Optical Thickness (AOT) and the Angström exponent above clouds in an operational way. This method was applied to various regions of the world and time period. Large mean AOTs above clouds at 0.865 μm (>0.3) are retrieved for oceanic regions near the coasts of South Africa and California (>0.1) that correspond to biomass burning aerosols whereas even larger mean AOTs above clouds for mineral dust particles (>0.6) are also retrieved near the coasts of Senegal (for June–August 2008). For these regions and time period, the direct AAC radiative forcing is likely to be significant. The final aim of this work is the global monitoring of the aerosol above clouds properties and the estimation of the direct aerosol radiative forcing in cloudy scenes.

scholarly journals Structure and Statistical Analysis of the Microphysical Properties of Generating Cells in the Comma Head Region of Continental Winter Cyclones

2014 ◽  
Vol 71 (11) ◽  
pp. 4181-4203 ◽  
Author(s):  
David M. Plummer ◽  
Greg M. McFarquhar ◽  
Robert M. Rauber ◽  
Brian F. Jewett ◽  
David C. Leon
Keyword(s):  
Liquid Water ◽  
Water Saturation ◽  
Supercooled Water ◽  
Particle Growth ◽  
Liquid Water Content ◽  
Head Region ◽  
Temperature Range ◽  
Microphysical Properties ◽  
The Impact ◽  
In Cells

Abstract This paper presents analyses of the microphysical structure of cloud-top convective generating cells at temperatures between −10° and −55°C across the comma head of 11 continental cyclones, using data collected by the W-band Wyoming Cloud Radar and in situ instrumentation aboard the National Science Foundation (NSF)/NCAR C-130. A case study of one cyclone is presented, followed by statistical analyses of the entire dataset. Ice particle number concentrations averaged 1.9 times larger inside generating cells compared to outside, and derived ice water contents and median mass diameters averaged 2.2 and 1.1 times larger in cells, respectively. Supercooled water was directly measured at temperatures between −31.4° and −11.1°C, with the median and 95th-percentile liquid water content increasing from ~0.09 to 0.12 g m−3 and 0.14 to 0.28 g m−3 over this temperature range, respectively. Liquid water was present in 26% of observations within cells and 18% of observations between cells over the same temperature range, and it was nearly ubiquitous at temperatures above −16°C. The larger ice particle concentrations in cells are consistent with greater ice production in convective updrafts. The increased mass and diameter of the ice particles demonstrate that generating cells provide environments favorable for enhanced particle growth. The impact of water saturation and supercooled water in the cells was evident, with rapid particle growth by diffusion and sometimes riming apparent, in addition to aggregation. Turbulent mixing lessened the observed differences between cells and surrounding regions, with supercooled water observed within and between cells, similar habits within and between cells, and rimed particles evident even in ice-phase conditions.

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