The Edition 2 (Ed2) cloud property retrieval algorithm system was upgraded and applied to the MODerate-resolution Imaging Spectroradiometer (MODIS) data for the Clouds and the Earth's Radiant Energy ...System (CERES) Edition 4 (Ed4) products. New calibrations for solar channels and the use of the 1.24-<inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula> channel for cloud optical depth (COD) over snow improve the daytime consistency between Terra and Aqua MODIS retrievals. Use of additional spectral channels and revised logic enhanced the cloud-top phase retrieval accuracy. A new ice crystal reflectance model and a CO 2 -channel algorithm retrieved higher ice clouds, while a new regional lapse rate technique produced more accurate water cloud heights than in Ed2. Ice cloud base heights are more accurate due to a new cloud thickness parameterization. Overall, CODs increased, especially over the polar (PO) regions. The mean particle sizes increased slightly for water clouds, but more so for ice clouds in the PO areas. New experimental parameters introduced in Ed4 are limited in utility, but will be revised for the next CERES edition. As part of the Ed4 retrieval evaluation, the average properties are compared with those from other algorithms and the differences between individual reference data and matched Ed4 retrievals are explored. Part II of this article provides a comprehensive, objective evaluation of selected parameters. More accurate interpretation of the CERES radiation measurements has resulted from the use of the Ed4 cloud properties.
The effect of aerosols on the properties of clouds is a large source of uncertainty in predictions of weather and climate. These aerosol‐cloud interactions depend critically on the ability of aerosol ...particles to form cloud droplets. A challenge in modeling aerosol‐cloud interactions is the representation of interactions between turbulence and cloud microphysics. Turbulent mixing leads to small‐scale fluctuations in water vapor and temperature that are unresolved in large‐scale atmospheric models. To quantify the impact of turbulent fluctuations on cloud condensation nuclei (CCN) activation, we used a high‐resolution Large Eddy Simulation of a convective cloud chamber to drive particle‐based cloud microphysics simulations. We show small‐scale fluctuations strongly impact CCN activity. Once activated, the relatively long timescales of evaporation compared to fluctuations causes droplets to persist in subsaturated regions, which further increases droplet concentrations.
Plain Language Summary
Increases in cloud droplet number concentrations from human emissions of aerosol particles modify cloud properties, which strongly impacts Earth's energy balance. Large Eddy Simulations and Earth System Models are used to quantify these aerosol‐cloud interactions, but the spatial and temporal resolution of these models is too coarse to represent the impact of turbulence at the smallest scales. In this study, we show that small‐scale turbulent fluctuations lead to cloud droplet formation even when air is, on average, subsaturated, which would be impossible in conventional models of cloud microphysics. Our findings suggest that models that neglect turbulent fluctuations in supersaturation will underestimate cloud condensation nuclei activity under specific supersaturation regimes, which will may lead to error in modeled cloud properties.
Key Points
Small‐scale turbulence leads to variability in the supersaturation experienced by aerosol particles and cloud droplets within clouds
Turbulent fluctuations increase cloud droplet formation at low supersaturation levels in comparison with uniform environmental conditions
Atmospheric models that neglect supersaturation variability due to turbulence may underestimate the number concentration of cloud droplets
Abstract
Marine cloud brightening (MCB) is a geoengineering approach to counteract climate change by the deliberate seeding of sea salt aerosol particles that, once they activated to cloud droplets, ...directly increase cloud reflectance and hence global albedo. However, a large fraction of the seeded aerosol may remain interstitial, i.e., unactivated particles among cloud droplets. Because the consideration of interstitial aerosol optical properties usually requires computationally expensive simulations of the entire particle spectrum and direct Mie calculations, we develop a simple parameterization to be used with computationally efficient bulk and even bin cloud microphysical schemes that do not treat the unactivated aerosol explicitly. Using parcel and large-eddy simulations with highly detailed Lagrangian cloud microphysics and direct Mie calculations as a reference, we show that the parameterization captures the variability in the interstitial aerosol extinction successfully. By applying the parameterization to typical MCB cases, we find that the consideration of interstitial aerosol extinction is important for the assessment of MCB in shallow clouds with weak updrafts, in which only a small fraction of aerosol particles is activated to cloud droplets.
Significance Statement
The optical properties of clouds are not only determined by the number and size of cloud droplets. Unactivated aerosol particles, so-called interstitial aerosol, can contribute substantially to the optical thickness of shallow clouds with weak updrafts in aerosol-laden conditions. The consideration of interstitial aerosol optical thickness has been computationally challenging, but the new parameterization presented here allows for an efficient representation in various types of cloud models. The parameterization is shown to be an important addition for the assessment of marine cloud brightening (MCB), a potential geoengineering technique to counteract global warming by increasing the cloud albedo through the deliberate seeding of aerosol.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Coincident instantaneous broadband radiation budget measurements from Clouds and Earth's Radiant Energy System and cloud vertical structure information from CloudSat‐Cloud‐Aerosol Lidar and Infrared ...Pathfinder Satellite Observations radar‐lidar observations are combined to study the relationship of cloud vertical structure to top‐of‐atmosphere energy balance fluctuations. Varying optical and physical thickness of high ice clouds produces most of the covariation between albedo and outgoing longwave radiation in regions of tropical convection. Rainy cores of tropical convective clouds have a negative impact on the radiation balance, while nonprecipitating anvil clouds have a positive effect. The effect of anvil clouds on the radiative heating profile is to warm near cloud base and cool near cloud top, and to reduce the radiative cooling rate in the clear air below the cloud. The cooling rate in the clear air below the anvil is reduced to small values for moderately thick anvils, and the driving of instability in the anvil itself also saturates for relatively thin clouds. It is hypothesized that the dependence of radiative heating on cloud thickness may be important in driving the distribution of tropical cloud structures toward one that produces net neutrality of the cloud radiative effect at the top‐of‐the‐atmosphere, as is found in regions of deep convection over ocean areas with high and relatively uniform surface temperatures. This idea is tested with a single‐column model, which indicates that cloud‐radiation interactions affect anvil cloud properties, encouraging further investigation of the hypothesis.
Plain Language Summary
Satellite data are used to show that tropical convective clouds have a small net effect on the energy balance because the cooling effect of thick tropical clouds is offset by the warming effect of thin upper level clouds that are connected to the thick clouds. Why these two types of clouds cancel each other out nearly exactly in the warm regions of the tropics is unknown. The effect of radiative transfer on the anvil clouds seems to be a possible contributor to the abundance of thin high clouds that give the positive influences on the radiation budget at the top of the atmosphere.
Key Points
Anvil cloud properties determine the radiation balance in the convective tropics
The effect of tropical clouds on the energy balance depends on the distribution of anvil cloud optical depth
Anvil cloud radiative interactions influence anvil cloud properties
The high variability of aerosol particle concentrations, sizes and chemical composition makes their description challenging in atmospheric models. Aerosol–cloud interaction studies are usually ...focused on the activation of accumulation mode particles as cloud condensation nuclei (CCN). However, under specific conditions Aitken mode particles can also contribute to the number concentration of cloud droplets (Nd), leading to large uncertainties in predicted cloud properties on a global scale. We perform sensitivity studies with an adiabatic cloud parcel model to constrain conditions under which Aitken mode particles contribute to Nd. The simulations cover wide ranges of aerosol properties, such as total particle number concentration, hygroscopicity (κ) and mode diameters for accumulation and Aitken mode particles.
Building upon the previously suggested concept of updraft (w)- and aerosol-limited regimes of cloud droplet formation, we show that activation of Aitken mode particles does not occur in w-limited regimes of accumulation mode particles. The transitional range between the regimes is broadened when Aitken mode particles contribute to Nd, as aerosol limitation requires much higher w than for aerosol size distributions with accumulation mode particles only. In the transitional regime, Nd is similarly dependent on w and κ. Therefore, we analyze the sensitivity of Nd to κ, ξ(κ), as a function of w to identify the value combinations above which Aitken mode particles can affect Nd. As ξ(κ) shows a minimum when the smallest activated particle size is in the range of the “Hoppel minimum” (0.06 µm ≤ Dmin ≤0.08 µm), the corresponding (w–κ) pairs can be considered a threshold level above which Aitken mode particles have significant impact on Nd. This threshold is largely determined by the number concentration of accumulation mode particles and by the Aitken mode diameter. Our analysis of these thresholds results in a simple parametric framework and criterion to identify aerosol and updraft conditions under which Aitken mode particles are expected to affect aerosol–cloud interactions. Our results confirm that Aitken mode particles likely do not contribute to Nd in polluted air masses (urban, biomass burning) at moderate updraft velocities (w≤3 m s−1) but may be important in deep convective clouds. Under clean conditions, such as in the Amazon, the Arctic and remote ocean regions, hygroscopic Aitken mode particles can act as CCN at updrafts of w<1 m s−1.
Aerosol effects on micro/macrophysical properties of marine stratocumulus clouds over the western North Atlantic Ocean (WNAO) are investigated using in situ measurements and large-eddy simulations ...(LES) for two cold-air outbreak (CAO) cases (28 February and 1 March 2020) during the Aerosol Cloud Meteorology Interactions over the Western Atlantic Experiment (ACTIVATE). The LES is able to reproduce the vertical profiles of liquid water content (LWC), effective radius reff and cloud droplet number concentration Nc from fast cloud droplet probe (FCDP) in situ measurements for both cases. Furthermore, we show that aerosols affect cloud properties (Nc, reff, and LWC) via the prescribed bulk hygroscopicity of aerosols (¯k) and aerosol size distribution characteristics. Nc, reff, and liquid water path (LWP) are positively correlated to ¯κ and aerosol number concentration (Na) while cloud fractional cover (CFC) is insensitive to ¯κ and aerosol size distributions for the two cases. The realistic changes to aerosol size distribution (number concentration, width, and the geometrical diameter) with the same meteorology state allow us to investigate aerosol effects on cloud properties without meteorological feedback. We also use the LES results to evaluate cloud properties from two reanalysis products, ERA5 and MERRA-2. Compared to LES, the ERA5 is able to capture the time evolution of LWP and total cloud coverage within the study domain during both CAO cases while MERRA-2 underestimates them.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This paper provides a description of the integrated representation for the cloud processes in the Community Atmosphere Model, version 5 (CAM5). CAM5 cloud parameterizations add the following unique ...characteristics to previous versions: 1) a cloud macrophysical structure with horizontally nonoverlapped deep cumulus, shallow cumulus, and stratus in each grid layer, where each of which has its own cloud fraction, and mass and number concentrations for cloud liquid droplets and ice crystals; 2) stratus–radiation–turbulence interactions that allow CAM5 to simulate marine stratocumulus solely from grid-mean relative humidity without relying on a stability-based empirical formula; 3) prognostic treatment of the number concentrations of stratus liquid droplets and ice crystals, with activated aerosols and detrained in-cumulus condensates as the main sources and with evaporation, sedimentation, and precipitation of stratus condensate as the main sinks; and 4) radiatively active cumulus and snow. By imposing consistency between diagnosed stratus fraction and prognosed stratus condensate, unrealistically empty or highly dense stratus is avoided in CAM5. Because of the activation of the prognostic aerosols and the parameterizations of the radiation and stratiform precipitation production as a function of the cloud droplet size, CAM5 simulates various aerosol indirect effects as well as the direct effects: that is, aerosols affect both the radiation budget and the hydrological cycle.
Detailed analysis of various simulations indicates that CAM5 improves upon CAM3/CAM4 in global performance as well as in physical formulation. However, several problems are also identified in CAM5, which can be attributed to deficient regional tuning, inconsistency between various physics parameterizations, and incomplete treatment of physics. Efforts are continuing to further improve CAM5.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6 cloud observations (MYD06) at 1 km are collocated with daytime CloudSat‐Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite ...Observations (CALIPSO) (C‐C) cloud vertical structures (2B‐CLDCLASS‐LIDAR). For 2007–2010, over 267 million C‐C cloud profiles are used to (1) validate MODIS cloud mask and cloud multilayer flag and (2) cross‐reference between C‐C cloud types and MODIS cloud regimes defined by joint histograms of cloud top pressure (CTP) and cloud optical depth (τ). Globally, of total observations, C‐C reports 27.1% clear and 72.9% cloudy, whereas MODIS reports 30.0% confidently clear and 58.7% confidently cloudy, with the rest 7.1% as probably clear and 4.2% as probably cloudy. Agreement between MODIS and C‐C is 77.8%, with 20.9% showing both clear and 56.9% showing both cloudy. The 9.1% of observations are clear in MODIS but cloudy in C‐C, indicating clouds missed by MODIS; 1.8% of observations are cloudy in MODIS but clear in C‐C, likely due to aerosol/dust or surface snow layers misidentified by MODIS. C‐C reports 47.4/25.5% single‐layer/multilayer clouds, while MODIS reports 26.7/14.0%. For C‐C single‐layer clouds, ~90% of tropical MODIS high (CTP < 440 hPa) and optically thin (τ < 3.6) clouds are identified as cirrus and ~60% of high and optically thick (τ > 23) clouds are recognized as deep convective in C‐C. Approximately 70% of MODIS low‐level (CTP > 680 hPa) clouds are classified as stratocumulus in C‐C regardless of region and optical thickness. No systematic relationship exists between MODIS middle‐level (680 < CTP < 440 hPa) clouds and C‐C cloud types, largely due to different definitions adopted.
Key Points
The 20.9/56.9% of collocated MODIS and C‐C are identified as clear/cloudy; 9.1/1.8% are clear/cloudy in MODIS but cloudy/clear in C‐C
The 17.7/7.7% of MODIS/C‐C observation are both single layer/multilayer; 8.7/6.3% are single layer/multilayer in MODIS but multilayer/single layer in C‐C
For single‐layer cases, ~70% of MODIS low‐level clouds are classified as Sc in C‐C regardless of optical thickness
The Moderate Resolution Imaging Spectroradiometer (MODIS) C6 L3 and the European Centre for Medium‐Range Weather Forecasts (ECMWF) ERA‐Interim reanalysis data from 2003 to 2016 are employed to study ...aerosol‐cloud correlations over three industrial regions and their adjacent oceans, as well as explore the impact of meteorological conditions on the correlations. The analysis focusing on liquid and single‐layer clouds indicates an opposite aerosol‐cloud correlation between land and ocean; namely, cloud effective radius is positively correlated with aerosol index over industrial regions (positive slopes), but negatively correlated over their adjacent oceans (negative slopes), for a quasi‐constant liquid water path. The positive slopes are relatively large under low lower‐tropospheric stability (LTS; weakly stable condition), but much weaker or even become negative under high LTS (stable conditions) and high liquid water path. The occurrence frequency of cloud top height (CTH) and LTS suggests that positive correlations are more likely corresponding to relatively high CTH and low LTS, while negative to low CTH and high LTS.
Plain Language Summary
Aerosol‐cloud interactions play an important role in climate prediction, but remain to have large uncertainties. The major industrial regions generally exhibit relatively high aerosol concentrations, and how aerosols impact cloud properties, and how aerosol‐cloud interactions modulate cloud and precipitation processes, is an interesting and challenging topic. In this study, a new retrieval from long‐term satellite data is employed to examine the correlation between aerosol concentration and cloud droplet size over the three industrial regions and their adjacent oceans. The results show the opposite aerosol‐cloud correlation between land and ocean. Positive slopes are more likely associated with high cloud top height and low lower‐tropospheric stability. This finding is very useful to the communities for an accurate prediction of weather and climate.
Key Points
Cloud effective radius is positively correlated to aerosol index over three industrial regions, but negatively over adjacent oceans
Positive slopes over land are relatively large under low lower‐tropospheric stability (LTS), but much weaker or negative under high LTS
Positive correlations possibly correspond to relatively high cloud top height (CTH) and low LTS, while negative to low CTH and high LTS
Marine low clouds are one of the greatest sources of uncertainty for climate projection. We present an observed climatology of cloud albedo susceptibility to cloud droplet number concentration ...perturbations (S0) with changing sea surface temperature (SST) and estimated inversion strength for single‐layer warm clouds over the North Atlantic Ocean, using eight years of satellite and reanalysis data. The key findings are that SST has a dominant control on S0 in the presence of co‐varying synoptic conditions and aerosol perturbations. Regions conducive to aerosol‐induced darkening (brightening) clouds occur with high (low) local SST. Higher SST significantly hastens cloud‐top evaporation with increasing aerosol loading, by accelerating entrainment and facilitating entrainment drying. In a global‐warming‐like scenario where aerosol loading is reduced, less cloud darkening is expected, mainly as a result of reduced entrainment drying. Our results imply a less positive low‐cloud liquid water path feedback in a warmer climate with decreasing aerosol loading.
Plain Language Summary
Low clouds over the ocean are a poorly quantified component of the climate system. Here we use eight years of space‐based measurements and meteorological data to quantify how the brightness of single‐layer low clouds over the North Atlantic Ocean might respond to cloud droplet number concentration perturbations in a warmer world. We find that under higher sea surface temperatures, increases in drop number tend to reduce cloud brightness by accelerating evaporation of cloud water. Thus in a warmer world, low clouds will reflect less energy to space in response to an increase in aerosol loading. If aerosol sources decrease then we expect more robust clouds and more offsetting of greenhouse gas warming.
Key Points
Sea surface temperature (SST) has a strong influence on the relative occurrence of aerosol‐induced brightness of clouds over the North Atlantic Ocean
Aerosol perturbations are locally confined and have less influence on the brightness of clouds compared to SST
In a warmer climate where aerosol loading is reduced, we expect a less positive liquid water path feedback
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