The cloud-base temperature (CBT) is one of the parameters that dominates the cloudy sky surface downward longwave radiation (SDLR). However, CBT is rarely available at regional and global scales, and ...its application in estimating cloud sky SDLR is limited. In this study, a framework to globally estimate cloud sky SDLR during both daytime and nighttime is proposed. This framework is composed of three parts. First, a global cloudy property database was constructed by combing the extracted cloud vertical structure (CVS) parameters from the active CloudSat data and cloud properties from passive MODIS data. Second, the empirical methods for estimating cloud thickness (CT) under ISCCP cloud classification system and MODIS cloud classification system were developed. Additionally, the coefficients of CERES CT estimate models were refitted using the constructed cloud property database. With the estimated CT and reanalysis data, calculating the CBT is straightforward. The accuracy of the estimated CT for ISCCP cloud type is compared with the existing studies that were conducted at local scales. Our CT estimate accuracy is comparable to that of the existing studies. According to the validation results at ARM NSA and SGP stations, the CT estimated by the developed CT model for MODIS cloud type is better than that estimated by the original CERES CT model. Finally, the cloudy sky SDLR values were derived by feeding the estimated CBT and other parameters to the single-layer cloud model (SLCM). When validated by the ground measured SDLR collected from the SURFRAD network, the bias and RMSE are 5.42 W∙m−2 and 30.3 W∙m−2, respectively. This accuracy is comparable to the evaluation results of the mainstream SDLR products (Gui et al. 2010), the new evaluation results of SLCMs (Yu et al. 2018), and the accuracy of a new cloudy sky SDLR estimate method (Wang et al. 2018). All the derived CBTs improve the SDLR estimate accuracy more than the SLCM that directly uses cloud-top temperature (CTT). We will collect more ground measurements and continue to validate the developed framework in the future.
•We propose a framework to globally estimate cloud sky SDLR during both daytime and nighttime.•The CT estimation models are developed for ISCCP cloud type and MODIS cloud type.•The cloudy sky SDLR is retrieved by the SLCM using the CBT derived from CT.•The bias and RMSE of the derived cloudy sky SDLR over SURFRAD are 5.42 W∙m−2 and 30.3 W∙m−2.
Variability and vertical structure of optically thin veil clouds over the stratocumulus to cumulus transition (SCT) are investigated using spaceborne satellite observations. Optically thin veil ...clouds, defined as the low clouds with cloud base >1 km that do not fully attenuate Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) lidar signal, comprise ∼30% of the low clouds over the SCT. It is found that optically thin veil clouds are geometrically thin with cloud thickness ∼200 m and commonly reside in the upper boundary layer with average cloud base >1.5 km. Satellite observations reveal pronounced relationships between optically thin veil clouds, strong precipitation, deep planetary boundary layer (PBL) height, and low‐cloud droplet number concentration. The results are in agreement with the hypothesis that the low optical thickness of veil clouds over the SCT is contingent on the low‐cloud droplet number concentration caused by strong precipitation scavenging occurring in active cumulus, a process of which efficiency is strongly dependent on maximum condensate amount in updrafts and thus is highly constrained by planetary boundary layer height.
Plain Language Summary
Optically thin veil cloud, a common feature over the stratocumulus to cumulus transition, has been found to be strongly associated with strong precipitation and low droplet concentration in the deep boundary layer based on multiple airborne satellite observations. This result supports the hypothesis that boundary layer depth, which controls cloud thickness and available liquid condensate in updrafts, could be a crucial factor regulating coalescence scavenging and thus the formation of optically thin veil cloud in the stratocumulus to cumulus transition regions.
Key Points
Optically thin veil clouds comprise 30% of the low cloud cover in the subtropical Sc‐Cu transition
There are pronounced relationships among veil clouds, strong precipitation, low‐cloud droplet concentration, and deep boundary layer height in the Sc‐Cu transition
Veil clouds in the Sc‐Cu transition are geometrically thin with average cloud depth of 200 m, and commonly reside in the upper PBL with average cloud base of 1.5 km
The challenges of cloud forensics have been well-documented by both researchers and government agencies (e.g., U.S. National Institute of Standards and Technology), although many of the challenges ...remain unresolved. In this article, we perform a comprehensive survey of cloud forensic literature published between January 2007 and December 2018, categorized using a five-step forensic investigation process. We also present a taxonomy of existing cloud forensic solutions, with the aim of better informing both the research and practitioner communities, as well as an in-depth discussion of existing conventional digital forensic tools and cloud-specific forensic investigation tools. Based on the findings from the survey, we present a set of design guidelines to inform future cloud forensic investigation processes, and a summary of digital artifacts that can be obtained from different stakeholders in the cloud computing architecture/ecosystem.
Here, we use 16‐year satellite and reanalysis data in combination with a multivariate regression model to investigate how aerosols affect cloud fraction (CF) over the East Coast of the United States. ...Cloud droplet number concentrations (Nd), cloud geometrical thickness, lower tropospheric stability, and relative humidity at 950 hPa (RH950) are identified as major cloud controlling parameters that explain 97% of the variability in CF. Nd is shown to play an important role in regulating the dependence of CF on RH950. The observed annual‐mean CF shows no significant trend due to the cancellation from the opposite trends in Nd and RH950. The multivariate regression model revealed that the decline in Nd alone would lead to about a 20% relative decline in CF. Our results indicate the significant aerosol effects on CF and suggest the need to account for pollution‐induced cloud changes in quantifying cloud feedback based on long‐term observations.
Plain Language Summary
Understanding how aerosols affect cloud cover is critical for reducing the large uncertainty of the radiative forcing associated with aerosol‐cloud interactions for future climate projections. By choosing a region (East Coast of the United States) that has experienced a significant decline in cloud droplet number concentrations due to pollution control in recent decades, we quantify aerosol effects on cloud amount based on long‐term satellite observation and reanalysis data in combination with a statistical regression model that isolates aerosol effects from meteorology influences. Our results reveal significant effects of aerosols on cloud amount and indicate the need to account for changes in cloud microphysics for constraining and quantifying cloud feedback based on long‐term observations.
Key Points
Nd plays an important role in regulating the dependence of cloud fraction (CF) on RH950
The observed annual‐mean CF shows no significant trend due to the cancellation from the opposite trends in Nd and RH950
When fixing the meteorology, we found significant aerosol effects on cloud amount based on long‐term satellite observations
Active vegetation fires in south‐eastern (SE) Europe resulted in a notable increase in the number concentration of aerosols and cloud condensation nuclei (CCN) particles at two high latitude ...locations—the SMEAR IV station in Kuopio, Finland, and the Zeppelin Observatory in Svalbard, high Arctic. During the fire episode aerosol hygroscopicity κ slightly increased at SMEAR IV and at the Zeppelin Observatory κ decreased. Despite increased κ in high CCN conditions at SMEAR IV, the aerosol activation diameter increased due to the decreased supersaturation with an increase in aerosol loading. In addition, at SMEAR IV during the fire episode, in situ measured cloud droplet number concentration (CDNC) increased by a factor of ∼7 as compared to non‐fire periods which was in good agreement with the satellite observations (MODIS, Terra). Results from this study show the importance of SE European fires for cloud properties and radiative forcing in high latitudes.
Plain Language Summary
Wildfires are large sources of aerosol particles and affect human health and climate. Aerosols from fires are transported long distances in the atmosphere and affect the aerosol and cloud properties at places far from the actual sources. In this study, we measured the long‐range transported (LRT) fire air masses from south‐eastern (SE) Europe at a northern European and a high Arctic site. LRT fire emissions from SE Europe increase the aerosol number and mass loading in Finland and even in the high Arctic. Results show that the effect of fire emissions on aerosol hygroscopicity depends on the properties of both the LRT fires and the background aerosols at a given location. The cloud properties analysis in eastern Finland shows that despite high hygroscopicity and increased CCN activity, the aerosol activation diameter for clouds increased during the fire episode. This is due to the depletion of available water vapor in clouds due to the increased aerosol loading. Satellite observations show an increase in cloud droplet number concentration during the fire episode confirming the effect of LRT fires on cloud properties in eastern Finland. This study can improve the understanding of the effect of LRT fires on aerosol and cloud properties at remote locations.
Key Points
Vegetation fires from southern Europe enhance aerosol and cloud condensation nuclei concentrations in northern Europe and the high Arctic
A contrary trend in aerosol hygroscopicity is observed at these two locations during a strong fire episode as compared to non‐fire periods
Cloud droplet number concentrations in liquid clouds show strong response to fire aerosol both in in situ and satellite observations
While the short-term relationship between clouds and aerosols is well known, no adequate data is available to verify the longer-term, annual to decadal, relationship. It is important to quantify the ...aerosol-cloud interaction (ACI) for mitigating uncertainty in climate prediction. Here the long-term ACI over the mid-to-high latitudes of the Northern Hemisphere was analyzed by using seasonally-resolved ion fluxes reconstructed from a southeastern Greenland ice core (SE-Dome ice core) as aerosol proxies, and satellite-based summer cloud amount between 1982 and 2014. As a result, SO
flux in the ice core shows significant positive correlation with total cloud amounts (
) and cloud droplet concentration (
) in the summer over the southeastern Greenland Sea, implying that the sulfate aerosols may contribute to the variability of
via microphysical cloud processes. Significant positive correlations are persistent even under the constrained conditions when cloud formation factors such as relative humidity, air temperature at cloud height, and summer North Atlantic Oscillation are limited within ± 1σ variability. Hence sulfate aerosols should control the interannual variability of summer
In terms of decadal changes,
was approximately 3-5% higher in the 1960s-1970s than in the 1990s-2000s, which can be explained by changes in the,
flux preserved in the SE-Dome ice core.
A brief review of the Internet history reveals the fact that the Internet evolved after the formation of primarily independent networks. Similarly, interconnected clouds, also called
Inter-cloud
, ...can be viewed as a natural evolution of cloud computing. Recent studies show the benefits in utilizing multiple clouds and present attempts for the realization of an Inter-cloud or federated cloud environment. However, cloud vendors have not taken into account cloud interoperability issues, and each cloud comes with its own solution and interfaces for services. This survey initially discusses all the relevant aspects motivating cloud interoperability. Furthermore, it categorizes and identifies possible cloud interoperability scenarios and architectures. The spectrum of challenges and obstacles that the Inter-cloud realization is faced with are covered, a taxonomy of them is provided, and fitting enablers that tackle each challenge are identified. All these aspects require a comprehensive review of the state of the art, including ongoing projects and studies in the area. We conclude by discussing future directions and trends toward the holistic approach in this regard.
Marine cloud brightening is a proposal to counteract global warming by increasing sea salt aerosol emissions. In theory, this increases the cloud droplet number concentration of subtropical marine ...stratocumulus decks, increasing cloud brightness and longevity. However, this theoretical progression remains uncertain in coupled climate models, especially the response of liquid water path and cloud fraction to aerosol seeding. We use the GFDL CM4 climate model to simulate marine cloud brightening following the published G4sea‐salt protocol, in which sea salt aerosol emissions are uniformly increased over 30 S–30 N in addition to standard forcings from a SSP2‐4.5 future warming scenario. The perturbed radiative and cloud responses are temporally stable though spatially heterogeneous, and direct scattering by the added sea salt predominates over changes to cloud reflectance. In fact, feedbacks in the coupled simulation lead to a net warming, rather than cooling, response by clouds.
Plain Language Summary
With calls for climate action rising, some countries and groups may be looking at counteracting global warming. As reducing emissions of greenhouse gases remains elusive, and while the results of climate change manifest in extreme events and weather records, state or private actors may look for active engineering solutions which remain hypothetical and not fully scientifically understood. Using premier climate models at NOAA GFDL, we examine one form of climate engineering, marine cloud brightening, aimed at increasing radiation reflected back to space by increasing sea salt aerosol emissions in the marine tropics. We find the climate response to a protocol of this scheme temporally stable over the time period of the simulation, though spatially uncertain. Moreover, the response is largely dominated by effects resulting from the direct interactions between aerosol particles and solar radiation, and not via clouds. Our results paint a more nuanced picture than previous studies and as such raise more questions and uncertainties about proposals for marine cloud brightening, at least through the prism of state‐of‐the‐art climate models.
Key Points
Temporally stable climate response to increased sea salt aerosol in GFDL’s AM4 and CM4 models following the G4sea‐salt protocol
Dominant role of direct aerosol effects in both models as the indirect aerosol–cloud effects are counterbalanced by cloud feedbacks in CM4
Uncertain spatial radiative and cloud responses necessitating further constraining to yield detailed mechanistic understanding
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