In July 2012, a heat wave swept across Greenland, resulting in extensive melting of surface ice and flooding; this is shown to have been enhanced by liquid clouds forming in such a way that sufficient incoming shortwave radiation could penetrate to the surface while downwelling longwave radiation increased. The heatwave that struck Greenland in July 2012 melted surface ice across virtually the entire ice sheet, causing extensive flooding. Ice core data suggest that such events occur only about once every 150 years on average, with the last one in 1889. Ralf Bennartz and colleagues have examined the physical mechanisms behind the 2012 event. At high elevations thin liquid-bearing clouds had an important role in enhancing surface warming, as they were optically thick enough and low enough to significantly enhance the downwelling infrared flux at the surface. At the same time they were thin enough to allow sufficient solar radiation through to raise surface temperatures above the melting point. Melting of the world’s major ice sheets can affect human and environmental conditions by contributing to sea-level rise. In July 2012, an historically rare period of extended surface melting was observed across almost the entire Greenland ice sheet1,2, raising questions about the frequency and spatial extent of such events. Here we show that low-level clouds consisting of liquid water droplets (‘liquid clouds’), via their radiative effects, played a key part in this melt event by increasing near-surface temperatures. We used a suite of surface-based observations3, remote sensing data, and a surface energy-balance model. At the critical surface melt time, the clouds were optically thick enough and low enough to enhance the downwelling infrared flux at the surface. At the same time they were optically thin enough to allow sufficient solar radiation to penetrate through them and raise surface temperatures above the melting point. Outside this narrow range in cloud optical thickness, the radiative contribution to the surface energy budget would have been diminished, and the spatial extent of this melting event would have been smaller. We further show that these thin, low-level liquid clouds occur frequently, both over Greenland and across the Arctic, being present around 30–50 per cent of the time3,4,5,6. Our results may help to explain the difficulties that global climate models have in simulating the Arctic surface energy budget7,8,9, particularly as models tend to under-predict the formation of optically thin liquid clouds at supercooled temperatures6—a process potentially necessary to account fully for temperature feedbacks in a warming Arctic climate.

https://projects.iq.harvard.edu/files/climate/files/bennartz_nature_2013.pdf

July 2012 Greenland melt extent enhanced by low-level liquid clouds

Accurate observations of cloud microphysical properties are needed for evaluating and improving the representation of cloud processes in climate models and better estimate of the Earth radiative budget. However, large differences are found in current cloud products retrieved from ground-based remote sensing measurements using various retrieval algorithms. Understanding the differences is an important step to address uncertainties in the cloud retrievals. In this study, an in-depth analysis of nine existing ground-based cloud retrievals using ARM remote sensing measurements is carried out. We place emphasis on boundary layer overcast clouds and high level ice clouds, which are the focus of many current retrieval development efforts due to their radiative importance and relatively simple structure. Large systematic discrepancies in cloud microphysical properties are found in these two types of clouds among the nine cloud retrieval products, particularly for the cloud liquid and ice particle effective radius. Note that the differences among some retrieval products are even larger than the prescribed uncertainties reported by the retrieval algorithm developers. It is shown that most of these large differences have their roots in the retrieval theoretical bases, assumptions, as well as input and constraint parameters. This study suggests the need to further validate current retrieval theories and assumptions and even the development of new retrieval algorithms with more observations under different cloud regimes.

/pdf/toward-understanding-of-differences-in-current-cloud-2sdvvcofyx.pdf

Toward understanding of differences in current cloud retrievals of ARM ground-based measurements

New space-borne active sensors make it possible to observe the three-dimensional structure of clouds. Here we use CALIPSO lidar observations together with a lidar simulator to evaluate the cloudiness simulated by a climate model: modeled atmospheric profiles are converted to an ensemble of subgrid-scale attenuated backscatter lidar signals from which a cloud fraction is derived. Except in regions of persistent thick upper-level clouds, the cloud fraction diagnosed through this procedure is close to that actually predicted by the model. A fractional cloudiness is diagnosed consistently from CALIPSO data at a spatiotemporal resolution comparable to that of the model. The comparison of the model's cloudiness with CALIPSO data reveals discrepancies more pronounced than in previous model evaluations based on passive observations. This suggests that space-borne lidar observations constitute a powerful tool for the evaluation of clouds in large-scale models, including marine boundary-layer clouds.

https://hal.archives-ouvertes.fr/hal-00423478/document

Use of CALIPSO lidar observations to evaluate the cloudiness simulated by a climate model

This paper investigates the mechanisms of convective cloud organization by precipitation-driven cold pools over the warm tropical Indian Ocean during the 2011 Atmospheric Radiation Measurement (ARM) Madden-Julian Oscillation (MJO) Investigation Experiment / Dynamics of the MJO (AMIE/DYNAMO) field campaign. A high-resolution regional model simulation is performed using the Weather Research and Forecasting model during the transition from suppressed to active phases of the November 2011 MJO. The simulated cold pool lifetimes, spatial extent and thermodynamic properties agree well with the radar and ship-borne observations from the field campaign. The thermodynamic and dynamic structures of the outflow boundaries of isolated and intersecting cold pools in the simulation and the associated secondary cloud populations are examined. Intersecting cold pools last more than twice as long, are twice as large, 41% more intense (measured by buoyancy), and 62% deeper than isolated cold pools. Consequently, intersecting cold pools trigger 73% more convective clouds than isolated ones. This is possibly due to stronger outflows that enhance secondary updraft velocities by up to 45%. However, cold pool-triggered convective clouds grow into deep convection not because of the stronger secondary updrafts at cloud base, but rather due to closer spacing (aggregation) between clouds and larger cloud clusters more » that formed along the cold pool boundaries when they intersect. The close spacing of large clouds moistens the local environment and reduces entrainment drying, allowing the clouds to further develop into deep convection. Implications to the design of future convective parameterization with cold pool-modulated entrainment rates are discussed. « less

/pdf/mechanisms-of-convective-cloud-organization-by-cold-pools-598ee7of4n.pdf

Mechanisms of convective cloud organization by cold pools over tropical warm ocean during the AMIE/DYNAMO field campaign

The Greenland ice sheet has become one of the main contributors to global sea level rise, predominantly through increased meltwater runoff. The main drivers of Greenland ice sheet runoff, however, remain poorly understood. Here we show that clouds enhance meltwater runoff by about one-third relative to clear skies, using a unique combination of active satellite observations, climate model data and snow model simulations. This impact results from a cloud radiative effect of 29.5 (±5.2) W m(-2). Contrary to conventional wisdom, however, the Greenland ice sheet responds to this energy through a new pathway by which clouds reduce meltwater refreezing as opposed to increasing surface melt directly, thereby accelerating bare-ice exposure and enhancing meltwater runoff. The high sensitivity of the Greenland ice sheet to both ice-only and liquid-bearing clouds highlights the need for accurate cloud representations in climate models, to better predict future contributions of the Greenland ice sheet to global sea level rise.

/pdf/clouds-enhance-greenland-ice-sheet-meltwater-runoff-1zt5tneek7.pdf

Clouds enhance Greenland ice sheet meltwater runoff

Many of the clouds important to the Earth's energy balance, from the Tropics to the Arctic, contain small amounts of liquid water. Longwave and shortwave radiative fluxes are very sensitive to small perturbations of the cloud liquid water path (LWP), when the LWP is small (i.e., < 100 g m−2; clouds with LWP less than this threshold will be referred to as “thin”). Thus, the radiative properties of these thin liquid water clouds must be well understood to capture them correctly in climate models. We review the importance of these thin clouds to the Earth's energy balance, and explain the difficulties in observing them. In particular, because these clouds are thin, potentially mixed phase, and often broken (i.e., have large 3D variability), it is challenging to retrieve their microphysical properties accurately. We describe a retrieval algorithm intercomparison that was conducted to evaluate the issues involved. The intercomparison used data collected at the Atmospheric Radiation Measurement (ARM) Southern G...

Thin Liquid Water Clouds: Their Importance and Our Challenge

Many of the clouds important to the Earth's energy balance, from the tropics to the Arctic, are optically thin and contain liquid water Longwave and shortwave radiative fluxes are very sensitive to small perturbations of the cloud liquid water path (LWP) when the liquid water path is small (ie, < g/sq m) and, thus, the radiative properties of these clouds must be well understood to capture them correctly in climate models We review the importance of these thin clouds to the Earth's energy balance, and explain the difficulties in observing them In particular, because these clouds are optically thin, potentially mixed-phase, and often (ie, have large 3-D variability), it is challenging to retrieve their microphysical properties accurately We describe a retrieval algorithm intercomparison that was conducted to evaluate the issues involved The intercomparison included eighteen different algorithms to evaluate their retrieved LWP, optical depth, and effective radii Surprisingly, evaluation of the simplest case, a single-layer overcast cloud, revealed that huge discrepancies exist among the various techniques, even among different algorithms that are in the same general classification This suggests that, despite considerable advances that have occurred in the field, much more work must be done, and we discuss potential avenues for future work

/pdf/optically-thin-liquid-water-clouds-their-importance-and-our-24go0fjoc7.pdf

Optically Thin Liquid Water Clouds: Their Importance and Our Challenge

Traditionally, analyses of satellite data have been limited to interpreting the radiances in terms of single layer clouds. Generally, this results in significant errors in the retrieved properties for multilayered cloud systems. Two techniques for detecting overlapped clouds and retrieving the cloud properties using satellite data are explored to help address the need for better quantification of cloud vertical structure. The first technique was developed using multispectral imager data with secondary imager products (infrared brightness temperature differences, BTD). The other method uses microwave (MWR) data. The use of BTD, the 11-12 micrometer brightness temperature difference, in conjunction with tau, the retrieved visible optical depth, was suggested by Kawamoto et al. (2001) and used by Pavlonis et al. (2004) as a means to detect multilayered clouds. Combining visible (VIS; 0.65 micrometer) and infrared (IR) retrievals of cloud properties with microwave (MW) retrievals of cloud water temperature Tw and liquid water path LWP retrieved from satellite microwave imagers appears to be a fruitful approach for detecting and retrieving overlapped clouds (Lin et al., 1998, Ho et al., 2003, Huang et al., 2005). The BTD method is limited to optically thin cirrus over low clouds, while the MWR method is limited to ocean areas only. With the availability of VIS and IR data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and MW data from the Advanced Microwave Scanning Radiometer EOS (AMSR-E), both on Aqua, it is now possible to examine both approaches simultaneously. This paper explores the use of the BTD method as applied to MODIS and AMSR-E data taken from the Aqua satellite over non-polar ocean surfaces.

/pdf/multilayered-clouds-identification-and-retrieval-for-ceres-4580f5yabl.pdf

Multilayered Clouds Identification and Retrieval for CERES Using MODIS

Life Cycle of Deep Convective Systems in a Lagrangian Framework: from Mid-latitude to Tropics

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Thin Liquid Water Clouds: Their Importance and Our Challenge

Optically Thin Liquid Water Clouds: Their Importance and Our Challenge

Multilayered Clouds Identification and Retrieval for CERES Using MODIS

Life Cycle of Deep Convective Systems in a Lagrangian Framework: from Mid-latitude to Tropics