scholarly journals Greenland Clouds Observed in CALIPSO-GOCCP: Comparison with Ground-Based Summit Observations

2017 ◽  
Vol 30 (15) ◽  
pp. 6065-6083 ◽  
Author(s):  
Adrien Lacour ◽  
Helene Chepfer ◽  
Matthew D. Shupe ◽  
Nathaniel B. Miller ◽  
Vincent Noel ◽  
...  
Keyword(s):  
Cloud Cover ◽  
Vertical Structure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ground Level ◽  
Detection Methods ◽  
Total Cloud Cover ◽  
Ice Clouds ◽  
Passive Sensors ◽  
Lidar Observations

Spaceborne lidar observations from the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations ( CALIPSO) satellite provide the first-ever observations of cloud vertical structure and phase over the entire Greenland Ice Sheet. This study leverages CALIPSO observations over Greenland to pursue two investigations. First, the GCM-Oriented CALIPSO Cloud Product ( CALIPSO-GOCCP) observations are compared with collocated ground-based radar and lidar observations at Summit, Greenland. The liquid cloud cover agrees well between the spaceborne and ground-based observations. In contrast, ground–satellite differences reach 30% in total cloud cover and 40% in cloud fraction below 2 km above ground level, due to optically very thin ice clouds (IWC < 2.5 × 10−3 g m−3) missed by CALIPSO-GOCCP. Those results are compared with satellite cloud climatologies from the GEWEX cloud assessment. Most passive sensors detect fewer clouds than CALIPSO-GOCCP and the Summit ground observations, due to different detection methods. Second, the distribution of clouds over the Greenland is analyzed using CALIPSO-GOCCP. Central Greenland is the cloudiest area in summer, at +7% and +4% above the Greenland-wide average for total and liquid cloud cover, respectively. Southern Greenland contains free-tropospheric thin ice clouds in all seasons and liquid clouds in summer. In northern Greenland, fewer ice clouds are detected than in other areas, but the liquid cloud cover seasonal cycle in that region drives the total Greenland cloud annual variability with a maximum in summer. In 2010 and 2012, large ice-sheet melting events have a positive liquid cloud cover anomaly (from +1% to +2%). In contrast, fewer clouds (−7%) are observed during low ice-sheet melt years (e.g., 2009).

scholarly journals How Well Are Clouds Simulated over Greenland in Climate Models? Consequences for the Surface Cloud Radiative Effect over the Ice Sheet

2018 ◽  
Vol 31 (22) ◽  
pp. 9293-9312 ◽  
Author(s):  
A. Lacour ◽  
H. Chepfer ◽  
N. B. Miller ◽  
M. D. Shupe ◽  
V. Noel ◽  
...  
Keyword(s):  
Cloud Cover ◽  
Climate Models ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ground Level ◽  
Radiative Effect ◽  
Cloud Properties ◽  
Cloud Radiative Effect ◽  
Primary Driver ◽  
Cloud Radiative Effects

Using lidar and radiative flux observations from space and ground, and a lidar simulator, we evaluate clouds simulated by climate models over the Greenland ice sheet, including predicted cloud cover, cloud fraction profile, cloud opacity, and surface cloud radiative effects. The representation of clouds over Greenland is a central concern for the models because clouds impact ice sheet surface melt. We find that over Greenland, most of the models have insufficient cloud cover during summer. In addition, all models create too few nonopaque, liquid-containing clouds optically thin enough to let direct solar radiation reach the surface (−1% to −3.5% at the ground level). Some models create too few opaque clouds. In most climate models, the cloud properties biases identified over all Greenland also apply at Summit, Greenland, proving the value of the ground observatory in model evaluation. At Summit, climate models underestimate cloud radiative effect (CRE) at the surface, especially in summer. The primary driver of the summer CRE biases compared to observations is the underestimation of the cloud cover in summer (−46% to −21%), which leads to an underestimated longwave radiative warming effect (CRELW = −35.7 to −13.6 W m−2 compared to the ground observations) and an underestimated shortwave cooling effect (CRESW = +1.5 to +10.5 W m−2 compared to the ground observations). Overall, the simulated clouds do not radiatively warm the surface as much as observed.

scholarly journals Decreasing cloud cover drives the recent mass loss on the Greenland Ice Sheet

2017 ◽  
Vol 3 (6) ◽  
pp. e1700584 ◽  
Author(s):  
Stefan Hofer ◽  
Andrew J. Tedstone ◽  
Xavier Fettweis ◽  
Jonathan L. Bamber
Keyword(s):  
Mass Loss ◽  
Cloud Cover ◽  
Ice Sheet ◽  
Greenland Ice Sheet

Simulations of dated radiostratigraphy for the Greenland ice sheet

2020 ◽  
Author(s):  
Andreas Born ◽  
Alexander Robinson
Keyword(s):  
Degrees Of Freedom ◽  
Vertical Structure ◽  
Three Dimensional ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Climate Forcing ◽  
Lagrangian Description ◽  
Glacial Cycle ◽  
Dynamic Constraints ◽  
The Last Glacial

&lt;p&gt;As layers of accumulated snow compact into ice and start to flow under its own weight, their deformations are recorded in the vertical structure of the glacier. Therefore, the isochronal stratigraphy of the Greenland ice sheet provides comprehensive dynamic constraints, which, with adequate methods, can be used to calibrate ice sheet models and greatly improve their accuracy.&lt;br&gt;&lt;br&gt;We present the first three-dimensional ice sheet model that explicitly resolves isochrones. Individual layers of accumulation do not exchange mass with each other as the flow of ice deforms them, resembling the Lagrangian description of flow in the vertical dimension, while lateral flow within each layer is Eulerian. Direct comparison with dated radiostratigraphy is used to filter an ensemble of simulations of the Greenland ice sheet. The abundant information implied by the shape of the three-dimensional layering enables us to constrain a large number of degrees of freedom. The mismatch in the thickness of certain isochrones is used to calibrate the climate forcing of different periods of the last glacial cycle.&lt;/p&gt;

scholarly journals Importance of Orography for Greenland Cloud and Melt Response to Atmospheric Blocking

2020 ◽  
Vol 33 (10) ◽  
pp. 4187-4206 ◽  
Author(s):  
L. C. Hahn ◽  
T. Storelvmo ◽  
S. Hofer ◽  
R. Parfitt ◽  
C. C. Ummenhofer
Keyword(s):  
High Pressure ◽  
Cloud Cover ◽  
Large Scale ◽  
Climate Model ◽  
Model Simulation ◽  
Regional Climate ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Atmospheric Blocking ◽  
Circulation Anomalies

AbstractMore frequent high pressure conditions associated with atmospheric blocking episodes over Greenland in recent decades have been suggested to enhance melt through large-scale subsidence and cloud dissipation, which allows more solar radiation to reach the ice sheet surface. Here we investigate mechanisms linking high pressure circulation anomalies to Greenland cloud changes and resulting cloud radiative effects, with a focus on the previously neglected role of topography. Using reanalysis and satellite data in addition to a regional climate model, we show that anticyclonic circulation anomalies over Greenland during recent extreme blocking summers produce cloud changes dependent on orographic lift and descent. The resulting increased cloud cover over northern Greenland promotes surface longwave warming, while reduced cloud cover in southern and marginal Greenland favors surface shortwave warming. Comparison with an idealized model simulation with flattened topography reveals that orographic effects were necessary to produce area-averaged decreasing cloud cover since the mid-1990s and the extreme melt observed in the summer of 2012. This demonstrates a key role for Greenland topography in mediating the cloud and melt response to large-scale circulation variability. These results suggest that future melt will depend on the pattern of circulation anomalies as well as the shape of the Greenland Ice Sheet.

scholarly journals Parameterizing the Difference in Cloud Fraction Defined by Area and by Volume as Observed with Radar and Lidar

2005 ◽  
Vol 62 (7) ◽  
pp. 2248-2260 ◽  
Author(s):  
Malcolm E. Brooks ◽  
Robin J. Hogan ◽  
Anthony J. Illingworth
Keyword(s):  
Wind Shear ◽  
General Circulation ◽  
Cloud Cover ◽  
Cloud Fraction ◽  
Vertical Resolution ◽  
Ice Clouds ◽  
Cloud Radar ◽  
The Mean ◽  
The Difference ◽  
Lidar Observations

Abstract Most current general circulation models (GCMs) calculate radiative fluxes through partially cloudy grid boxes by weighting clear and cloudy fluxes by the fractional area of cloud cover (Ca), but most GCM cloud schemes calculate cloud fraction as the volume of the grid box that is filled with cloud (Cυ). In this paper, 1 yr of cloud radar and lidar observations from Chilbolton in southern England, are used to examine this discrepancy. With a vertical resolution of 300 m it is found that, on average, Ca is 20% greater than Cυ, and with a vertical resolution of 1 km, Ca is greater than Cυ by a factor of 2. The difference is around a factor of 2 larger for liquid water clouds than for ice clouds, and also increases with wind shear. Using Ca rather than Cυ, calculated on an operational model grid, increases the mean total cloud cover from 53% to 63%, and so is of similar importance to the cloud overlap assumption. A simple parameterization, Ca = [1 + e(−f )(C−1υ − 1)]−1, is proposed to correct for this underestimate based on the observation that the observed relationship between the mean Ca and Cυ is symmetric about the line Ca = 1 − Cυ. The parameter f is a simple function of the horizontal (H) and vertical (V) grid-box dimensions, where for ice clouds f = 0.0880 V 0.7696 H−0.2254 and for liquid clouds f = 0.1635 V 0.6694 H−0.1882. Implementing this simple parameterization, which excludes the effect of wind shear, on an independent 6-month dataset of cloud radar and lidar observations, accounts for the mean underestimate of Ca for all horizontal and vertical resolutions considered to within 3% of the observed Ca, and reduces the rms error for each individual box from typically 100% to approximately 30%. Small biases remain for both weakly and strongly sheared cases, but this is significantly reduced by incorporating a simple shear dependence in the calculation of the parameter f, which also slightly improves the overall performance of the parameterization for all of the resolutions considered.

scholarly journals Decreasing cloud cover drives the recent mass loss on the Greenland Ice Sheet

2017 ◽  
Author(s):  
Stefan Hofer ◽  
Andrew Tedstone ◽  
Xavier Fettweis ◽  
Jonathan Bamber
Keyword(s):  
Mass Loss ◽  
Cloud Cover ◽  
Ice Sheet ◽  
Greenland Ice Sheet

scholarly journals Assessment of Cloud Cover Characteristics in Satellite Datasets and Reanalysis Products for Greenland

2008 ◽  
Vol 21 (9) ◽  
pp. 1837-1849 ◽  
Author(s):  
J. A. Griggs ◽  
J. L. Bamber
Keyword(s):  
Cloud Cover ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Atmospheric Model ◽  
Reanalysis Data ◽  
Department Of Energy ◽  
Balance Model ◽  
Radiation Balance ◽  
Moderate Resolution ◽  
Moderate Resolution Imaging Spectroradiometer

Abstract Clouds have an important controlling influence on the radiation balance, and hence surface melting, over the Greenland ice sheet and need to be classified to derive reliable albedo estimates from visible imagery. Little is known, however, about the true cloud cover characteristics for the largest island on Earth, Greenland. Here, an attempt is made to address this knowledge gap by examining cloud characteristics, as determined by three complementary satellites sensors: the Advanced Very High Resolution Radiometer (AVHRR), the Along Track Scanning Radiometer-2 (ATSR-2), and the Moderate Resolution Imaging Spectroradiometer (MODIS). The first provides a multidecadal time series of clouds, albedo, and surface temperature, and is available, in the form of the extended AVHRR Polar Pathfinder dataset (APP-x), as a homogeneous, consistent dataset from 1982 until 2004. APP-x data, however, are also the most challenging to cloud classify over snow-covered terrain, due to the limited spectral capabilities of the instrument. ATSR-2 permits identification and classification using stereophotogrammetric techniques and MODIS has enhanced spectral sampling in the visible and thermal infrared but over more limited time periods. The spatial cloud fractions from the three sensors are compared and show good agreement in terms of both magnitude and spatial pattern. The cloud fractions, and inferred patterns of accumulation, are then assessed from three commonly used reanalysis datasets: NCEP–NCAR, the second NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP-II), and the 40-yr ECMWF Re-Analysis (ERA-40). Poor agreement between the reanalysis datasets is found. NCEP–DOE AMIP-II produces a cloud fraction similar to that observed by the satellites. NCEP–NCAR and ERA-40, however, bear little similarity to the cloud fractions derived from the satellite observations. This suggests that they may produce poor accumulation estimates over the ice sheet and poor estimates of radiation balance. Using these reanalysis data to force a mass balance model of the ice sheet, without appropriate downscaling and correction for the substantial biases present, may, therefore, produce substantial errors in surface melt rate estimates.

scholarly journals The impact of cloud cover on the net radiation budget of the Greenland ice sheet

2002 ◽  
Vol 34 ◽  
pp. 141-149 ◽  
Author(s):  
F. G. L. Cawkwell ◽  
J. L. Bamber
Keyword(s):  
Cloud Cover ◽  
Stereo Matching ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Cloud Amount ◽  
Radiation Budget ◽  
Radiation Balance ◽  
Net Radiation ◽  
Radiation Fluxes ◽  
The Impact

AbstractEnergy-balancemodels driven by radiation and turbulent heat fluxes have been widely applied to predicting the response of the Greenland ice sheet to climate change. However, a lack of knowledge of the temporal and spatial distribution of cloud amount and type has necessitated the use of parameterizations or statistical models of cloud cover. This deficiency results in large uncertainties in both shortwave and longwave radiation fluxes. Stereo-matching of nadir and forward view AlongTrack Scanning Radiometer-2 (ATSR-2) image pairs has been shown to be a reliable method of retrieving cloud top height, and further cloud properties can be derived from thermal imagery allowing classification into cloud type. A 1 year cloud record for a transect across southern Greenland derived from stereo-matching is presented here, and comparisons are made with climate re-analysis data and ground observations. The cloud-cover data were used in a simple radiative transfer model, and the impact of clouds on the net radiation fluxes was found to be considerable. Different cloud scenarios produced up to 40 Wm–2 difference in net radiation balance. In the ablation zone, where the albedo is lower and most variable, the sensitivity to cloud-cover fraction was less marked, but the higher spatial resolution of the ATSR-2 cloud record was reflected by a much more varied trend in radiation balance. Whether the net radiation balance increases or decreases with increased cloud cover was found to be a function of the cloud amount and type and also the surface albedo. The sensitivity of the model to a ±5% change in cloud amount was found to be comparable to a 1 K change in temperature. This clearly demonstrates the importance of reliable, quantitative cloud data in mass-balance and other glaciological studies.

scholarly journals Future projections of the Greenland ice sheet energy balance driving the surface melt, developed using the regional climate MAR model

2012 ◽  
Vol 6 (4) ◽  
pp. 2265-2303 ◽  
Author(s):  
B. Franco ◽  
X. Fettweis ◽  
M. Erpicum
Keyword(s):  
Energy Balance ◽  
Cloud Cover ◽  
Surface Albedo ◽  
Regional Climate ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Heat Fluxes ◽  
Near Surface ◽  
Global Circulation Models ◽  
Surface Melt

Abstract. In this study, 25 km-simulations are performed over the Greenland ice sheet (GrIS) throughout the 20th and 21st centuries, using the regional climate model MAR forced by four RCP scenarios from two CMIP5 global circulation models, in order to investigate the projected changes of the surface energy balance (SEB) components driving the surface melt. Analysis of 2000–2100 melt anomalies compared to melt results over 1980–1999 reveals an exponential relationship of the GrIS surface melt rate simulated by MAR to the near-surface temperature (TAS) anomalies, mainly due to the surface albedo positive feedback associated with the extension of bare ice areas in summer. On the GrIS margins, the future melt anomalies are rather driven by stronger sensible heat fluxes, induced by enhanced warm air advections over the ice sheet. Over the central dry snow zone, the increase of melt surpasses the negative feedback from heavier snowfall inducing therefore a decrease of the summer surface albedo even at the top of the ice sheet. In addition to the incoming longwave flux increase associated to the atmosphere warming, MAR projects an increase of the cloud cover decreasing the ratio of the incoming shortwave versus longwave radiation and dampening the albedo feedback. However, it should be noted that this trend in the cloud cover is contrary to that simulated by ERA-INTERIM-forced MAR over current climate, where the observed melt increase since the 1990's seems rather to be a consequence of more anticyclonic atmospheric conditions. Finally, no significant change is projected in the length of the melt season. This timing highlights the importance of solar radiation in the melt SEB.

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