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In situ observations of snowfall over the Antarctic Ice Sheet are scarce. Currently, continent-wide assessments of snowfall are limited to information from the Cloud Profiling Radar on board the CloudSat satellite, which has not been evaluated up to now. In this study, snowfall derived from CloudSat is evaluated using three ground-based vertically profiling 24 GHz precipitation radars (Micro Rain Radars: MRRs). Firstly, using the MRR long-term measurement records, an assessment of the uncertainty caused by the low temporal sampling rate of CloudSat (one revisit per 2.1 to 4.5 days) is performed. The 10–90th-percentile temporal sampling uncertainty in the snowfall climatology varies between 30 % and 40 % depending on the latitudinal location and revisit time of CloudSat. Secondly, an evaluation of the snowfall climatology indicates that the CloudSat product, derived at a resolution of 1 ∘ latitude by 2 ∘ longitude, is able to accurately represent the snowfall climatology at the three MRR sites (biases < 15 %), outperforming ERA-Interim. For coarser and finer resolutions, the performance drops as a result of higher omission errors by CloudSat. Moreover, the CloudSat product does not perform well in simulating individual snowfall events. Since the difference between the MRRs and the CloudSat climatology are limited and the temporal uncertainty is lower than current Climate Model Intercomparison Project Phase 5 (CMIP5) snowfall variability, our results imply that the CloudSat product is valuable for climate model evaluation purposes. [ABSTRACT FROM AUTHOR]

We performed five high resolution (2.8 km) decadal convection permitting scale (CPS) climate simulations over Belgium using the COSMO-CLM regional climate model and examined the future changes in daily precipitation extremes compared to coarser resolution simulations. The CPS model underestimates the higher percentiles during both seasons, however, some improvements in the higher percentile values are noticed during the summer season. Analysis of three future climate simulations indicates that the CPS model modifies the future signals of daily precipitation extremes compared to their forcing non-CPS simulations during summer. During this season, the increase (decrease) in the daily precipitation extremes is stronger in the CPS compared to the non-CPS simulations. During winter, no significant changes between CPS and non-CPS were found. [ABSTRACT FROM AUTHOR]

The large regional summer warming on the east coast of the northern Antarctic Peninsula (AP), which has taken place since the mid-1960s, has previously been proposed to be caused by a trend in the Southern Hemisphere Annular Mode (SAM). The authors utilize a high-resolution regional atmospheric model climatology (14-km grid spacing) to study the mechanisms that determine the response of the near-surface temperature to an increase in the SAM (ΔT/ΔSAM). Month-to-month variations in near-surface temperature and surface pressure are well represented by the model. It is found that north of ∼68°S, ΔT/ΔSAM is much larger on the eastern (lee) side than on the western (windward) side of the barrier. This is because of the enhanced westerly flow of relatively warm air over the barrier, which warms (and dries) further as it descends down the lee slope. The downward motion on the eastern side of the barrier causes a decrease in surface-mass balance and cloud cover. South of ∼68°S, vertical deflection across the barrier is greatly reduced and the contrast in ΔT/ΔSAM between the east and west sides of the barrier vanishes. In the northeastern part of the AP, the modeled ΔT/ΔSAM distribution is similar to the distribution derived from satellite infrared radiometer data. The region of strongest modeled temperature sensitivity to the SAM is where ice shelf collapse has recently taken place and does not extend farther south over the Larsen-C Ice Shelf. [ABSTRACT FROM AUTHOR]

Since the mid-1960s, rapid regional summer warming has occurred on the east coast of the northern Antarctic Peninsula, with near-surface temperatures increasing by more than 2°C. This warming has contributed significantly to the collapse of the northern sections of the Larsen Ice Shelf. Coincident with this warming, the summer Southern Hemisphere Annular Mode (SAM) has exhibited a marked trend, suggested by modeling studies to be predominantly a response to anthropogenic forcing, resulting in increased westerlies across the northern peninsula. Observations and reanalysis data are utilized to demonstrate that the changing SAM has played a key role in driving this local summer warming. It is proposed that the stronger summer westerly winds reduce the blocking effect of the Antarctic Peninsula and lead to a higher frequency of air masses being advected eastward over the orographic barrier of the northern Antarctic Peninsula. When this occurs, a combination of a climatological temperature gradient across the barrier and the formation of a föhn wind on the lee side typically results in a summer near-surface temperature sensitivity to the SAM that is 3 times greater on the eastern side of the peninsula than on the west. SAM variability is also shown to play a less important role in determining summer temperatures at stations west of the barrier in the northern peninsula (∼62°S), both at the surface and throughout the troposphere. This is in contrast to a station farther south (∼65°S) where the SAM exerts little influence. [ABSTRACT FROM AUTHOR]

The sensitivity of the surface mass balance of the Antarctic ice sheet to a change in temperature and a change in the sea ice extent is studied with a regional atmospheric climate model (RACMO) using a horizontal grid spacing of 55 km. The model is driven at its lateral boundaries by the reanalyses from the European Centre for Medium-Range Weather Forecasts. Sea ice extent and sea surface temperature are prescribed from observations. A control integration is performed for the 5-yr period 1980-84. In a 5-yr sensitivity run, the model is forced by a 2-K increase in temperature at the sea surface and at the lateral boundaries of the model domain, and a reduction in the sea ice extent. The relative humidity at the lateral boundaries is kept constant. The calculated surface mass balance of the grounded Antarctic ice is found to increase by 30% due to the 2K warming and the retreat of the sea ice. This value is two to three times as large as previous estimates, which were based on simplified atmospheric models and on statistical relations between the surface temperature and the surface mass balance. Additional sensitivity runs show that applying the forcing throughout the atmosphere in the lateral boundary zone has a more significant effect than applying the forcing at the sea surface, especially for the interior of the ice sheet. If only an increase in the sea surface temperature or a retreat of the sea ice is prescribed, the increase in temperature and specific humidity is restricted to the lowest 2 km of the atmosphere above the ocean. Sensitivity runs with forcings in the range of -5 to +10 K indicate that the commonly used assumption stating that the surface mass balance responds in proportion to the change in continental saturation specific humidity at the inversion height is an oversimplification. [ABSTRACT FROM AUTHOR]