Because of the scarcity of meteorological observations, the precipitation climate on the Tibetan Plateau and surrounding regions (TP) has been insufficiently documented so far. In this study, the characteristics and basic features of precipitation on the TP during an 11-yr period (2001-11) are described on monthly-to-annual time scales. For this purpose, a new high-resolution atmospheric dataset is analyzed, the High Asia Reanalysis (HAR), generated by dynamical downscaling of global analysis data using the Weather Research and Forecasting (WRF) model. The HAR precipitation data at 30-and 10-km resolutions are compared with both rain gauge observations and satellite-based precipitation estimates from the Tropical Rainfall Measurement Mission (TRMM). It is found that the HAR reproduces previously reported spatial patterns and seasonality of precipitation and that the highresolution data add value regarding snowfall retrieval, precipitation frequency, and orographic precipitation. It is demonstrated that this process-based approach, despite some unavoidable shortcomings, can improve the understanding of the processes that lead to precipitation on the TP. Analysis focuses on precipitation amounts, type, seasonality, and interannual variability. Special attention is given to the links between the observed patterns and regional atmospheric circulation. As an example of an application of the HAR, a new classification of glaciers on the TP according to their accumulation regimes is proposed, which illustrates the strong spatial variability of precipitation seasonality. Finally, directions for future research are identified based on the HAR, which has the potential to be a useful dataset for climate, glaciological, and hydrological impact studies.

Abstract.Meteorological observations over the Tibetan Plateau (TiP) are scarce, and precipitation estimations over this remote region are difficult. The constantly improving capabilities of numerical weather prediction (NWP) models offer the opportunity to reduce this problem by providing precipitation fields and other meteorological variables of high spatial and temporal resolution. Longer time periods of years to decades can be simulated by NWP models by successive model runs of shorter periods, which can be described by the term "regional atmospheric reanalysis". In this paper, we assess the Weather Research and Forecasting (WRF) models capacity in retrieving rain-and snowfall on the TiP in such a configuration using a nested approach: the simulations are conducted with three nested domains at spatial resolutions of 30, 10, and 2 km. A validation study is carried out for a one-month period with a special focus on one-week (22-28 October 2008), during which strong rain-and snowfall was observed on the TiP. The output of the model in each resolution is compared to the Tropical Rainfall Measuring Mission (TRMM) data set for precipitation and to the Moderate Resolution Imaging Spectroradiometer (MODIS) data set for snow extent. TRMM and WRF data are then compared to weather-station measurements. Our results suggest an overall improvement from WRF over TRMM with respect to weather-station measurements. Various configurations of the model with different nesting and forcing strategies, as well as physical parameterisation schemes are compared to propose a suitable design for a regional atmospheric reanalysis over the TiP. The WRF model showed good accuracy in simulating snow-and rainfall on the TiP for a one-month simulation period. Our study reveals that there is nothing like an optimal Correspondence to: F. Maussion (fabien.maussion@tu-berlin.de) model strategy applicable for the high-altitude TiP, its fringing high-mountain areas of extremely complex topography and the low-altitude land and sea regions from which much of the precipitation on the TiP is originating. The choice of the physical parameterisation scheme will thus be always a compromise depending on the specific purpose of a model simulation. Our study demonstrates the high importance of orographic precipitation, but the problem of the orographic bias remains unsolved since reliable observational data are still missing. The results are relevant for anyone interested in carrying out a regional atmospheric reanalysis. Many hydrological analyses and applications like rainfall-runoff modelling or the analysis of flood events require precipitation rates at daily or even hourly intervals. Thus, our study offers a process-oriented alternative for retrieving precipitation fields of high spatio-temporal resolution in regions like the TiP, where other data sources are limited.

The ice cap Vestfonna in the northern Svalbard archipelago is one of the largest ice bodies of the European Arctic (∼2400 km2), but little is known about its mass balance. We model the climatic mass balance of the ice cap for the period September 2000 to August 2009 on a daily basis. Ablation is calculated by a spatially distributed temperature‐radiation‐index melt model. Air temperature forcing is provided by ERA‐Interim data that is downscaled using data from an automatic weather station operated on the ice cap. Spatially distributed net shortwave radiation fluxes are obtained from standard trigonometric techniques combined with Moderate Resolution Imaging Spectroradiometer‐based cloud cover and surface albedo information. Accumulation is derived from ERA‐Interim precipitation data that are bias corrected and spatially distributed as a function of elevation. Refreezing is incorporated using the Pmax approach. Results indicate that mass balance years are characterized by short ablation seasons (June to August) and correspondingly longer accumulation periods (September to May). The modeled, annual climatic mass balance rate shows an almost balanced mean of −0.02 ± 0.20 m w.e. yr−1 (meters water equivalent per year) with an associated equilibrium line altitude of 383 ± 54 m above sea level (mean ± one standard deviation). The mean winter balance is +0.32 ± 0.06 m w.e. yr−1, and the mean summer balance −0.35 ± 0.17 m w.e. yr−1. Roughly one fourth of total surface ablation is retained by refreezing indicating that refreezing is an important component of the mass budget of Vestfonna.

Calving is an important mass-loss process at ice sheet and marine-terminating glacier margins, but identifying and quantifying its principal driving mechanisms remains challenging. Hansbreen is a grounded tidewater glacier in southern Spitsbergen, Svalbard, with a rich history of field and remote sensing observations. The available data make this glacier suitable for evaluating mechanisms and controls on calving, some of which are considered in this paper. We use a full-Stokes thermomechanical 2D flow model (Elmer/Ice), paired with a crevasse-depth calving criterion, to estimate Hansbreen's front position at a weekly time resolution. The basal sliding coefficient is re-calibrated every 4 weeks by solving an inverse model. We investigate the possible role of backpressure at the front (a function of ice mélange concentration) and the depth of water filling crevasses by examining the model's ability to reproduce the observed seasonal cycles of terminus advance and retreat. Our results suggest that the ice-mélange pressure plays an important role in the seasonal advance and retreat of the ice front, and that the crevasse-depth calving criterion, when driven by modeled surface meltwater, closely replicates observed variations in terminus position. These results suggest that tidewater glacier behavior is influenced by both oceanic and atmospheric processes, and that neither of them should be ignored.

In this article, the results of an investigation into the air temperature conditions on Svalbard in the period 1 September 2010 to 31 August 2011 are presented. For this period, parallel temperature measurements have been made as many as in 30 sites. On the basis of this unique set of data it was possible to study, in detail, the spatial distribution of different thermal characteristics [mean temperature, diurnal temperature range (DTR), day‐to‐day variability, degree of climate continentality, etc.] in Svalbard. Such knowledge of the whole of Svalbard was not previously available with sufficient accuracy for all areas. High resolution maps showing the spatial distribution of all studied thermal characteristics were also produced and analysed. Analysis of surface temperature data shows that the markedly coldest area throughout the whole year was northern Svalbard, and in particular its eastern side (Nordaustlandet). On the other hand, the highest temperatures were recorded in western part of Spitsbergen. The greatest spatial decreasing rate of temperature in Svalbard throughout the whole year was observed in a southwest (SW)–northeast (NE) direction. The distribution of mean seasonal and annual temperature reduced to sea level on Svalbard differs from the distribution based on surface temperatures. Spring, and in particular winter, saw the greatest DTRs (4–7 and 6–9 °C, respectively), while the lowest were observed in summer (3.0–3.5 °C). In all seasons, the highest DTR were mainly noted in the NE part of Svalbard, while the lowest were in its SW part. The lowest continentality of climate (30%) is clearly seen in the south‐western part of Svalbard, while the highest values (above 43%) stretch from the western part of Nordaustlandet to the area of Sveagruva in the central‐eastern part of Spitsbergen. The NORA10 hindcast temperature data differ significantly from measured data for some seasons and areas and need bias corrections when used in climatology.