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Quantifying the Added Value of Convection-Permitting Climate Simulations in Complex Terrain: a Systematic Evaluation of WRF Over the Himalayas

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Quantifying the Added Value of Convection-Permitting Climate Simulations in Complex Terrain: a Systematic Evaluation of WRF Over the Himalayas Earth Syst. Dynam., 8, 507–528, 2017 https://doi.org/10.5194/esd-8-507-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Quantifying the added value of convection-permitting climate simulations in complex terrain: a systematic evaluation of WRF over the Himalayas Ramchandra Karki1,2, Shabeh ul Hasson1,3, Lars Gerlitz4, Udo Schickhoff1, Thomas Scholten5, and Jürgen Böhner1 1Center for Earth System Research and Sustainability, Institute of Geography, University of Hamburg, Bundesstraße 55, 20146 Hamburg, Germany 2Department of Hydrology and Meteorology, Government of Nepal, 406 Naxal, Kathmandu, Nepal 3Department of Space Sciences, Institute of Space Technology, Islamabad 44000, Pakistan 4Section Hydrology, GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany 5Soil Science and Geomorphology, University of Tübingen, Department of Geosciences, Rümelinstrasse 19–23, 72070 Tübingen, Germany Correspondence to: Ramchandra Karki ([email protected]) Received: 27 March 2017 – Discussion started: 5 April 2017 Accepted: 31 May 2017 – Published: 5 July 2017 Abstract. Mesoscale dynamical refinements of global climate models or atmospheric reanalysis have shown their potential to resolve intricate atmospheric processes, their land surface interactions, and subsequently, re- alistic distribution of climatic fields in complex terrains. Given that such potential is yet to be explored within the central Himalayan region of Nepal, we investigate the skill of the Weather Research and Forecasting (WRF) model with different spatial resolutions in reproducing the spatial, seasonal, and diurnal characteristics of the near-surface air temperature and precipitation as well as the spatial shifts in the diurnal monsoonal precipitation peak over the Khumbu (Everest), Rolwaling, and adjacent southern areas. Therefore, the ERA-Interim (0.75◦) reanalysis has been dynamically refined to 25, 5, and 1 km (D1, D2, and D3) for one complete hydrological year (October 2014–September 2015), using the one-way nested WRF model run with mild nudging and parameter- ized convection for the outer but explicitly resolved convection for the inner domains. Our results suggest that D3 realistically reproduces the monsoonal precipitation, as compared to its underestimation by D1 but overestima- tion by D2. All three resolutions, however, overestimate precipitation from the westerly disturbances, owing to simulating anomalously higher intensity of few intermittent events. Temperatures are generally reproduced well by all resolutions; however, winter and pre-monsoon seasons feature a high cold bias for high elevations while lower elevations show a simultaneous warm bias. Unlike higher resolutions, D1 fails to realistically reproduce the regional-scale nocturnal monsoonal peak precipitation observed in the Himalayan foothills and its diurnal shift towards high elevations, whereas D2 resolves these characteristics but exhibits a limited skill in reproducing such a peak on the river valley scale due to the limited representation of the narrow valleys at 5 km resolution. Nonetheless, featuring a substantial skill over D1 and D2, D3 simulates almost realistic shapes of the seasonal and diurnal precipitation and the peak timings even on valley scales. These findings clearly suggest an added value of the convective-scale resolutions in realistically resolving the topoclimates over the central Himalayas, which in turn allows simulating their interactions with the synoptic-scale weather systems prevailing over high Asia. Published by Copernicus Publications on behalf of the European Geosciences Union. 508 R. Karki et al.: Quantifying the added value of convection-permitting climate simulations over the Himalayas 1 Background lation modes over the complex Himalayan terrain largely de- pends upon fine-scale climate model simulations featuring a Featuring a complex terrain along a 2500 km arc, the Hi- satisfactory representation of the present climate (Hasson et malaya barrier contributes extensively to redefining the re- al., 2016c), usually obtained through dynamical downscaling gional climate: (1) by controlling the cold and dry air advec- of reanalysis products. tion from central Asia and (2) by enhancing and redistribut- The Weather Research and Forecasting (WRF) model is ing the incident precipitation (Norris et al., 2015). Hence, a proven tool to dynamically downscale at very high res- the Himalayas play a crucial role in defining the regions’ olutions (< 10 km) in a complex mountainous terrain (Ras- hydrometeorology and hydrology, which in turn ensures the mussen et al., 2011; Viale et al., 2013; Jiménez and Dudhia, food and water security of 1.6 billion people and the sustain- 2013). Unlike temperature, WRF does not necessarily show able development of the agrarian economies downstream. an improvement for precipitation when going to high resolu- Since Nepal is situated in the center of the Himalayan tions, and there are regional differences. For example, over arc, its climate also results from a sophisticated interac- the Tibetan plateau (TP), Maussion et al. (2014) have shown tion between the steep and complex Himalayan terrain and that the spatial patterns, interannual variability, and season- the two large-scale circulation modes: the south Asian sum- ality of precipitation are reproduced well by WRF simula- mer monsoon and the high-level baroclinic westerlies. The tions performed at 30 and 10 km, with the latter one show- hydro-climatology of Nepal is dominated by the south Asian ing relative improvements. But over the central Himalayas summer monsoon that provides more than 80 % of the to- – where topography is relatively complex as compared to tal annual precipitation, as winter is mostly dry and cold TP – their 10 km simulation suggests an underestimation of (Shrestha, 2000; Karki et al., 2017). Being a monsoon- monsoonal precipitation by half. By contrast, over the cen- dominated mountainous region, Nepal is highly susceptible tral Himalayas, the 6.7 km convection-permitting WRF sim- to the developments of heavy rainfall that trigger hydro- ulation by Norris et al. (2016) suggests an overestimation meteorological disasters, threaten the food and water secu- up to twice that of observed monsoonal precipitation. Fo- rity and cause the loss of life. Such problems of food and cusing on the small Langtang catchment in Nepal, Collier water security and disaster management in Nepal and the and Immerzeel (2015) even illustrated the poor performance Himalayas seems to be further exacerbated by the observed of 5 km convection-permitting WRF simulations featuring a warming and its altitude-dependent intensification, the thin- higher underestimation of monsoonal precipitation compared ning and retreat of glaciers, the formation and expansion to 25 km resolution simulations with parameterized convec- of glacial lakes, the shift of tree line to higher elevations, tion. Such regionally distinct WRF results could be due to the changes in precipitation seasonality, and the increasing fre- extremely heterogeneous terrain and varied microclimates of quency and severity of floods and droughts (Karki et al., the Himalayas. 2009, 2017; Soncini et al., 2016; Schickhoff et al., 2015; The diurnal distribution of precipitation is another im- Hasson et al., 2016a). Such vulnerability and exposure of the portant characteristic of the monsoonal precipitation, as it mountainous communities to the adverse socioeconomic im- controls the circulation characteristics (latent heat release) pacts of changing climate and its induced disasters greatly and affects the precipitation magnitude (Sato et al., 2008; highlights the importance of understanding their drivers and Bhatt et al., 2014; Shrestha and Desar, 2014). Hence, a bet- their subsequent impacts on various sectors of life for de- ter imitation of the diurnal cycle characteristics by the cli- vising local-scale adaptation strategies. This requires, above mate models can ensure their realistic representation of the all, fine-scale climatic information, particularly of surface monsoonal precipitation and particularly their topoclimatic air temperature (hereinafter temperature) and precipitation, driving factors. However, at coarser horizontal resolutions which are also amongst the essential inputs to impact assess- (> 15 km), WRF has shown a deficiency in simulating even ment models (Böhner and Lehmkuhl, 2005; Karmacharya et large-scale midnight to early-morning peak precipitation, ev- al., 2016; Gerlitz et al., 2016). ident in the Himalayan foothills (Bhate et al., 2012). Al- However, meteorological observations are too sparse in though a 12 km WRF run broadly reproduces the principal the region, while spatially complete gridded (interpolated shape of the diurnal cycle but fails to capture the lull pre- or remotely sensed) observations, reanalysis products, and cipitation hour, the timing of the diurnal peak, and its south- regional-to-global scale climate models’ datasets are too ward migration (Bhatt et al., 2014). Besides horizontal res- coarse to represent the topoclimates over the deep and broad olution, convection parameterizations can also greatly affect river valleys and over the steep slopes and ridges of the com- the climate model’s performance in reproducing the distri- plex Himalayan terrain (Lang and Barros, 2004; Palazzi et butional characteristics of diurnal precipitation. Investigating al., 2013; Gerlitz et al., 2016; Hasson et al., 2016b). Conse-
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