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Future changes in precipitation, in particular extremes, are among the most impact-relevant consequences of a warming climate driven by increases in atmospheric greenhouse gas concentrations. Still, climate model projections of future changes in regional and local precipitation remain uncertain. This is in part due to inabilities of climate models to properly represent important atmospheric moist processes, such as convection, as well as surface properties like complex terrain, primarily since these models are typically run at relatively coarse horizontal resolution. The application of a new generation of kilometer-scale ”convection-permitting” models (CPMs), which treat deep convection explicitly, has led to a step-change improvement in simulating precipitation, especially short-duration local intense events. Therefore, CPMs have proven to be valuable tools in understanding precipitation in present climate and its response to rising global temperatures.Here, the performance of the HARMONIE-Climate (HCLIM) CPM in a regional context has been investigated as well as the added value of this model in comparison with HCLIM run with standard grid resolution of ~10 km. In the present climate, the HCLIM CPM applied over the Nordic region outperforms both the coarser-scale HCLIM and a global reanalysis data set, especially for precipitation on sub-daily time scales in summer when precipitation is often convective. This is corroborated in a study investigating how precipitation is related to large-scale atmospheric circulation, which revealed differences between the HCLIM CPM and its coarser counterpart in convection-dominated circulation types in summer. By improving the frequency and intensity distributions, the wet bias seen in the coarser HCLIM version is reduced by the CPM while also better capturing intense precipitation events, but also improvements in the partitioning between snow and rain in complex terrain.In projections of a future warmer climate, the HCLIM CPM sim

High-impact, localized intense rainfall episodes represent a major socio-economic problem for societies worldwide, and at the same time these events are notoriously difficult to simulate properly in climate models. Here, the authors investigate how horizontal resolution and model formulation influence this issue by applying the HARMONIE regional climate model (HCLIM) with three different setups; two using convection parameterization at 15 and 6.25 km horizontal resolution (the latter within the “grey-zone” scale), with lateral boundary conditions provided by ERA-Interim reanalysis and integrated over a pan-European domain, and one with explicit convection at 2 km resolution (HCLIM2) over the Alpine region driven by the 15 km model. Seven summer seasons were sampled and validated against two high-resolution observational data sets. All HCLIM versions underestimate the number of dry days and hours by 20-40%, and overestimate precipitation over the Alpine ridge. Also, only modest added value were found of “grey-zone” resolution. However, the single most important outcome is the substantial added value in HCLIM2 compared to the coarser model versions at sub-daily time scales. It better captures the local-to-regional spatial patterns of precipitation reflecting a more realistic representation of the local and meso-scale dynamics. Further, the duration and spatial frequency of precipitation events, as well as extremes, are closer to observations. These characteristics are key ingredients in heavy rainfall events and associated flash floods, and the outstanding results using HCLIM in convection-permitting setting are convincing and encourage further use of the model to study changes in such events in changing climates.

There are well-known difficulties to run numerical weather prediction (NWP) and climate models at resolutions traditionally referred to as 'grey-zone' (similar to 3-8 km) where deep convection is neither completely resolved by the model dynamics nor completely subgrid. In this study, we describe the performance of an operational NWP model, HARMONIE, in a climate setting (HCLIM), run at two different resolutions (6 and 15 km) for a 10-yr period (1998-2007). This model has a convection scheme particularly designed to operate in the 'grey-zone' regime, which increases the realism and accuracy of the time and spatial evolution of convective processes compared to more traditional parametrisations. HCLIM is evaluated against standard observational data sets over Europe as well as high-resolution, regional, observations. Not only is the regional climate very well represented but also higher order climate statistics and smaller scale spatial characteristics of precipitation are in good agreement with observations. The added value when making climate simulations at similar to 5 km resolution compared to more typical regional climate model resolutions is mainly seen for the very rare, high-intensity precipitation events. HCLIM at 6 km resolution reproduces the frequency and intensity of these events better than at 15 km resolution and is in closer agreement with the high-resolution observations., AuthorCount:4