A physically based precipitation separation algorithm for convection‐permitting models over complex topography

Abstract

A new algorithm is described that can separate precipitation output from convection‐permitting models into three different types of precipitation: (a) convective, (b) stratiform, and (c) orographically enhanced precipitation. The algorithm is based on physical processes that underlie these types of precipitation and it is applicable over both ocean and land surfaces. It is particularly well suited for mountainous areas or other regions exhibiting complex terrain. The algorithm's performance is first demonstrated for a selection of well‐understood weather events and then for a 10‐year convection‐permitting climate simulation over Norway. The algorithm correctly separates convection embedded in frontal systems from stratiform precipitation and also properly identifies orographically enhanced precipitation when the frontal systems interact with local orography. The results suggest that this can be a powerful new tool for investigating characteristics of precipitation in convection‐permitting climate simulations, particularly in a climate change context and as researchers move towards models that explicitly resolve convection and its related processes.

Replies to review

“…These systems need to be better described, both their identification procedure and physical meaning. There is a recent paper by Poujol et al (2020) on a separation between convective and stratiform precipitation. It might be interesting to check if the precipitation within high pressure systems can be classified as convective using their approach.…”

“…We thank the reviewer for pointing us to the paper by Poujol et al (2020), as their classification approach looks very promising. However, we do not think that it would add much to the characterization of the high-pressure area precipitation, because it is fairly obvious to us (from extensive visual analysis of precipitation fields during method development) that most of this precipitation is convective.…”

“…We have added such a remark to the end of the "Conclusions". New: Finally, methods that separate precipitation types like convective and stratiform (e.g., Poujol et al, 2020) could be combined with our feature-based attribution, which would enable a more in-depth characterization of the different front-cyclone-relative precipitation components C7 6.3 [ Fig. 3]: Blue filling of cold fronts is very similar to precipitation 0.2-1 mm/h, please use different colours.…”

Attribution of precipitation to cyclones and fronts over Europe in a kilometer-scale regional climate simulation

“…In the global mean, precipitation is expected to increase at a rate of 2 % per degree of global-mean warming, but changes in short-term precipitation are likely to occur at much faster rates (Trenberth, 1999;Held and Soden, 2006;Schneider et al, 2010). In the last decade, huge progress has been made in realistically simulating the hydrological cycle with highresolution climate models, including the spatial distribution of precipitation, its diurnal cycle, and the statistics of extreme events (e.g., Hohenegger et al, 2008;Kendon et al, 2012;Ban et al, 2014Ban et al, , 2015Prein et al, 2015;Clark et al, 2016; S. Rüdisühli et al: Attribution of precipitation to cyclones and fronts over Europe Keller et al, 2016;Leutwyler et al, 2017). A major part of this progress is due to the step change of simulating deep convection explicitly instead of using a parameterized representation.…”

“…A major part of this progress is due to the step change of simulating deep convection explicitly instead of using a parameterized representation. In their systematic comparison of climate model simulations with parameterized or explicit convection, Prein et al (2015) found that "Improvements [when using explicit convection] are evident mostly for climate statistics related to deep convection, mountainous regions, or extreme events. "…”