Evaluation of daily precipitation statistics and monsoon onset/retreat over western Sahel in multiple data sets

Abstract

number of its physics modules with the limited-area version of GEM forecast model. The evaluation is focused on various daily precipitation statistics (maximum number of consecutive wet days, number of moderate and very heavy precipitation events, precipitation frequency distribution) and on the monsoon onset and retreat over the Sahel region. We find that the CRCM5 has a good representation of daily precipitation statistics over the southern Sahel, with spatial distributions close to GPCP dataset. Some differences are observed in the northern part of the Sahel, where the model is characterised by a dry bias. CanRCM4 and the ERA-Interim and MERRA reanalysis products overestimate the number of wet days over Sahel with a shift in the frequency distribution toward smaller daily precipitation amounts than in observations. Both RCMs and reanalyses have difficulties in reproducing the local onset date over the Sahel region. Nevertheless, the large-scale features of the monsoon precipitation evolution over West Africa are well reproduced by the RCMs, whereas the northern limit of the rainy bands is less accurately reproduced. Both RCMs exhibit an overall good representation of the local retreat index over the Sahel region.

West African monsoon intraseasonal activity and its daily precipitation indices in regional climate models: diagnostics and challenges

“…Diaconescu et al (2015) give details on the background concepts supporting the selection of these indices, while Gachon et al (2007) show the climatology of such indices using numerous stations over West Africa. The Root Mean Square Error (RMSE, against ARC2 used as reference dataset) is used as the metric and the information is summarized using a condensed performance matrix (e.g.…”

“…Vellinga et al 2013;Fitzpatrick et al 2015), two main features can be used for detecting the onset: (1) the large-scale onset that focuses on the shift (or jump) of the sustained rainy band from its Guinean position (~5°N) to a Sudanese position (~10°N), (2) the local onsets are related to more agricultural purposes and represent the effective start of sustained rainfall at a specific location. The large-scale precipitation onset has been quite widely analyzed, in particular in CORDEX simulations (among others, Gbobaniyi et al 2013;Nikulin et al 2012;Diaconescu et al 2015) and most RCMs tend to improve the onset occurrence in comparison with ERAI and GCMs. The local onset is more complicated to reproduce since precipitation structures are more affected by smaller scale processes in this case (Marteau et al 2011).…”

“…In this paper, the focus is on local onset and the intention is to see how good RCM skills are at smaller scales, and hence whether they can be used in agricultural domains, for instance. The local onset computation is based on the method of Diaconescu et al (2015) here, which takes its inspiration from a study by Liebmann and Marengo (2001). An onset function is defined, for each grid point, as the cumulative sum of daily precipitation anomalies from day 1 (1 st January) to day 365 (31 st December).…”

“…The WAM local onset/retreat is another intraseasonal event of great interest but, until now, it has been the subject of only a few comprehensive assessments, especially at local scale (in contrast with large-scale onset) in RCM simulations. Diaconescu et al (2015) pointed out the difficulties of RCM simulations as well as reanalyses to simulate the local onset over the Sahel region. In this paper, in order to obtain a more comprehensive idea of the intraseasonality in RCMs and its impacts (quantitatively), a set of daily precipitation indices are evaluated against satellite observations and presented as a performance matrix.…”

“…Parry et al (2007) highlighted the necessity of understanding and representing these daily rainfall statistics since they are often more useful (than seasonal means) for impact and adaptation studies. Diaconescu et al (2015) used two ERAI-driven RCMs to analyze the local onset and retreat of the WAM, as well as the daily precipitation statistics such as the occurrences of wet days, consecutive wet and dry days, and extreme rainfall. Their study reveals the difficulty that reanalyses, such as ERAI, have in adequately representing these indices, in particular the statistics of events with precipitation higher than 10 mm day −1 .…”