Large discrepancies in the representation of compound long-duration dry and hot spells over Europe in CMIP5
- 1School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, United Kingdom
- 2Department of Computational Hydrosystems, Helmholtz Centre for Environmental Research—UFZ, Leipzig, Germany
- 3University of Birmingham, Edgbaston, Birmingham, B152TT, United Kingdom
- 4Wegener Center for Climate and Global Change, University of Graz, Graz, Austria
- 5Institute for Environmental Studies (IVM), Vrije Universiteit Amsterdam, Amsterdam, the Netherlands
- 1School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, United Kingdom
- 2Department of Computational Hydrosystems, Helmholtz Centre for Environmental Research—UFZ, Leipzig, Germany
- 3University of Birmingham, Edgbaston, Birmingham, B152TT, United Kingdom
- 4Wegener Center for Climate and Global Change, University of Graz, Graz, Austria
- 5Institute for Environmental Studies (IVM), Vrije Universiteit Amsterdam, Amsterdam, the Netherlands
Abstract. Long-duration dry spells in combination with temperature extremes during summer have led to extreme impacts on society and ecosystems in the past. Such events are expected to become more frequent due to increasing temperatures as a result of anthropogenic climate change. However, there is little information on how long-duration dry and hot spells are represented in global climate models (GCMs). In this study, we evaluate 33 CMIP5 GCMs in their representation of long-duration dry spells and temperatures during dry spells. We define a dry spell as a consecutive number of days with daily precipitation less than 1mm. CMIP5 models tend to underestimate the persistence of dry spells in Northern Europe while a large variability exists between model estimates in Central and Southern Europe where models have contrasting biases. Our results indicate that this variability in model estimates is due to inherent model differences and not internal variability. In Northern Europe, differences in the representation of persistent dry spells are related to the representation of persistent anticyclonic conditions. We also find a large spread in the representation of temperature extremes during dry spells. In Central and Southern Europe this spread in temperature extremes between models is related to the representation of dry spells, where models that produce longer dry spells also produce higher temperatures, and vice versa. Overall, there are large discrepancies in the representation of long-duration dry and hot events in the CMIP5 ensemble where the simulated climates vary from models with shorter-cooler dry spells to models with longer-hotter dry spells. This information is important to consider when interpreting the plausibility of future projections from climate models.
Colin Manning et al.
Status: open (extended)
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RC1: 'Comment on wcd-2022-15', Anonymous Referee #1, 14 Apr 2022
reply
Review of manuscript submitted to Weather and Climate Dynamics
Manuscript identification number: wcd-2022-15
Title: Large discrepancies in the representation of compound long-duration dry and hot
spells over Europe in CMIP5
Authors: Colin Manning, Emanuele Bevacqua, Martin Widmann, Douglas Maraun, Anne F. Van Loon
Recommendation: Rejection
General Comments
The authors analyse the links between the occurrence of anticyclones, dry spells and heat waves in a large ensemble of CMIP5 GCM simulations for the historical period. Additionally, ERA5 and E-OBS data is considered in the evaluation. The authors conclude that the discrepancies between the GCMs identified in terms of the duration of dry spells and extreme temperatures are related with the GCM biases regarding temperature and precipitation themselves. While the topic of the manuscript is surely interesting and I acknowledge that there was a huge amount of data processing involved, there are also severe shortcomings, the main one being the way that “anticyclones” are considered, which is methodological not sound (see major point). Given that this was the only aspect broadly related with atmospheric dynamics (the core of WCD), I’d argue that the manuscript in its present form is also out of scope of the journal. For both reasons, and given that this shortcoming is pretty fundamental to the whole analysis, I’d like to recommend its rejection in the present form (as the whole calculations and analysis would need to be redone).
This say, I’d strongly encourage the authors to take up this task and resubmit the manuscript for further evaluation. If resubmitted to WCD, the aspect of atmospheric dynamics should be strongly strengthened so it fits the scope of the journal (e.g. also links to blocking, synoptic weather types, anticyclone dynamics). If the authors do not which to change the focus of the manuscript so strongly, I’d suggest the resubmission to a different journal – notably NHESS, which focus primarily on impacts. I would be willing to review the paper again upon resubmission.
Main Comment
The main shortcoming in the present study is the way “anticyclones” are dealt with. For me, an anticyclone is a high-pressure centre with clockwise rotating winds and large-scale divergence at the surface. At upper levels, it is typically associated with a cut off low / ridge / blocking system, where upper-level convergence occurs, thus leading to large-scale subsidence in the area of the surface high-pressure centre. As described e.g. in a recent review paper published in WCD (Kautz et al. 2022; https://doi.org/10.5194/wcd-3-305-2022), a persistent anticyclone / blocking over Central Europe in the summer typically leads to heat waves and dry spells collocated with the centre of the system (see their Figure 2b). However, on both flanks of the system you often observe heavy precipitation associated e.g. with moist air intrusions on the western flank of the system (same figure).
Given this, I am really puzzled that the authors “define an anticyclone" as local exceedance of MSLP above 1012 hPa over five days. This value is below the average mean MSLP for a considerable part of Europe (particularly in the summer months!), and even below the global average MSLP value! Given the often quite stationary weather conditions in the summer, five days is also no real constraint. So we are not even looking at above average pressure conditions. And of course, this has nothing to do with atmospheric dynamics and cannot provide any insight either on the exact location of the anticyclone and where different types of extremes may be expected. If a simple metric is needed, I’d use chose something based on MSLP anomalies to the monthly mean (or summer) MSLP fields, thus as an indication of the anomalous circulation associated with the high-pressure anomaly.
This explains why several of the following evaluations/results do not really meet the expectations (e.g. compared to what you'd expect for blocking, e.g. around lines 265) or are not really understandable (at least to me), e.g. Figs 3 and low collocation of dry spells and anticyclones for Southern Europe (as acknowledged by the authors also in the text), and several other figures, particularly Figure 8. While I do like the idea of the paper (which is my I accepted the review), I do thing the methodology is flawed and the presented results are thus unfortunately not sound.
Minor comments
a) It is not clear to me why version 16.0 of E-OBS is being used, as we are currently on version 24.0 (see https://www.ecad.eu/download/ensembles/download.php). There have been quite a few important updates since.
b) I think it is a very strong statement to say that the “combination of dry spells and extreme temperatures” has not yet been assessed in CMIP5 models. Please weaken the statement.
c) The description of the results is often not understandable, e.g. the description of figure 3b (likes 228-236). Please enhance.
d) Several of the references are missing page ranges, issue numbers, particularly for AGU journals, please enhance.
Colin Manning et al.
Colin Manning et al.
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