Climate change in the Mediterranean region poses critical issues on the future evolution of total precipitation [1
], and of its extremes, their intensification with different trends at the sub-regional scale [7
Particularly for precipitation, dynamical downscaling through the coupled regional climate model plays an important role to improve climate information at the regional fine-scale, since it modulates information produced by the global climate model (GCM), combining planetary scale processes with regional scale processes. Further, several studies show that to explicitly resolve the two-way interactions at the atmosphere–ocean interface influences the short time evolution of the system [13
] and, generally, enhances the quality of climate simulations in the region of interest [5
]. Here, we used a regional coupled model where two-way interaction is fully represented.
The role of resolution is often addressed for precipitation extremes and the need for high resolution is often advocated [16
]. Benefits from improving resolution are shown, particularly over complex orography, such as the Alps [17
]. However, a very high resolution (capable to explicitly resolve convection) is needed and other model components (mainly microphysics) have to be considered [19
]. Some aspects of precipitation changes with global warming (such as winter rainfall intensity) have been found to show robust features persistent across different models’ resolutions, but other seasons (summer) and extremes depend significantly on model resolution [22
] (for the southern UK).
The main novelty of this work is to analyze the simulation of precipitation using two downscaling of the same GCM, both covering the whole Mediterranean region (Figure 1
a) and carried out with the same model (COSMOMed) [23
]. One downscaling, at a horizontal grid resolution of about 0.44 degs (~50 Km), is named LRRCM (low-resolution regional climate model) and the other, at about 0.11 degs (~12 Km), is named HRRCM (high-resolution RCM). Both downscaling consider the RCP4.5 and RCP8.5 scenarios, that is the highest emission (RCP8.5) and the rather moderate (RCP4.5) scenarios. Since horizontal resolution is the only difference between the two regional simulations, this set of experiments represents a very interesting opportunity to assess the effective role of model grid resolution in simulating climate change over areas characterized by complex topographical features. The RCM simulations include an interactive ocean component, so that the atmospheric circulation and the Mediterranean Sea circulation are fully coupled.
The paper consists of 6 sections. Section 2
“Data and Methods” describes the models used and the simulations that have been carried out (Section 2.1
), the indices used for describing the characteristics of precipitation (Section 2.2
), areas (Figure 1
b) and boxes (Figure 1
c) within the model domain used for analyzing the results (Section 2.3
) and specifically the probability of precipitation as a function of its daily intensity (Section 2.3
). Results are split in two sections. Section 3
describes the effect of resolution on the precipitation indices: total precipitation (Section 3.1
), frequency of wet days and daily precipitation intensity (Section 3.2
), precipitation extremes and intense events (Section 3.3
). This analysis is carried out considering the spatial distribution of these indices across the Mediterranean region. The effect of resolution on the probability of precipitation (Section 4.1
) and of its intensity (Section 4.2
) is described in Section 4
, aggregating data over six boxes, selected to represent land areas with complex orography and sea surface areas. Results are discussed in Section 5
and main conclusions are shortly presented in Section 6
5. Summary and Discussion
In this paper, we analyzed the impact of climate change on intense and extreme rain events over the Mediterranean region, including the Mediterranean Sea and the surrounding areas, which has been identified as one of the main climate change hotspots [1
]. This study adopted two different horizontal grid resolutions, named HRRCM (0.11 degs) and LRRCM (0.44 degs), in dynamical downscaling of the same GCM (0.75 degs) to which the two RCMs are also compared.
Increasing resolution (HRRCM against LRRCM) increases total precipitation(TOT_PREC) above orography and along coastlines, where precipitation has maxima, while it decreases it above flat areas and sea surface (Figure 2
, top row). Increases (decreases) of total precipitation with resolution are due to the increase(decrease) of both the frequency of wet days (Wet_days) and the average daily precipitation intensity (SDII, Figure 2
, second and third row). Correspondingly, also extremes of precipitation (95th and 99th percentile, RRwn95 and RRwn99, respectively) increase above orography and along coastlines, while they decrease above flat areas and sea surface (Figure 5
) with increasing resolution. Similarly, the amount of total precipitation during extreme events increase with resolution above orography, and decreases in the narrow surrounding areas. The areas where increasing resolution decreases total precipitation, its average daily intensity and extremes are much larger than those where resolution has the opposite effect. However, over most of points the difference are not significant (Table 2
In comparison with GCM, HRRCM produces higher precipitation also over North Africa, parts of the sea surface and of flat areas (Figure 2
) so that differences are positive over most points (Table 2
). Further, the RCMs produce a larger number of wet days than GCM, over most of the model domain and larger precipitation during extreme events.
The spatial patterns of future total precipitation change are similar in the three simulations, with GCM producing slightly weaker patterns than the two RCMs. In fact (Table 4
and Table 5
), the fraction of points with non significant change is largest for the GCM. Changes are characterized as a bimodal pattern, with decrease over the central and southern areas and an increase in some northern areas.
In the future total precipitation will decrease over most the Mediterranean region (except at its northern boundary) with changes that are largest in the high emission scenario RCP8.5 and at the end of this century (2071–2100). Magnitude and spatial distribution of total precipitation change do not appreciably depend on resolution. Differences among the two scenarios are not large at mid of the century (2021–2050), but they are substantially different at the end of the century (2071–2100), when RCP8.5 produces a 40% reduction, which is approximately twice as the RCP4.5 scenario. The future decrease of total precipitation is slightly weaker in the GCM.
In the northern areas the future increase of total precipitation is associated with a decrease of wet days and an increase of average daily mean precipitation. In the southern areas the daily precipitation does not change in the future, but the number of wet days decreases. Therefore, the future increase of total precipitation in the northern areas is associated with an increase of the average intensity of precipitation. The future decrease of precipitation in the southern areas (and most of the domain) is associated with a decrease of the number of wet days. All these climate change signals weakly depend on the resolution.
The spatial patterns of future changes in the number of wet days and of average daily precipitation intensity are very similar for all simulation, being slightly larger for the GCM than for the two RCMs, suggesting that other model features are more important than the considered increase of resolution. Additionally, the bimodal pattern of strong future increase (weak future decrease) in the number of intense precipitation events and of the corresponding total amount of precipitation in the northern areas (southern) is similar in all simulations (Figure 4
, rows 4 and 5). This suggests that the future changes in the amount of precipitation during intense events is mainly associated to a change in the number of events.
Six boxes, four corresponding to land areas with complex orography (Alps, North-West Mediterranean coast, South Italy, Greece and Anatolia) and two sea areas (Central Med Sea and Levantine basin), have been considered for analyzing the effect of resolution on probability of wet days, weak, medium, strong and extreme daily precipitation intensity.
The probability of wet days (Figure 6
) does not exhibit differences statistically significant between the two RCMs (HRRCM and LRRCM), except for south Italy area, where HRRCM shows a rain probability significantly larger than LRRCM (Figure 6
a). Increasing resolution increases the probability of strong and extreme precipitation over land areas with complex orography and reduces it over sea.
When precipitation occurs, all simulations agree that the probability of weak precipitation will decrease and of medium, strong and extreme precipitation will increase (Figure 7
). Increasing resolution tends to reduce these future changes, with the exception of the Greece and Anatolia box, where HRRCM produces a significantly larger increase of strong and extreme precipitation probability. These changes are conditional to considering only wet days.
The effect of model resolution on precipitation changes over the Mediterranean Region is an important issue addressed in the current scientific literature. Our study contributes to this debate using exactly the same model at two different resolutions with a domain that covers the whole Mediterranean region. This model implementation allows us to understand the role of the model resolution, considering the whole Mediterranean region and the differences among its various parts. Specifically, the comparison among HRRCM and LRRCM offers this opportunity, because the differences between HRRCM and LRRCM depend only on resolution.
Our study confirms previous results on the effect of resolution on precipitation characteristics. Over areas with large orographic features, increasing resolution in the RCM increases total precipitation, intensity of daily precipitation, precipitation extremes, and the fraction of total precipitation during intense events. Increasing the resolution over the large flat areas and over the sea has the opposite effect. These conclusions are valid considering the comparison between the two RCMS at different resolutions and also comparing them to the GCM. In fact, high resolution increases the vertical speed long the upwind slope of mountains and flow convergence along coastlines. In several areas of the Mediterranean region, these two effects may occur simultaneously. This is compensated by a decrease of precipitation further upwind over flat areas and the sea. However, when comparing RCM to GCM, differences are caused also by different models of physics. This latter factor is, unfortunately, a source of confusion that is not possible to avoid. In fact, it is currently not possible to use the same model for global and regional simulations and to push the resolution of a global model to 0.11degs. Completing centennial simulations for multiple scenarios would require huge computational resources (in practice not available).
Climate change projections suggest significant future precipitation changes, whose magnitude increases with the emission scenario and in time. Our analysis shows that the following projected changes are robust with respect to the considered range of model resolution. In fact, they depend weakly on resolution and on the used RCM or GCM:
Over most of the Mediterranean, total precipitation will decrease in association with the decrease of the wet day frequency;
Only in some areas at the northern border of the Mediterranean, total precipitation will increase in association with the increase of intensity of daily precipitation;
The average intensity of precipitation events, the fraction of precipitation during intense events and frequency of intense events will increase over the northern Mediterranean. The same indices will decrease over sparse areas in the south Mediterranean;
During wet days, the probability of weak precipitation will decrease, whereas the probability of medium, strong and extreme precipitation will increase. In other terms, the frequency of rainy days will decrease, but they will be characterized with events more intense than in the present climate.
Our results show that the effect of increasing resolution of the RCM has a strong geographical connotation. Increasing resolution from LRRCM to HRRCM produces significant changes on:
The present and future probability of wet days in southern Italy, which is significantly higher in HRRCM than in LRRCM and will decrease significantly more in the future in the former than in the latter;
Considering only wet days, the future changes of probability is in most areas significantly smaller in HRRCM (with the exception of strong and intense precipitation in Greece and Levantine Basin where increasing resolution has the opposite effect).
Changes are larger in the GCM than in both the RCMs. This is consistent with the reduction of the effect of climate change on precipitation with increasing resolution. However, the differences between GCM and RCMs are not due only to resolution, because many of the models’ parameterizations are different. Indeed, the resolution of LRRCM and GCM are comparable and their differences are, to large extent, likely the effects of the different parameterizations.
Our results show that high resolution (11 km) simulations may not be needed if only changes of precipitation totals at the large scale are investigated. However, actual precipitation values in areas with steep orography features depend strongly on resolution. Further, high resolution is advised when computing probability of daily events, especially for strong and intense precipitations. In conclusion, this study shows that resolution is relevant, though its effects are not always significant. However, it should be noted that the resolution range explored in this study (up to 11 km) is still not sufficiently high to show strong effects of non-hydrostatic processes and local scale convection. Advantages from even higher resolutions are plausible, but their assessment is outside of the scope of this study, and is left for future investigations. Further, these results, obtained with the COSMOMed model, should be compared with studies based on other RCMs to reach robust conclusions, excluding that they are linked to this specific model.