Sensitivity of a regional climate model on the simulation of high intensity rainfall events over the Arabian Peninsula and around Jeddah (Saudi Arabia)
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As a step towards the adoption and use of the regional climate model (RegCM3) for the simulation of intense rainfall events over the Arabian Peninsula, this study examines its sensitivity to domain size, boundary location, forcing fields, and resolution. In the climatological results, RegCM3 performs well in reproducing the annual and the seasonal mean precipitation as well as the contrast between wet and dry years in terms of the amounts and locations of the rainbands. In addition, simulations are performed for two cases of intense rainfall events in the Jeddah area and surroundings using a combination of three domains and two boundary forcings at 50 km. The results show that different combinations of these parameters provide different skills for the regional model. However, RegCM3 performs relatively better when ERA40 (NNRP2) is used at the boundaries in the smaller domain (larger domain), indicating the importance of the stronger (relatively weaker) influence of boundary forcing needed to capture these intense rainfall events around Jeddah. This may be explained by the fact that around that region, RegCM3 produces, in the smaller domain, higher relative humidity and stronger wind vectors closer to the reanalyses when nested within the ERA40, while it shows its best performance with the larger domain when driven by NNRP2. It is also shown that the use of high resolution does not systematically improve the simulation of such events, although some encouraging results were produced.
KeywordsRainfall Event Regional Climate Model Rain Gauge Wind Vector Arabian Peninsula
The rainfall in most parts of Saudi Arabia is scant, irregular, and unreliable, falling generally from October through April (Atlas 1984). During all other months, there is almost no rainfall except in the south-western areas of the country (Almazroui 2010). Due to mountainous areas, the spatial variation of the rainfall in the south-western region is high. This area is climatologically different from the rest of the country. It is characterized by precipitation events throughout the whole year because of the topographically driven convective rain (Al-Mazroui 1998; Abdullah and Al-Mazroui 1998). The annual total rainfall in the north of this area, particularly in Jeddah (21.71° N, 39.18° E), ranges from 50 to 100 mm, occurring mainly during the winter season. In general, only a few rainfall events are sufficient to produce this amount in this semi-arid region (Köppen 1936).
The area’s flash flooding is mainly due to convective storm-related events, which can cause a great deal of damage in arid and semi-arid regions such as Saudi Arabia. Several devastating flash floods have recently occurred (e.g., on 25 November 2009 at Jeddah and on 5 May 2010 at Riyadh, among many others), suggesting a strong need to emphasize research aimed at improving extreme rainfall prediction. Reliable predictions of extreme rainfall events remain a difficult challenge for numerical modelers. The tools for an accurate prediction of high-impact weather, such as severe thunderstorms and heavy rainfall events (Litta et al. 2007) that may cause flash flooding, include regional climate models.
Recently, regional climate models (RCMs) have become a useful tool for downscaling global climate simulations and/or reanalysis providing regional climate details when nested within the boundaries of such global datasets for regional climate studies (Giorgi et al. 1994; Leung et al. 1999; Sylla et al. 2009). In this approach, domain size and the location of lateral boundaries, resolution, and the quality of the forcing fields have a strong impact in the simulations produced by the regional model. In fact, earlier studies have shown the importance of these parameters in such simulations. Jones et al. (1995) argued that the regional domain must be large enough to allow the full development of fine-scale features over the region. In the same vein, Seth and Giorgi (1998) showed that the lateral boundaries must be placed well outside the region of interest to avoid an unrealistic response to internal forcings, while a smaller domain confines the interior solution more toward the coarse driving fields, which may also yield an unrealistic response to internal higher resolution forcing (Giorgi and Mearns 1999). In addition, Nobre et al. (2001) noted that their simulations were sensitive to both the model resolution and the location of the lateral boundaries, while Shiao and Juang (2006) demonstrated that the simulated large-scale circulation and rainfall distribution are more sensitive to the domain size than to the horizontal resolution. Recently, Sylla et al. (2010) argued that the use of better reanalyses at the boundaries in their relatively large domain might have contributed to improving their simulations. From this discussion, it is clearly apparent that domain size, the location of lateral boundaries, the resolution considered, and also the quality of the forcing fields are of critical importance for the performance of the regional climate model.
Therefore, to adopt the RegCM3 for the simulation of rainfall events in and around Jeddah, its sensitivity to all those parameters has to be addressed. Towards this goal, the objective of this study is to assess the capability of RegCM3 to reproduce specific cases of intense rainfall events in and around Jeddah (Saudi Arabia) by examining its sensitivity to the domain size, the location of lateral boundaries, the resolution, and also the quality of the forcing fields. The next section begins with the model description and simulations.
2 Data and methodology
2.1 Model description
The International Centre for Theoretical Physics (ICTP) Regional Climate Model, RegCM3 (Giorgi et al. 1993a, b; Pal et al. 2007), is used in this study. RegCM3 is a primitive equation, sigma vertical coordinate model based on the hydrostatic dynamical core of the NCAR/PSU’s mesoscale meteorological model MM5 (Grell et al. 1994). Radiation is represented by the CCM3 parameterization of Kiehl et al. (1996) and the planetary boundary layer scheme is by Holtslag et al. (1990). Interactions between the land surface and the atmosphere are described using the Biosphere Atmosphere Transfer Scheme (BATS1E; Dickinson et al. 1993). The scheme of Zeng et al. (1998) is used to represent fluxes from water surfaces. Convective precipitation is calculated with the scheme of Grell (1993) applying the Fritsch and Chappell (1980) closure assumption. Resolvable precipitation processes are treated with the subgrid explicit moisture scheme (SUBEX) of Pal et al. (2000), which is a physically based parameterization including subgrid scale cloud fraction, cloud water accretion, and evaporation of falling raindrops.
RegCM3 has been used for a wide range of applications (e.g., Pal et al. 2007), including regional climate change (e.g., Giorgi et al. 2004; Diffenbaugh et al. 2005), water resources (e.g., Pal et al. 2000), and seasonal prediction (e.g., Rauscher et al. 2006; Seth et al. 2007), but it has also been used to study specific cases of meteorological events (e.g., Zakey et al. 2006; Yuan et al. 2008; Santese et al. 2010; Camara et al. 2010; Tchotchou and Kamga 2010). In that same vein, RegCM3 is used for the simulation of some case studies of high intensity rainfall events occurring over the Arabian Peninsula and more specifically over Jeddah and its surroundings. In particular, we have studied the effect of domain size and the (strong and weak) influence of boundary forcings on the performance of such a model to capture those events.
2.2 Experimental design and data
Model initial and lateral boundary conditions were created using two reanalyses, which are NNRP2 (Kalnay et al. 1996; Kistler et al. 2001) and ERA40 (Uppala et al. 2005) at the 2.5° × 2.5° gridded and 6-hourly temporal resolution. These data, which are derived from various observational sources including rawinsondes, surface marine data, aircraft data, surface land synoptic data, satellite sounder data, special sensing microwave imager, and satellite cloud drift winds, are interpolated onto the model grid and used in the corresponding simulations.
Sea surface temperature (SST), used to force RegCM3 (in all cases), is obtained from the National Oceanic and Atmospheric Administration Optimum Interpolation (OI) SST. This OISST analysis is produced weekly on a 1° grid. These analyses were based on ship and buoy SST data supplemented with satellite SST retrievals. OISST is the most updated observed SST field and it resolves equatorial upwelling and fronts (Reynolds and Smith 1994; Reynolds et al. 2002, 2007).
The regional climate model performance was first evaluated from the climatological point of view (1998–2009), at the annual and seasonal timescales as well as for wet and dry years, using the NNRP2 reanalysis over the large Arabian Peninsula domain before focusing on some targeted rainfall events for the sensitivity study. To reinforce the results, two events have been considered for this sensitivity study. They occurred on 8 January 1999 and 31 December 2001. The simulations for the former and latter cases began on 1 October 1998 and 1 September 2001, respectively, and were completed for 5 months in order to place the results in a broader perspective. For more convenience, the summary of all simulations performed and their names are shown in Table 1.
The summary of all simulations performed and their names
RegCM3 driven by ERA40 in domain A at 50 km
RegCM3 driven by ERA40 in domain A at 25 km
RegCM driven by ERA40 in domain B at 50 km
RegCM driven by ERA40 in domain C at 40 km
RegCM3 driven by NNRP2 in domain A at 50 km
RegCM3 driven by NNRP2 in domain A at 25 km
RegCM3 driven by NNRP2 in domain B at 50 km
RegCM3 driven by NNRP2 in domain C at 50 km
The degree to which the regional climate model simulates mean rainfall climatology over the Arabian Peninsula has to be examined before assessing its performance in order to capture the specific cases of intense rainfall events occurring in the region under different configurations.
3.1 Mean rainfall climatology
In general, RegCM3 shows a fairly good performance in representing the rainfall climatology over the Arabian Peninsula; however, it has a tendency to exhibit consistently larger amounts, compared to TRMM, in regions experiencing wetter conditions. It is thus worth investigating how well it performs during wet and dry years.
Overall, RegCM3 is capable of reproducing the annual and the seasonal mean climatology of precipitation over the Arabian Peninsula as well as the contrast between wet and dry years in terms of the amounts and locations of the rainbands. From this analysis, it is evident that over the Arabian Peninsula, observed rainfall amounts are very low, mostly less than 200 mm/year (0.55 mm/day) for the annual mean climatology and less than 250 mm/year (0.68 mm/day) for the wet year. Only some small areas located at the interface between the Red Sea and Indian Ocean experience the larger rainfall amounts, exceeding sometimes 400 mm/year (1.09 mm/day). Due to these very low amounts over most of these regions, this study relating to precipitation climatology over the Arabian Peninsula may provide only a limited understanding of the phenomena. Therefore, a more fruitful investigation should examine targeted specific cases of the high intensity rainfall events that occur in the regions once in a while and that cause a great deal of damage. In addition, it is clear that the regional model has a tendency to overestimate the maxima and to extend areas of dry conditions, hence confining the higher amounts to some localized regions. It is thus a good tool for studying those specific cases of high intensity rainfall events striking the region.
3.2 Sensitivity to boundary location and forcing
In general, for the first cases, ERA40-RegCM_A50 and NNRP2-RegCM_B50 display a better spatial distribution, while ERA40-RegCM_A50, ERA40-RegCM_B50, NNRP2-RegCM_A50, and NNRP2-RegCM_B50 show a closer amount of rainfall when compared to the TRMM and the Jeddah rain gauge datasets. For the second case studied, EAR40-RegCM_B50, NNRP2-RegCM_A50, NNRP2-RegCM_B50, and NNRP2-RegCM_C50 provided encouraging results in reproducing better spatial distributions of rainfall when compared to the TRMM data. Note that switching B to C can cause differences in the simulations, but these can be ascribed to model internal variability (Giorgi and Bi 2000), and in this regard, analysis in the next section will be focused mainly on inter-comparison simulations performed over domains A and B (with the better performing reanalysis forcings).
When the rainfall is extracted near the Jeddah station, only ERA40-RegCM_A50, ERA40-RegCM_B50, and NNRP2-RegCM_B50 capture reasonable rainfall peaks close to both TRMM and the Jeddah rain gauge amounts. This indicates that different combinations of domain sizes and boundary forcings provide different performances for the regional climate model RegCM3 in simulating these rainfall events. In addition, they suggest that the simulation of high intensity rainfall events over the studied region is more sensitive to domain size than boundary forcing. Furthermore, ERA40-RegCM_A50 and NNRP2-RegCM_B50 provide the best performances (although NNRP2-RegCM_A50 is by no means poor) in indicating the role of the center of the domain. The better representation of these events using a smaller (relatively bigger) domain in the case of ERA40 (NNRP2) indicates that the stronger (weaker compared to ERA40) influence and constraint of lateral boundary forcings are critical in capturing these features.
3.3 Synoptic features during the cases
In general, when RegCM3 is driven by ERA40, simulation provides lower values for relative humidity for the larger domain (domain B) than when the smaller domain is used. In the case of NNRP2 boundary forcing, relative humidity is overestimated; it is higher than the values provided by RegCM3 driven by ERA40 most of the time. In combination with the results found above, it is revealed that RegCM3 driven by ERA40 is relatively better in simulating wind vectors and relative humidity in and around Jeddah when the smaller domain is used. In contrast, using NNRP2 as boundary forcing, RegCM3 performs better in the larger domain (domain B). This may explain why the stronger influence of large-scale features in ERA40 and their relatively weaker influence in NNRP2 (than in the case of ERA40) lead to a better simulation of high intensity rainfall events in and around Jeddah.
3.4 Sensitivity to resolution
In general, using high resolution does not guarantee better simulation of high rainfall intensity events in Jeddah. However, the fine-scale features (small maxima of rainfall) could help to set up more accurate amounts of rainfall depending on the proximity of the grid points to the region of interest.
4 Discussion and conclusion
In this paper, the sensitivity of the regional climate model (RegCM3) in simulating heavy rainfall events over the Jeddah region of Saudi Arabia using different domain sizes and lateral boundary forcings is reported. Domain choice, and therefore the location of the lateral boundaries, is of critical importance in the issue of regional climate modeling over the Arabian Peninsula, where data are sparse and large-scale synoptic features have a strong influence on the regional climate. In addition, as simulated precipitation results from a sum of physical processes, involving water vapor advection from lateral boundaries and moisture sources from the surface, the driving large-scale fields may play an important role.
Before conducting the sensitivity study for the model to simulate extreme rainfall events, the regional model’s performance was assessed for the 12-year climatology. RegCM3 performed well in simulating the annual and the seasonal mean climatology of precipitation over the Arabian Peninsula as well as the contrast between the wet and dry years (in terms of the amount and locations of the rainbands). It has also been shown that the observed rainfall amounts are very low and that the sensitivity study vis-à-vis simulated precipitation may be better achieved using targeted specific cases of intense rainfall events. Notwithstanding this proviso, the results indicate that different combinations of domain sizes and boundary forcings provide different skills for RegCM3 in simulating heavy rainfall events around Jeddah and that ERA40-RegCM_A50 and NNRP2-RegCM_B50 show the best performances although NNPRP2-RegCM_A50 is quite encouraging. These indicate that RegCM3 captures adequately the simulation of intense rainfall events better both in the small domain when ERA40 is used at the boundaries and in the larger domain when NNRP2 is forcing the regional model. It is noted that these domains have the same center. The performance of RegCM3 is poor if the larger domain is used, with the center shifted towards the east in both forcings and case studies. This attests to the important role of the domain center and therefore the locations of lateral boundaries in capturing the heavy rainfall events in and around Jeddah, Saudi Arabia.
To address the issue of the role of large-scale fields in the performance of RegCM3 in simulating these events using ERA40 in the small domain and NNRP2 in the larger domain, 925 hPa relative humidity and superimposed wind vectors for each event were considered. The results reveal that RegCM3 driven by ERA40 simulates higher values of relative humidity and stronger wind vectors around Jeddah within the smaller domain (rather than the larger domain), thus providing its best performance. However, when NNRP2 reanalyses are placed at the boundaries, RegCM3 reproduces those large-scale features in the larger domain better and especially in Jeddah and the surrounding regions. This corroborates the stronger (relatively weaker) influence of lateral boundary forcings on the regional climate model needed to capture such events when ERA40 (NNRP2) is used.
Finally, to highlight the relative role of resolution in the simulations of these high intensity rainfall events around Jeddah, high-resolution simulations were performed over domain A with RegCM3 forced by ERA40 and NNRP2, respectively. Note that the regional climate model had better performance in domain A in the case of ERA40 than NNRP2. It was shown that the high resolution remarkably improves the simulation of the amount of rainfall around Jeddah and the surroundings by splitting the maxima around the Red Sea coastlines and extending them further south. Although other cases show differences in the amounts of rainfall simulated in Jeddah, they do not show any consistent improvements. Therefore, high resolution does not always guarantee increased performance for the regional climate model in simulating high intensity rainfall events, except if the fine-scale features that showed split rainfall maxima, are shifted towards the region of interest.
The author would like to acknowledge the Presidency of Meteorology and Environment (PME) of Saudi Arabia for providing the rain gauge data. The RegCM3 group of ICTP, Trieste, Italy is acknowledged for providing the model with LBCs. TRMM data were acquired from their website at http://trmm.gsfc.nasa.gov.
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