Influence of initial soil moisture in a Regional 1 Climate Model study over West Africa . Part 1 : 2 Impact on the climate mean

12 The impact of soil moisture initial conditions on the mean climate over West Africa was 13 examined using the latest version of the Regional Climate Model of the International Centre for 14 Theoretical Physics (RegCM4) at a horizontal resolution of 25 km × 25 km. The soil moisture 15 reanalysis of the European Centre Meteorological Weather Forecast’s reanalysis of the 20th 16 century ERA20C is used to initialize the control experiment, while its minimum and maximum 17 values over the entire domain are used to establish the initial dry and wet soil moisture 18 conditions respectively (hereafter dry and wet experiments). For the control, the wet and dry 19 experiments, an ensemble of five runs from June to September are performed. In each 20 experiment, we analyzed the two idealized simulations most sensitive to the dry and wet soil 21 moisture initial conditions. The impact of soil moisture initial conditions on precipitation in West 22 Africa is linear over the Central and West Sahel where dry (wet) experiments lead to rainfall 23 decrease (increase). The strongest precipitation increase is found over the West Sahel for wet 24 experiments with a maximum change value of approximately 40%, while the strongest 25 precipitation decrease is found for dry experiments over Central Sahel with a peak of change of 26 approximately −4%. The sensitivity of soil moisture initial condition can persist for three to four 27 months (90-120 days) depending on the region. However, the influence on precipitation is no 28 longer than one month (between 15 and 30 days). The strongest temperature decrease is located 29 over the Central and West Sahel with a maximum change of approximately −1.5 °C in wet 30 experiments, while the strongest temperature increase is found over the Guinea Coast and 31 Central Sahel for the dry experiments, with a maximum of change around 0.6°C. A significant 32

The impact of soil moisture initial conditions on the mean climate over West Africa was 13 examined using the latest version of the Regional Climate Model of the International Centre for to have a much stronger impact on daily maximum temperature variability than on daily mean 59 temperature variability, but generally has small effects on daily minimum temperature, except in 60 the eastern Tibetan Plateau. They showed that soil moisture has a prominent contribution to 61 precipitation variability in many parts of western China.  Zhang et al., 2008b). However, at local and regional scales, the land-atmosphere coupling studies 68 with AGCMs, present significant uncertainties (Xue et al. 2010 where the ability of the model to reproduce the climate mean has been validated. The 83 descriptions of the model and experimental setup used in this study are presented in Section. 2; 84 in the Section 3, the influence of wet and dry soil moisture initial conditions on the subsequent 85 climate mean is analyzed and discussed; and in Section 4 the main conclusions are presented.

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While this Part I investigates the impacts on the climate mean, the Part II of this article will be 87 focused on the influence of soil moisture initial conditions on climate extremes. Research (NCAR) Community Climate Model Version three (CCM3) (Kiehl et al., 1996). 99 Aerosols representation is from Zakey et al. (2006) and Solmon et al. (2006). The large-scale 100 precipitation scheme is from Pal et al. (2000) and the moisture scheme is the SUBgrid EXplicit 101 moisture scheme (SUBEX). The SUBEX take into account the sub-grid scale cloud variability, 102 and the accretion processes and evaporation for stable precipitation following the work of

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For the two years most sensitive to soil moisture initial conditions, the Student t-test is used to 169 compare the significance of the difference between a wet or dry sensitivity test (sample 1) and 170 the control (sample 2) in assuming that our two samples are independent and in considering that   (Fig.4a, c).  in magnitude than that of dry experiments over most studied domains (Fig. 7). For dry 258 experiments, the strongest daily precipitation response (about −4mm.day -1 ), is found over the 259 Guinea Coast in the run JJAS 2003 (Fig. 7c). While for the wet experiments, the strongest impact 260 on the daily precipitation is more than 8mm.day -1 and it is found over the West Sahel and the 261 Guinea Coast (Fig. 7b, c, respectively). It is worth to note that the impact of initial soil moisture 262 conditions on daily precipitation is much shorter than the duration of the impact on daily soil 263 moisture. The significant impact on daily precipitation is found only for wet experiments, and 264 9 did not last more than 15 days in large parts of the study domain, excepted over wetter sub-265 region of Guinea Coast where it lasts approximately one month. We noted that the precipitation 266 peaks over West Sahel and Guinea Coast (Fig. 7b and c, respectively) during August and 267 September coincide with fluctuation in the daily soil moisture impact (Fig.6b and c). This  The impacts on relative humidity and air temperature (Fig.8 and Fig.9, respectively) Fig.8a and Fig. 9a).

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For the upper troposphere, the significant impact on relative humidity and temperature is found 281 only for wet experiments, and exhibited a drying and a warming over most of studied domains 282 ( Fig.8 and Fig.9). This impact for the wet experiments was also reported by Hong and Pal

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For the dry experiments (Fig. 10a, c), we found that the moistening of the lower atmosphere 288 decreases over most of the study domain. However, the strong wind magnitude changes over the 289 Atlantic Ocean bring the moistening from the ocean to the Guinea Coast and West Sahel. This 290 can explain the precipitation increase over these sub-regions in the dry experiments. Over 291 Central Sahel, the strong decrease in precipitation seems to be associated with the decrease of 292 specific humidity which is particularly notable in the run JJAS 2003 (Fig.4a). Conversely, for the 293 wet experiments (Fig.10b, d), an increase in the moistening of the atmosphere is found mainly 294 over the Sahel band while further South, a decrease of the specific humidity is simulated over 295 Guinea Coast. The strong change in wind magnitude shifts the moistening from the North to the 296 South, leading to precipitation increase over most part of study domain (Fig.4 b and d). These   (Table 3).  Figure 13. The impact on temperature is linear 334 over the Central Sahel, Guinea Coast and the whole West African domain (Fig.13a, c and d). The to temperature increase (Fig.13, Table2). 343 We now analyze the influence of soil moisture initial conditions anomalies on land energy 344 balance, particularly on the surface fluxes sensible and latent heat. Figure 14 shows changes in respect to the control exhibits significant increase (decrease) of the sensible heat (Fig.14).  (Fig. 15). The impact in wet experiments is strong over Central and West Sahel 353 compared to the dry experiments, but not for Guinea Coast (Fig. 15, Table 2). In the dry 354 experiments, the strongest sensible heat flux increase is found over Guinea Coast, with 355 maximum change about 9.18 W.m -2 during JJAS 2004 (see Table 2). In the wet experiments, the  Table 2).   (Table 2).

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We then examined the impact on the stability of the PBL of the soil moisture initial conditions.  (Fig.18 a and c, respectively). For the wet experiments, a PBL 378 decrease is found over most of the studied domains. The PDF of PBL changes (Fig. 19) show

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The impact of the soil moisture initial conditions on the subsequent summer (JJAS) mean 393 climate over West Africa was explored using the RegCM4-CLM45. In particular, the aim of this 394 study was to investigate how soil moisture initialization at the beginning of the rainy season may   The temperature at 2m is more sensitive to the anomalies of initial soil moisture condition than

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This study is the first investigating the impact of soil moisture initial conditions in West Africa.

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However, this study is based on idealized experiments: sensitivity experiments such as "wet" and 444 "dry" ones conducted in this study were not intended to simulate real climate since such 445 extremes are very rare. Moreover, this study is very specific to RegCM4. In the future, an 446 investigation using different RCMs in a multi-model framework will contribute to better quantify