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The water cycle over a given region is governed by many complex multiscale interactions and feedbacks, and their representation in climate models can vary in complexity. To understand which of the key processes require better representation, evaluation and validation of all components of the simulated water cycle are required. Adequate assessing of the simulated hydrological cycle over a given region is not trivial because observations for various water cycle components are seldom available at the regional scale. In this paper, a comprehensive validation method of the water budget components over a river basin is presented. In addition, the sensitivity of the hydrological cycle in the Canadian Regional Climate Model (CRCM) to a more realistic representation of the land surface processes, as well as radiation, cloud cover, and atmospheric boundary layer mixing is investigated. The changes to the physical parameterizations are assessed by evaluating the CRCM hydrological cycle over the Mississippi River basin. The first part of the evaluation looks at the basin annual means. The second part consists of the analysis and validation of the annual cycle of all water budget components. Finally, the third part is directed toward the spatial distribution of the annual mean precipitation, evapotranspiration, and runoff. Results indicate a strong response of the CRCM evapotranspiration and precipitation biases to the physical parameterization changes. Noticeable improvement was obtained in the simulated annual cycles of precipitation, evapotranspiration, moisture flux convergence, and terrestrial water storage tendency when more sophisticated physical parameterizations were used. Some improvements are also observed for the simulated spatial distribution of precipitation and evapotranspiration. The simulated runoff is less sensitive to changes in the CRCM physical parameterizations. [ABSTRACT FROM AUTHOR]

Thirteen regional climate model (RCM) simulations of June–July 1993 were compared with each other and observations. Water vapor conservation and precipitation characteristics in each RCM were examined for a 10° × 10° subregion of the upper Mississippi River basin, containing the region of maximum 60-day accumulated precipitation in all RCMs and station reports. All RCMs produced positive precipitation minus evapotranspiration (P - E > 0), though most RCMs produced P - E below the observed range. RCM recycling ratios were within the range estimated from observations. No evidence of common errors of E was found. In contrast, common dry bias of P was found in the simulations. Daily cycles of terms in the water vapor conservation equation were qualitatively similar in most RCMs. Nocturnal maximums of P and C (convergence) occurred in 9 of 13 RCMs, consistent with observations. Three of the four driest simulations failed to couple P and C overnight, producing afternoon maximum P. Further, dry simulations tended to produce a larger fraction of their 60-day accumulated precipitation from low 3-h totals. In station reports, accumulation from high (low) 3-h totals had a nocturnal (early morning) maximum. This time lag occurred, in part, because many mesoscale convective systems had reached peak intensity overnight and had declined in intensity by early morning. None of the RCMs contained such a time lag. It is recommended that short-period experiments be performed to examine the ability of RCMs to simulate mesoscale convective systems prior to generating long-period simulations for hydroclimatology. [ABSTRACT FROM AUTHOR]