Understanding the forcings exerted by large dams on local climate is key to establishing if artificial reservoirs inadvertently modify precipitation patterns in impounded river basins. Using a 30 year record of reanalysis data, the spatial gradients of atmospheric variables related to precipitation formation are identified around the reservoir shoreline for 92 large dams of North America. Our study reports that large dams influence local climate most in Mediterranean, and semi‐arid climates, while for humid climates the influence is least apparent. Clear spatial gradients of convective available potential energy, specific humidity and surface evaporation are also observed around the fringes between the reservoir shoreline and farther from these dams. Because of the increasing correlation observed between CAPE and extreme precipitation percentiles, our findings point to the possibility of storm intensification in impounded basins of the Mediterranean and arid climates of the United States.

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[1] The specific question that this study pursued is ''Have large dams modified the downwind frequency of rainfall in the mesoscale during growing season?'' Rigorously quality controlled precipitation data comprising 3055 stations from the Global Historical Climatology Network (GHCN) were analyzed with 92 large dams in the U.S. Using 30 years of atmospheric reanalysis data, the wind rose diagram for each dam was derived from wind data at the 850 mb level. Around 96 (78) GHCN downwind (upwind) precipitation stations were identified that were within 100 km (mesoscale) of dams. The Mediterranean and humid subtropical climates were found to have experienced the highest and statistically significant change in trend in precipitation frequency downwind and within 100 km of dams during the growing season. The warm summer continental climatic region was found to have exhibited the next most modification. Paired analyses were performed as a function of predam and postdam and at upwind and downwind locations. For Mediterranean climates, the stations studied were found to have experienced a generally weak trend in precipitation frequency before the construction of the selected dams and a systematically more impacted trend during the postdam period. However, using precipitation observations alone, the specific role played by irrigation dams could not be distinguished from other types of dams in this study. Analysis of humidity records, however, revealed that dams can increase the moistening of the air mass by about 5%-15% (in terms of vapor pressure) as it passes downwind, while the effect can also be marginal for other dams. In summary, our study reveals that it is easier to establish a physically intuitive connection between large dams and downwind frequency of rain, but it is much more difficult to demonstrate this connection consistently for all the downwind stations in the mesoscale without the use of additional geophysical data (e.g., topography, land use, and land cover patterns) and mesoscale atmospheric modeling.Citation: Degu, A. M., and F. Hossain (2012), Investigating the mesoscale impact of artificial reservoirs on frequency of rain during growing season, Water Resour. Res., 48, W05510,

The artificially created region around the “Land between the Lakes” (LBL) in Kentucky represents unique land use and land cover (LULC) heterogeneities. Over a distance of 100 km, the LULC comprises artificially created open water bodies (i.e., two parallel large run-on-river dams separated by the LBL), mountainous terrain, forest cover, and extensive agricultural land. Such heterogeneities increase (decrease) moisture supply and sensible heat, resulting in a differential air mass boundary that helps to initiate (inhibit) convection. Hence, the LBL can potentially modify precipitation formation. Historical anecdotes reveal a tendency for storms to dissipate or reintensify near the LBL. The specific scientific question pursued in this study is therefore the following: Has the unique development of two parallel run-on-river reservoirs and the surrounding LULC heterogeneity modified storm patterns in the region? Ten storm events during the growing season were selected. Two additional events, observed by the newly established high-resolution Kentucky Mesonet network, were also considered. Radar reflectivity images were visually inspected to understand the evolution of convective cells that originated or were modified near the LBL. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) Model was used to determine near-surface trajectories that led to the selected events. The spatial synoptic classification and merged Geostationary Operational Environmental Satellite (GOES) IR images were analyzed to determine the prevailing synoptic conditions on the event dates. Six storm events showed a pattern wherein the convective cells lost strength as it passed over the LBL in a northeasterly direction. In two events, Next Generation Weather Radar (NEXRAD) reflectivity imagery revealed enhancement of convection as the storm passed over the LBL toward the Mississippi valley. Further dissection of the storm morphology suggested that the thermodynamic environment may have played an important role for the eight events where modification of precipitation near LBL has been clearly observed.

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