terns (Kerr, 2011), and this is a significant weakness given the importance of drought to agriculture, water resources, food security, and geopolitical stability (Romm, 2011). Regional precipitation is controlled by atmospheric moisture convergence associated with large-scale and mesoscale circulation, but local forcing from the surface related to orography, land-surface heterogeneity, and precipitation recycling in general alter its amount and intensity, thereby modulating its spatial and temporal characteristics.
Projections of 21st-century regional precipitation trends are of particular societal interest. Climate models consistently agree that globally averaged annual mean precipitation will increase poleward of 45° latitude, as well as over the warmest parts of the tropical oceans (IPCC, 2007c). Held and Soden (2006) gave a simple theoretical argument for this behavior as a consequence of the increased water-holding capacity of a warmer atmosphere as well as increased rates of evaporation in a warmer world. In the subtropics and in some midlatitude regions, many models project drying trends, but the location and magnitude of projected drying vary between models. Model differences in regional precipitation trends have multifaceted causes, including grid resolution but also treatments of cumulus convection, air-sea interaction, land-surface processes, upper ocean dynamics, aerosols, cloud microphysics, and the simulated global climate sensitivity.
These factors interact. As discussed above, model representations of cloud physics, convective processes, orographic and frontal forcing, and land-surface exchanges (i.e., evapotranspiration) are still limited by model resolution as well as process understanding. Because hydrologic cycle processes are inherently multiscale, increasing model resolution to more explicitly represent finer-scale processes is important. Partly because of insufficient spatial resolution, models tend to “drizzle” a lot, overestimating the number of precipitation days but underestimating high-intensity precipitation events (e.g., days with rainfall totals in excess of 10 mm) (e.g., Dai, 2006). Spatial precipitation patterns are similarly blurred in climate models because of the limited ability to resolve strong orographic and frontal gradients.
Orography is an important forcing mechanism for precipitation worldwide. There are significant challenges in predicting both cold and warm season orographic precipitation fundamentally because of the myriads of scale interactions involved. For example, mountains can modulate large-scale circulation, causing changes in local moisture convergence, but local condensation and microphysical processes also influence flow stability upstream. In summer convective regimes, orography can induce convective storms that can organize onto larger spatial scales as they are blown downwind, challenging models’ ability to simulate the multiscale precipitation patterns (Houze, 2012). Resolution of snow versus rainfall in mountain regions is also critical for water