Coherent Cloudiness Variability from the Mountains to the Sea in the Western U.S

Cloud variability serves as the most important modulator of solar insolation reaching the surface—a dominant component in snowmelt energy balance. Understanding cloud variability is consequently critical for seasonal hydrologic forecasters in the western U.S., where over two-third of the water supply is derived from warm season mountain snowmelt. At present cloud variability over the mountain zones of the western U.S. has not been thoroughly explored. An outstanding challenge to such endeavors comes from the fact that mountainous terrains pose difficulties to any necessary surface measurement efforts. Thus, we utilize 19 years (1996-2014) of GOES visible albedo product with 4-km spatial and 30-minute temporal resolutions to address two questions: 1) How does cloudiness vary from daily to seasonally over the mountains of the western U.S.? 2) Are the mountain clouds only one part of a larger-scale system, or distinct from cloud patterns upstream over the lowland and coastal areas? Two domains are considered: high elevations (>800m) in which lowland and coastal clouds are excluded, and all elevations from offshore North Pacific to the Great Basin. Our focus is on California and on the spring and early summer period when snowmelt activity is at peak. Inter-annual variation of cloudiness, represented by the coefficient of variation of cloud albedo, reveals a clear eastward migration of areas of highest scaled variability from the coastal and valley areas in winter and spring to the highland and mountain regions in summer and autumn. This pattern also occurs across shorter to longer time-scales, with coefficient of variation ranging from 30-180% on daily scale to 5-40% on seasonal scale. Considering the spatial structure of anomalous cloudiness, rotated EOF (REOF) analyses of de-seasonalized daily cloud albedo in the high elevation domain yields patterns and temporal variations that are in accord with those from the all elevation domain. The monthly standard deviations of the amplitudes of the time varying patterns represented by the leading REOF modes exhibit strong variability in winter and spring. These results indicate that, to a large degree, mountain clouds co-vary with those upstream over the coastal and valley regions although the coastal and valley clouds include low stratiform clouds.