Variability of Cloud Cover and its Relation to Springtime Snowmelt and Runoff

Much of the variability in water supply and an important part of the uncertainty in water supply forecasts are driven by the variability in solar insolation, which in turn is modulated by cloud cover. Here we investigate the space/time variability of cloud and incoming radiation and how it may affect snowmelt and streamflow. We use NASA/NOAA Geostationary Operational Environmental Satellite (GOES 9-11 and 15) albedo product (α) spanning from 1996 to 2012 during the daytime (8-16 PST) over the westernmost U.S. (25-50 °N, 113-130 °W) with 4-km spatial and 30-minute temporal resolutions. Only elevations above 800 meters are included to avoid contaminations from coastal marine and low stratus clouds. A translation of cloud albedo (α_cloud) to incoming surface radiation yields results that are well correlated with time series from surface radiometers at selected Sierra Nevada locations. Cloud albedo varies considerably from day to day, month to month, and even from year to year. To determine the most prominent spatial α_cloud patterns and their temporal variability, we conduct a Rotated Empirical Orthogonal Function/Principal Component (REOF/PC) analysis. The 5 leading REOFs account for ~66% of the total α_cloud variance. The leading REOF (~19%) covers portions of the Sierra Nevada and the Cascades and has a relatively high variance during the springtime, indicating its pertinence to snowmelt. During spring and early summer, these anomalous cloud patterns exhibit significant influence on snowmelt and runoff (R² > 30%), with anomalously high α_cloud producing lower snowmelt/runoff and vice versa. Correlations reveal a strong change in the response to incoming radiation of snowmelt and streamflow over the seasonal transition from winter-to-spring-to-summer. Dry years and wet years exhibit considerable difference in cloud amount and pattern, with lower overall springtime cloudiness in drier years, which leads to higher surface radiation available for snowmelt.

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