David A. Robinson

Global Snow Cover Monitoring: An Update

Abstract Accurate monitoring of the large-scale dimensions of global snow cover is essential for understanding details of climate dynamics and climate change. Presently, such information is gathered individually from ground station networks and satellite platforms. Efforts are in progress to consolidate and analyze long-term station records from a number of countries. To gain truly global coverage, however, satellite-based monitoring techniques must be employed. A 27-year record of Northern Hemisphere continental snow cover produced by the National Oceanic and Atmospheric Administration (NOAA) is the longest such environmental record available. Records of Southern Hemisphere continental cover and snow on top of Arctic sea ice have been produced by similar means for a portion of this interval. The visible imagery charting technique used to generate these data provides information on snow extent but not on snow volume. Satellite microwave analyses over Northern Hemisphere lands show some promise in this reg...

Gridded North American monthly snow depth and snow water equivalent for GCM evaluation

Abstract Evaluation of snow cover in GCMs has been hampered by a lack of reliable gridded estimates of snow water equivalent (SWE) at continental scales. In order to address this gap, a snow depth analysis scheme developed by Brasnett (1999) and employed operationally at the Canadian Meteorological Centre (CMC), was applied to generate a 0.3° latitude/longitude grid of monthly mean snow depth and corresponding estimated water equivalent for North America to evaluate GCM snow cover simulations for the Atmospheric Model Intercomparison Project II (AMIP II) for the period 1979–96. Approximately 8000 snow depth observations per day were obtained from U.S. cooperative stations and Canadian climate stations for input to the analysis. The first‐guess field used a simple snow accumulation, aging and melt model driven by 6‐hourly values of air temperature and precipitation from the European Centre for Medium‐range Weather Forecasting (ECMWF) ERA‐15 Reanalysis with extensions from the Tropical Ocean Global Atmosphere (TOGA) operational data archive. The gridded snow depth and estimated SWE results agree well with available independent in situ and satellite data over mid‐latitudinal regions of the continent, and the snow depth climatology exhibited several improvements over Foster and Davy (1988). The monthly snow depth and estimated SWE climatologies are available for downloading from the Canadian Cryospheric Information Network (http://www.ccin.ca).

Maximum Surface Albedo of Seasonally Snow-Covered Lands in the Northern Hemisphere

Abstract Areally weighted clear sky surface albedo of snow-covered land in the middle and high latitudes of the Northern Hemisphere was measured from satellite imagery in A 1×1° latitude-longitude cells. The study area included 87% of the land polewards of 25°N, where Dickson and Posey found the probability of the seasonal occurrence of snow cover over −2.5 cm deep to be greater than zero. Albedo is 0.60 in Eurasia and 0.56 in North America, approximately 3.5 times greater than snow-free conditions. The highest average value for a 5° latitudinal zone is 0.77 at 70–75°N. The lowest is 0.43 at 60–75°N, which includes 0.36 in Eurasia and 0.58 in North America. The low albedo is due to the masking of snow covered ground by the canopy of coniferous forests. Data were obtained by image processor analyses of Defense Meteorological Satellite Program imagery. Scene brightness was converted to surface albedo by linear interpolation between bright and dark snow-covered surfaces with known albedo. The resulting chart...

Northern Hemisphere snow extent: regional variability 1972–1994

Snow cover is an important hydrologic and climatic variable due to its effects on water supplies, and on energy and mass exchanges at the surface. We investigate the kinematics and climatology of Northern Hemisphere snow extent between 1972 and 1994, and associated circulation patterns. Interannual fluctuations of North American and Eurasian snow extents are driven by both hemispheric scale signals, as well as signals from smaller ‘coherent’ regions, within which interannual fluctuations of snow extent are highly correlated. These regions cover only 2‐6% of the continental land area north of 20°N, yet during many months they explain more than 60% of the variance in continental snow extent. They are identified using Principal Components Analysis (PCA) of digitized snow extent charts obtained from the National Oceanic and Atmospheric Administration (NOAA). Significant month-to-month persistence is found over western North America and Europe during winter and spring. Geographically and seasonally dependent associations are identified between North American snow extent and atmospheric circulation patterns, surface air temperature, and snowfall. Over western North America, snow extent is associated with the longitudinal position of the North American ridge. Over eastern North America, snow extent is associated with a meridional oscillation in the 500-mb geopotential height field. These teleconnection patterns, derived using composite analyses, are associated with secondary modes of tropospheric variability during autumn and winter. During spring, snow extent becomes effectively decoupled from tropospheric dynamics. These results are useful for understanding the natural variability of the climate system, reconstructing pre-satellite era climate variability, evaluating climate models, and detecting climate change. Copyright

Seasonal Variability of Northern Hemisphere Snow Extent Using Visible Satellite Data

In this paper we use a satellite-derived data set to explore spatial and temporal variations of snow extent across Northern Hemisphere continents during the last three decades. These weekly visible-wavelength satellite maps of Northern Hemisphere snow extent produced by the National Oceanic and Atmospheric Administration constitute the longest consistently-derived satellite record of any environmental variable. We document the considerable intra-annual variability of snow extent, and show that during each month, fluctuations over relatively small areas are responsible for the majority of the year-to-year variability. Regions that cover less than 6% of Northern Hemisphere lands north of 20°N explain 62%–92% of the interannual variance across the continents. On average, snow was more extensive across both Eurasia and North America from the 1970s to middle 1980s than during the late 1980s to late 1990s. During late winter, spring and summer, snow extent has decreased since the middle 1980s, while during fall to middle winter, snow extent has remained relatively constant. Accurate information on continental snow extent is critical for weather and hydrologic forecasting; for understanding hemispheric-scale atmospheric circulation, thermal variations, and regional snow extent; and for using snow as a credible indicator of climate variability and change.

Atmospheric controls on Eurasian snow extent

Composite analyses, based on weekly snow-cover charts, temperature, sea level pressure, cyclone tracks and a rotated PCA of daily filtered 700 hPa geopotential height are used to examine relationships between the dominant modes of low-frequency atmospheric variability and mid-winter snow extent over the Eurasian continent. Two of the circulation modes examined have been identified previously and represent the North Atlantic Oscillation (NAO) and the Eurasian Type 1 (EU1) pattern. A third, termed the Siberian pattern (SIB), has not been identified previously, and describes variability in 700 hPa height over central Asia and southern Siberia. The most coherent snow-cover signals occur in the transient snow regions over Europe and south-western Asia, where variations in snow extent are largely controlled by temperature. Snow signals in east Asia are difficult to explain, but appear to be primarily determined by the availability of precipitation. For the NAO, snow-cover signals are largely restricted to central Europe. This result is initially surprising, as the NAO is associated with large temperature anomalies over a large part of the Eurasian continent. However, east of the Ural Mountains temperature anomalies in NAO extremes are confined to northern regions where mean temperatures are well below freezing, and air temperatures have little influence on snow extent. In extremes of the EU1 and SIB patterns, significant snow-cover signals are found in south-western Asia, where variability in the amplitude of the Eurasian wave train results in large differences in air temperature and cyclone activity over the transient snow regions. No coherent snow-cover signals are associated with extremes of the Siberian High. Copyright

Streamflow response to seasonal snow cover extent changes in large Siberian watersheds

[1] This study uses remotely sensed long-term (1966-1999) weekly snow cover extent data to investigate snowmelt runoff response to seasonal snow cover change in the large Siberian watersheds (Ob, Yenisei, and Lena basins). It quantified the seasonal cycles and variations of snow cover extent and river streamflow and identified a clear correspondence of river streamflow to seasonal snow cover extent change; i.e., an association of low streamflow with high snow cover extent during the cold season and an increase in discharge associated with a decrease of snow cover extent during the melt periods. This study also examined and compared the weekly mean streamflow with the weekly basin snow cover extent for the study period. The results revealed a very strong linkage between the streamflow and snow cover extent change during the spring melt season over the large Siberian watersheds and developed a statistically significant weekly runoff-snow cover relation. This relation suggests a practical procedure of using remotely sensed snow cover information for snowmelt runoff forecasting over the large northern watersheds. Analyses of extreme (high/low) streamflow cases (years) and the associated snow cover conditions indicate an association of high (low) flood peak with late (early) snowmelt in the Ob and Yenisei basins. Comparisons of snowmelt timing with peak flow show different associations between these two variables among the large Siberian rivers. These results demonstrate that the NOAA weekly snow cover extent data are useful for understanding and predicting streamflow changes in the Arctic regions. Snow cover water equivalent data/products obtained by remote sensing technology and in situ snow observations are currently being examined for what we expect will eventually improve hydrologic forecasts over the large northern watersheds.

Observed climate variability and change of relevance to the biosphere

In this paper we review the current instrumental evidence regarding climate variations and change during the 20th century emphasizing those changes that are likely to have direct interactions with the biosphere. Three basic questions are addressed: (1) Is the climate getting warmer, (2) is the hydrologic cycle changing, and (3) is the climate becoming more extreme. Based on global near-surface temperature measurements for the 20th century, it is clear that a warming of ∼0.5°C has occurred. More importantly for biospheric systems, however, are the observed asymmetric changes in daily maximum and minimum temperature, with the minimum temperatures increasing at a rate approximately twice that of the maximum temperature. Other temperature-sensitive measures, such as glacial and snow cover extent, reinforce the observed temperature trends. Examination of the hydrologic cycle indicates that changes also appear to be occurring, although less confidence can be placed on these analyses than those for temperature. Recent studies suggest that precipitation has increased in higher latitudes, particularly in the Northern Hemisphere. Increases in cloudiness, atmospheric water vapor, and changes in stream flow also suggest that changes to a more vigorous hydrologic cycle are taking place. The final question regarding climate extremes is much more difficult to assess due to a lack of high temporal resolution climate databases. Of the few studies that have been performed, however, there is evidence that precipitation extremes, particularly heavy rainfall events, are increasing in the United States and Australia, also suggesting an enhanced hydrologic cycle as the planet warms.

Changing Northern Hemisphere Snow Seasons

Abstract Spatial and temporal patterns in the onset, offset, and length of the snow season across Northern Hemisphere continents are examined for the period from 1967 to 2008. Full snow seasons (FSS) and core snow seasons (CSS) are defined based on the consistency of snow cover within a location over the course of the cold season. Climatologically, the seasonal onsets of FSS and CSS progress more rapidly across the continents than the slower spring northward offset. Average Northern Hemisphere FSS duration has decreased at a rate of 0.8 week decade−1 (5.3 days decade−1) between the winters of 1972/73 and 2007/08, while there is no significant hemispheric change in CSS duration. Changes in the FSS duration are attributed primarily to a progressively earlier offset, which has advanced poleward at a rate of 5.5 days decade−1. A major change in the trends of FSS offset and duration occurred in the late 1980s. Earlier FSS offsets, ranging from 5 to 25 days, and resultant abbreviated durations are observed in w...

Multiple Effects of Changes in Arctic Snow Cover

Snow cover plays a major role in the climate, hydrological and ecological systems of the Arctic and other regions through its influence on the surface energy balance (e.g. reflectivity), water balance (e.g. water storage and release), thermal regimes (e.g. insulation), vegetation and trace gas fluxes. Feedbacks to the climate system have global consequences. The livelihoods and well-being of Arctic residents and many services for the wider population depend on snow conditions so changes have important consequences. Already, changing snow conditions, particularly reduced summer soil moisture, winter thaw events and rain-on-snow conditions have negatively affected commercial forestry, reindeer herding, some wild animal populations and vegetation. Reductions in snow cover are also adversely impacting indigenous peoples’ access to traditional foods with negative impacts on human health and well-being. However, there are likely to be some benefits from a changing Arctic snow regime such as more even run-off from melting snow that favours hydropower operations.