Assessment of spring snow cover duration variability over northern Canada from satellite datasets
Introduction
Variability and change in the duration of snow cover during the spring season (or the date of snow cover disappearance) has important consequences for surface energy and water budgets over a range of scales (Serreze et al., 2002, Yang et al., 2003), as well as significant implications for cold region ecosystems (Chapin et al., 2005). A number of studies have documented recent trends toward earlier spring snowmelt across many regions of the NH (e.g. Brown, 2000, Dye, 2002) in response to enhanced spring warming that is due in part to a positive feedback from reduced snow cover (Groisman et al., 1994). The accurate monitoring of spring snow cover variability across the Canadian arctic and subarctic tundra is important because this region is expected to experience the largest cryospheric changes in response to increasing greenhouse gas forcing. In addition, snow cover variations over the high latitudes of North America (NA) contribute significantly to summer season variability in Northern Hemisphere (NH) snow cover extent (Fig. 1), which in turn, appears to play an important role in NH winter climate (Saunders et al., 2003).
The two main datasets for monitoring snow cover over this region of NA in a spatially continuous and temporally consistent manner are (1): weekly snow extent charts produced largely from optical satellite data by NOAA/NESDIS since 1966 (Robinson et al., 1993), and (2) satellite passive microwave data from the Scanning Multichannel Microwave Radiometer (SMMR; 1987–1987) and Special Sensor Microwave/Imager (SSM/I; 1987–present; Chang et al., 1990). The NOAA dataset has been utilized extensively for continental and hemispheric snow cover monitoring, and as input to climatological, hydrological, and numerical modelling studies (for example, Frei et al., 2003, Leathers and Robinson, 1993, Yang et al., 2003). While hemispheric information on snow extent and snow water equivalent (SWE) has been produced from the microwave time series (for example, Armstrong & Brodzik, 2001) most validation and application activities have been regional in nature (for example, Derksen & MacKay, 2006). Both the NOAA and microwave-derived datasets have received only limited evaluation in the Arctic and subarctic tundra regions during the dynamic and highly variable snowmelt period.
Recent studies have shown that there are several advantages of using active microwave Ku-band scatterometer data for snow cover monitoring based on the high sensitivity of these data to surface melt and the limited extent to which they are affected by vegetation cover (e.g. Nghiem and Tsai, 2001, Wang et al., 2005b). Wang et al. (2005b) were able to provide detailed maps of melt season duration over the Canadian high Arctic ice caps from QuikSCAT (QSCAT) Ku-band scatterometer data which suggests there is some potential for high resolution (∼ 5 km) mapping of snowmelt onset over high latitudes.
Wang et al. (2005a) identified several issues with snowmelt detection across the central Canadian tundra in the NOAA snow chart data record. In particular, the NOAA weekly dataset consistently overestimated snow cover extent during the spring melt period, with delays of up to 4 weeks in the identification of snowmelt when compared to snow cover extent estimates derived from passive microwave brightness temperatures and AVHRR imagery. This offset was attributed to several characteristics of the NOAA data record, including the use of a 50% snow covered area threshold to segment snow covered from snow free land, spring cloudiness, reduced availability of satellite imagery as data sources at higher latitudes, limited surface observations for validation, and differing interpretations of patchy snow cover by snow analysts. Reducing the uncertainty in high latitude snow extent and snow duration estimates during the melt period is essential because high latitude snow cover both forces (for example Gong et al., 2003a, Gong et al., 2003b, Watanabe and Nitta, 1999) and responds (for example Bamzai, 2003, Saito and Cohen, 2003) to atmospheric circulation. A number of studies have reported significant correlations between NH spring and summer snow covered area anomalies and subsequent NH winter circulation (Ellis and Hawkins, 2001, Gong et al., 2003a, Hawkins et al., 2002, Saunders et al., 2003) which appears to be related to large-scale wave activity fluxes and impacts on the polar vortex.
The main objective of this study was to carry out a detailed evaluation of snow cover datasets over northern Canada to obtain a clearer understanding of the strengths, and limitations of the various methods for investigating snow cover variability and change in a region that exerts an important influence on NH summer snow cover variability. This evaluation also included an assessment of the capability of the enhanced resolution QSCAT Ku-band scatterometer data to resolve spring snowmelt over the Canadian Arctic. Relationships between snow cover duration (SCD) and air temperature anomalies were also explored to assess and explain inter-dataset variability.
Section snippets
Study region
This paper looked at two regions: (1) the entire Canadian Arctic north of 60° N for evaluation of the ability of datasets to capture the spatial gradients in spring SCD; and (2) a sub-region (66–74 N, 80–120 W) north of the boreal tree line for evaluating interannual variability in spring SCD (the area occupied by first principal component for NH July snow cover extent from the NOAA weekly dataset shown in Fig. 1). This region is above the treeline, contains relatively large land masses
Data
This study used spring snow cover duration (SCD), defined as the number of days with snow cover on the ground from 1 April to 31 July, to investigate snow cover variability over northern Canada during the melt season. SCD is more easily defined than the end-date of continuous snow cover, avoids the problem of multiple end-dates due to late season transient snowfall events, and is strongly correlated (r > 0.8) with the end-date of continuous snow cover over the study domain. Analysis of snow cover
Intercomparison methodology
The first step of the evaluation process involved a comparison of coincident surface and satellite observations of mean spring SCD over the entire Canadian Arctic domain (60–90° N, 60–140° W) shown in Fig. 3. For the higher resolution datasets (passive microwave, IMS and QSCAT), station-satellite pairs were obtained from the nearest satellite pixel following Foster et al. (2005). The average station-grid distances are shown in Table 1 and were on the order of 10 km for the IMS and passive
Comparison of mean SCD at coincident grid points
The summary statistics for the nearest pairings of surface and satellite spring SCD are shown in Table 1. For the 2000–2004 period, the passive microwave, IMS and QSCAT all have similar r2 values but the IMS product had the highest slope (0.70) which means it was best at capturing the observed spatial gradient in SCD over the study domain. The NOAA weekly product had the lowest r2 value which is likely related to the difficulty deriving comparable observed information over the larger grid size
Conclusions and discussion
A comprehensive inter-comparison of melt season SCD datasets derived from satellite data was conducted for high latitude NA for the spring seasons of 1979–2004. Consistently gridded time series of comparable data (days between April 1 and July 31 with snow cover) were produced from the weekly NOAA snow chart record and daily IMS maps, as well as from satellite passive microwave and Ku-band scatterometer data. Careful treatment of in situ observations was required to produce an independent SCD
Acknowledgements
We are grateful to the data providers: The National Snow and Ice Data Center (EASE-Grid satellite passive microwave brightness temperatures and land/sea mask; daily IMS maps); NOAA Climate Prediction Center (daily IMS maps); Rutgers University Climate Laboratory Snow Data Resource Center (NOAA weekly snow charts); Meteorological Service of Canada (in situ snow data); The Microwave Earth Remote Sensing Laboratory, Brigham Young University (the enhanced resolution QuikSCAT data). Thanks to Bob
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Now with: Climate Processes Section, Atmospheric Science and Technology Directorate, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, Canada M3H 5T4.