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Satellite analysis of interannual variability and trends in the Northern Hemisphere annual snow-free period

Dennis G. Dye
Assistant Professor, Department of Geography
Boston University
675 Commonwealth Ave.,
Boston, MA 02215
Tel: (1)-617-353-4807 Fax: (1)-617-353-8399
E-mail: [email protected]
USA

Abstract
This paper examines interannual variability and trends in the timing and duration of the annual snow-free period in northern high latitude land areas and associated changes in vegetation-absorbed photosynthetically active radiation (APAR). Three satellite-derived time-series data sets were employed in the analysis: (1) NSIDC’s Northern Hemisphere EASE-Grid Weekly Snow Cover product for years 1971 to 1994, (2) normalized difference vegetation index (NDVI) data from the Advanced Very High Resolution Radiometer (AVHRR), and (3) incident PAR data from the Nimbus-7 Total Ozone Mapping Spectrometer (TOMS). Over the 24-year study period in land areas between 41° N and 75° N: (a) snow cover disappearance in spring advanced by an average of 5.0 days per decade, (b) snow cover onset in autumn became delayed by an average of 4.5 days per decade, and (c) the duration of the snow-free period has increased by an average of 8.7 days per decade. These results represent the average of statistically significant trends observed within 1° latitudinal zones. The observed spatial and temporal changes in snow cover are associated with significant increases in total APAR for the snow-free season. The increased APAR provides a basis for a mechanistic explanation of the reported “greening” of northern high latitude land areas.

Introduction
Snow cover over continental land areas increases the surface albedo by 30-80%. The increased albedo affects the Earth’s surface energy budget and poses a feedback on climate (Cohen and Rind, 1991; Harvey, 1988). The presence or absence of snow cover is thus a critical factor in determining boundary conditions for global atmospheric circulation models (Loth et al., 1993). Snow cover has an important influence on plant distributions and ecosystem functioning through its affect on the soil and near-surface air temperature regimes (Galen and Stanton, 1995; Walker et al., 1993; Bonan, 1992; Uemura, 1989) and by limiting the photosynthetically active radiation (PAR) flux to surface vegetation.

Previous studies of snow cover dynamics at continental or global scales have focused principally on variability or trends in the spatial extent of snow cover (e.g., Robinson et al., 1995; Bamzai and Kinter, 1997). Groisman et al. (1994) analyzed time-series of satellite-derived snow cover maps for the Northern Hemisphere and found that the annual spatial extent of snow cover has declined by approximately 10% in recent decades. Less attention has been given to variability or trends in the annual timing of snow cover, particularly at continental and global scales (Foster, 1989). The timing of snow cover disappearance in spring and snow cover onset in autumn determines the duration of the annual snow-free period, an important factor controlling the duration of the active growing season. Thus, in addition to direct feedback on climate, changes in the timing and/or duration of the annual snow-free period may affect climate indirectly by curtailing or enhancing annual terrestrial primary production and associated biosphere-atmosphere carbon exchange.

Improved information about the interannual dynamics of snow cover and its affect on terrestrial primary production is important for reliable modeling of past, present and future global climate. This research was designed to contribute toward that goal by 1) examining the interannual variability in the timing and duration of snow cover in high northern latitude land areas, and 2) investigating the affect of the observed patterns on vegetation-absorbed PAR (APAR), a fundamental of determinant of net primary production (e.g., Field, 1991).

Data and Methods

Data Sources
The Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent data product was obtained from the EOS Distributed Active Archive Center (DAAC) at the National Snow and Ice Data Center (NSIDC), University of Colorado, Boulder, CO. The data set was created by NSIDC from a digital version of the Weekly Northern Hemisphere Snow Charts from NOAA-NESDIS, after corrections by D. Robinson (Rutgers University). The original analog version of the NOAA-NESDIS snow charts were created through manual interpretation of visible-band images acquired by polar-orbiting and geostationary meteorological satellites. The analog snow charts delineate snow-covered land areas in a polar stereographic map projection. NOAA-NESDIS generated the digital version by overlaying an 89 x 89 grid on the polar stereographic map; grid cells with 50% or greater snow cover were assigned a value of 1, otherwise a value of 0 was assigned. NSIDC created the EASE-Grid snow cover product by mapping the digital NOAA-NESDIS product to new grid in an azimuthal equal area projection with 25 km grid cell resolution. Snow cover data for consecutive weeks from January 1971 through December 1994 were employed in this study.

The affect of changes in snow-free period on APAR was analyzed for the month of May for two sample years, 1983 and 1993. This analysis required two additional data fields: incident PAR, and the fraction of incident PAR absorbed by vegetation (fPAR). Monthly composite NDVI data produced as part of the NOAA/NASA Pathfinder AVHRR Land program were used to estimate fPAR for May of 1983 and 1993. Data on monthly total incident PAR produced in earlier research using ultraviolet reflectivity measurements from the Total Ozone Mapping Spectrometer (TOMS) (Dye and Shibasaki, 1995). The 11-year mean (1979-1989) of the monthly total PAR for May was employed to represent incident PAR for both 1983 and 1994.

Data Processing
To facilitate zonal analysis, the NSIDC EASE-Grid product was remapped to a linear projection (Plate Carée) with a 1° grid cell resolution. The total number of 25 km input cells that corresponded to each one-degree output cell ranged between eight and twenty. An individual 1° output cell was classified as snow-covered if 30% or more of the input cells were snow-covered. A nearest-neighbor procedure was employed for the remapping. The weekly snow cover data files were each assigned a sequential week number (1-52) within the calendar year, where the week number corresponds to a fixed sequence of Julian days (1-7, 8-15, etc.).

Quantifying Snow Cover Timing and Duration
The temporal characteristics of snow cover in each annual period was quantified on a grid cell basis with respect to two key variables: the week number of the last observed snow-cover in spring (Cdis) and the week number of the first-observed snow cover in autumn (Cons ). For this purpose the spring and autumn periods were defined as weeks in the January-July and August-December periods, respectively. The annual duration of the snow free period (Cdur , units in weeks) was computed as
Cdur, n=Cons, n- Cdis, n- 1 (1)
where n is the year and 1971£n£1994.

Estimation of fPAR and APAR
The monthly composite NDVI data was corrected for calibration differences between satellite sensors following the approach of Myneni et al. (1997). The global correction was applied based on observed deviations in the average NDVI for the Sahara Desert region. In this analysis, fPAR was assumed to be equal to the NDVI. Monthly APAR (MJ m-2) for May of 1983 and 1993 was computed as
APAR = fAPAR X PAR (2)
Statistical Analysis
Descriptive statistics (mean, standard deviation) were employed to quantify the average timing of snow cover and the magnitude of interannual variability between 41° N and 75° N. Aggregations representing three spatial scales were examined: local (per pixel), continental (North America, Eurasia), and hemispheric (combined land areas except Greenland). Zonal statistics were also computed for 1° latitudinal bands. Least-squares linear regression was used to quantify any temporal trends over the 24-year study period. Trends were considered to be statistically significant when p < 5.0 (95% confidence level). Zonal averages were determined by computing the mean for each row of grid 1° grid cells. Individual 1° zonal trends were computed by linear regression of the 24-year time series of zonal average values.

Results

Zonal Trends for Snow Cover
Zonal trend analysis indicates that the duration of snow cover in northern hemisphere land areas between 45° N and 75° N increased at a zonal average rate of 8.8 (1.7) days per decade between 1971 and 1994 (Fig. 1). The increased duration is a consequence of observed advance in the timing of snow cover disappearance in spring (-6.5 [0.7] days per decade) and delay of snow cover onset in autumn (+4.5 [0.9] days per decade). These average rates of change represent the mean of individual 1° zonal trend values between 45° N and 75° N that are statistically significant.

Figure 1. Zonal average trends (1971-1994) in the timing of snow cover disappearance, snow cover onset, and the duration of the annual snow-free period. All trends shown are significant at the 95% confidence level.

Satellite Analysis of Interannual Variability and Trends in the Northern Hemisphere Annual Snow-Free Period
Affects on Zonal APAR
As the duration of the snow-free period increases, additional PAR becomes available for interception by surface vegetation. This effect is observed across all latitudes (Fig. 2). Earlier vegetation growth distributed over larger areas results in increased APAR, which supports enhanced rates of photosynthesis and carbon assimilation.

Figure 2. Zonal total APAR for May of 1983 and 1993 as determined from satellite observations of snow cover, fPAR and incident PAR.
Conclusions
The trends toward increased duration of the annual snow-free period in high northern latitudes is in general agreement with trends reported for growing season length as determined from satellite observations of vegetation greenness (Myneni et al., 1997). These trends are associated with significant increases in the amount of PAR captured by vegetation during the snow-free season. Empirical and theoretical studies have shown that primary production varies in proportion to APAR (e.g., Field, 1991). The increased APAR, together with changes in temperature and moisture regimes, thus provide the basis for a mechanistic explanation of the reported “greening” of the northern high latitude land areas (Myneni et al., 1997).

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