HR: 11:00h
AN: C12A-03    [Abstracts]
TI: Spatial and Temporal Variations of the Annual Freezing/Thawing Index in the Northern Hemisphere
AU: * Frauenfeld, O W
EM: oliverf@kryos.colorado.edu
AF: CIRES/National Snow and Ice Data Center, University of Colorado 449 UCB, Boulder, CO 80309-0449 United States
AU: Zhang, T
EM: tzhang@nsidc.org
AF: CIRES/National Snow and Ice Data Center, University of Colorado 449 UCB, Boulder, CO 80309-0449 United States
AU: McCreight, J L
EM: mccreigh@nsidc.org
AF: CIRES/National Snow and Ice Data Center, University of Colorado 449 UCB, Boulder, CO 80309-0449 United States
AB: The annual freezing/thawing index (AF/TI) can be used to predict and map permafrost distribution and active layer extent, providing important information on climate variability. Reliable long-term measures of the freezing and thawing index are thus important variables for understanding and predicting high-latitude processes. The AF/TI is defined as the cumulative number of degree-days below/above $0\deg$C for a given time period. However, long-term daily air temperature measurements are not easily available and are prone to missing values which will influence the magnitude of the AF/TI. Monthly data are more easily obtained and less influenced by missing data issues. An important question regarding the use of monthly temperatures in the computation of AF/TI is how reliable the monthly approximation is. Using stations with long-term daily temperature observations at various locations in the Northern Hemisphere (NH) mid- to high-latitudes we assess the temporal reliability of AF/TI calculations based on monthly data, relative to the conventional daily computation. Gridded NH daily 2-m temperatures from the ECMWF 40+ year reanalysis (ERA-40), interpolated onto a 25x25 km grid, we assess the spatial accuracy. For both temperature products, the daily values are averaged to monthly resolution and AF/TI is computed based on daily, and monthly data. The accuracy of the monthly approximation is then assessed using a simple measure of percentage (relative) error. Based on station data, preliminary results indicate no significant changes in accuracy over time, but a latitudinal dependence of the accuracy of the monthly-based AF/TI, {\it e.g.}, the approximation works well in higher latitudes, but is less accurate south of $60\deg$N over North America. This is verified by the reanalysis temperatures, which provide a spatially continuous assessment over the NH. Freezing index is approximated well using monthly data (errors$<$10%) between the pole and $\sim$$50\deg$N over North America, and as far south as $\sim$$30\deg$N over central Asia. South of these regions and the oceans, freezing index based on monthly data is not reliable. Thawing index is approximated well over most Northern Hemisphere land areas, but not over the high-latitude oceans. Having verified regions where AF/TI calculations work well allows for the use of globally gridded monthly temperature products extending back to the beginning of the last century, and allows for the use of long-term monthly station records.
DE: 1823 Frozen ground
DE: 1827 Glaciology (1863)
DE: 1833 Hydroclimatology
DE: 1866 Soil moisture
DE: 1620 Climate dynamics (3309)
SC: Cryosphere [C]
MN: 2004 AGU Fall Meeting