Ninth International Conference on Permafrost ... - IARC Research

Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tFigure 3. Mean number of days of the near-surface soil freeze in theNorthern Hemisphere.Figure 2. Climatology of monthly area extent of the near-surfacesoil freeze in the Northern Hemisphere over the period from 1988through 2006, detected from the NSIDC Frozen Soil Algorithm.Over the majority of the middle latitude regions andcertainly whole high latitude regions, the near-surface soilexperiences soil freeze-thaw cycle every year. Near-surfacesoils occasionally experience freeze in summer months overhigh elevation mountain areas (Fig. 2). The preliminaryresults indicate that the long-term average maximumarea extent of the near-surface soil freeze-thaw, includingpermafrost regions, is about 65 x 10 6 km 2 or 68% of the landmass in the Northern Hemisphere (Fig. 2). The absolutemaximum area extent can be up to 76 x 10 6 km 2 or 80% ofthe land mass in the Northern Hemisphere.The number of days of the near-surface soil freeze variesfrom a few days in the middle or lower latitude region toseveral months over high elevation mountain regions andhigh latitude regions (Fig. 3). For High Arctic regions, suchas in Siberia and northern Canada, the near-surface soilexperiences up to nine months of freeze per year. As wemove southwards, the number of days of soil freeze per yeardecreases gradually with clear zonal characteristics (Fig. 3).Another feature is that mean length of a freeze-thaw cyclevaries from a few days in middle and low latitude regions,to several months in high elevation mountain and latituderegions. Autumn and spring seasons at high latitude regionsare very short, fluctuations of air temperature around 0°Care not as frequent as in the middle latitude regions. Whenthe near-surface soil freezes in autumn at high latitudes, it isexpected to be still frozen until next spring thaw.Meanwhile, frequency of the soil freeze-thaw cycle onaverage varies from more than 20 times in middle and lowlatitudes to less than 10 times in high mountain and elevationregions.Based on results from passive microwave satellite remotesensing data, we have not detected any significant trendsof changes in timing, duration, and frequency of the nearsurfacesoil freeze-thaw cycle in the Northern Hemispherefrom 1988–2006. However, further work is still neededto better validate the NSIDC Frozen Soil Algorithm. Thecurrent algorithm is validated using data from the contiguousUnited States. Data from other parts of the world are neededto further validate the algorithm.AcknowledgmentsWe thank Jeff Smith at the National Snow and Ice DataCenter (NSIDC), University of Colorado at Boulder, forpreparing datasets and graphs. This study was supportedby the U.S. National Aeronautics and Space Administration(NASA) grants 13721 and NNX06AE65G.ReferencesZhang, T. & Armstrong, R. L. 2001. Soil freeze/thaw cyclesover snow-free land detected by passive microwaveremote sensing. Geophysical Research Letters 28(5):763-766.Zhang, T., Armstrong, R.L. & Smith, J. 2003. Investigationof the near-surface soil freeze/thaw cycle in thecontiguous United States: Algorithm developmentand validation. J. Geophys. Res. 108(D22): 8860,doi:10.1029/2003JD003530.362