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ARS Home » Pacific West Area » Boise, Idaho » Northwest Watershed Research Center » Research » Publications at this Location » Publication #233029
Title: The impact of coniferous forest temperature on incoming longwave radiation to melting snow

Author
item POMEROY, JOHN - U. SASK
item Marks, Daniel
item ELLIS, C - U SASK
item LINK, TIM - U OF I
item HARDY, J - CRREL
item ROWLANDS, A - U WALES

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/9/2009
Publication Date: 5/22/2009
Citation: Pomeroy, J., D. Marks, T. Link, C. Ellis, J. Hardy, A. Rowlands and R. Granger, 2009, The Impact of Coniferous Forest Temperature on Incoming Longwave Radiation to Melting Snow, Hydrological Processes, 23(1-2):2513-2525, doi: 10.1002/hyp.7325.

Interpretive Summary: Thermal longwave radiation below a forest canopy is not the same as it is in the open. In order to effectively model the snowcover in mountain regions, we need to be able to account for this. Experiments showed that simple assumptions about the temperature of the forest being the same as the air were incorrect because the lower portions of the forest tended to be much warmer during the day and cooler at night. A more effective model was developed based on a multiple temperature component approach – one for the atmosphere and one for the canopy structure. The experiment showed that such a correction not necessary during cloudy or low sun conditions, but is important during clear conditions at high sun angles.

Technical Abstract: Experiments were conducted in Rocky Mountain evergreen forests of differing density, insolation and latitude to test whether air temperatures are suitable surrogates for canopy temperature in estimating sub-canopy longwave irradiance to snow. Under conditions of low to no insolation then air temperature generally was a good representation of canopy radiative temperature. However during high insolation, needle temperatures were well estimated by air temperature only in relatively dense canopies and exceeded air temperatures in discontinuous canopies. Tree trunks exceeded air temperatures in all canopies during high insolation, with the relatively hottest trunks associated with direct interception of sunlight, sparse canopy cover and dormant or dead trees. The exitance of longwave radiation from these relatively warm canopies exceeded that calculated assuming canopy temperature was equal to air temperature and the exceedance was strongly related to the extinction of shortwave radiation by the canopy. Estimates of sub-canopy longwave irradiance using two-component approaches to evaluate the contribution of canopy longwave exitance performed better than did estimates that used only air temperature and sky view. The two-energy component approach used a combination of air temperature and shortwave extinction by canopy to estimate the canopy longwave exitance. The two-thermal regime approach partitioned the canopy between needle exitance and trunk exitance calculated from surface temperature observations. There was little evidence that such corrections are necessary under cloudy or low solar insolation conditions, however the effect was important to sub-canopy longwave irradiance to snow during clear, sunlit conditions.