WINTER RADIATION EXTINCTION AND REFLECTION IN A BOREAL PINE CANOPY - MEASUREMENTS AND MODELING

Predicting the rate of snowmelt and intercepted snow sublimation in bo real forests requires an understanding of the effects of snow-covered conifers on the exchange of radiant energy. This study examined the am ount of intercepted snow on a jack pine canopy in the boreal forest of central Saskatchewan and the shortwave and net radiation exchange wit h this canopy, to determine the effect of intercepted snow and canopy structure on shortwave radiation reflection and extinction and net rad iation attenuation in st boreal forest. The study focused on clear sky conditions, which are common during winter in the continental boreal forest. Intercepted snow was found to have no influence on the clear-s ky albedo of the canopy, the extinction of short wave radiation by the canopy or ratio of net radiation at the canopy top to that at the sur face snow cover. Because of the low albedo of the snow-covered canopy, net radiation at the canopy top remains positive and a large potentia l source of energy for sublimation. The canopy albedo declines somewha t as the extinction efficiency of the underlying canopy increases. The extinction efficiency of short wave radiation in the canopy depends o n solar angle because of the approximately horizontal orientation of p ine branches. For low solar angles above the horizon, the extinction e fficiency is quite low and short wave transmissivity through the canop y is relatively high. As the solar angle increases, extinction increas es up to angles of about 50 degrees, and then declines. Extinction of short wave radiation in the canopy strongly influences the attenuation of net radiation by the canopy. Short wave radiation that is extingui shed by branches is radiated as long wave, partly downwards to the sno w cover. The ratio of net radiation at the canopy top to that at the s now cover surface increases with the extinction of short wave radiatio n and is negative for low extinction efficiencies. For the pine canopy examined, the daily mean net radiation at the snow cover surface beca me positive when daily mean solar angles exceeded 22 degrees in late M arch. Hence, canopy structure and solar angle control the net radiatio n at the snow cover surface during clear sky conditions and will gover n the timing and rate of snowmelt. Models of intercepted snow sublimat ion and forest snowmelt could beneficially incorporate the canopy radi ation balance, which can be extrapolated to stands of various canopy d ensities, coverage and heights in a physically based manner. Such mode ls could hence avoid 'empirical' temperature index measures that canno t be extrapolated with confidence.