How Net Radiation on Forested Snowpack Changes 2 across a Latitudinal Gradient 3

Abstract

Radiation is the major driver of snowmelt, and hence its estimation is critically important. 13 Net radiation reaching the forest floor is influenced by vegetation density. Previous studies in mid14 latitude conifer forests have confirmed that net radiation decreases and then subsequently increases 15 with increasing vegetation density, for clear sky conditions. This leads to existence of a net radiation 16 minimum at an intermediate vegetation density. With increasing cloud cover, the minimum 17 radiation shifts toward lower densities, sometimes resulting in a monotonically increasing radiation 18 with vegetation density. The net radiation trend, however, is expected to change across sites, 19 affecting the magnitude and timing of individual radiation components. This research explores the 20 variability of net radiation on snow-covered forest floor for different vegetation densities along a 21 latitudinal gradient. We especially investigate how the magnitude of minimum/maximum radiation 22 and the corresponding vegetation density change with the site geographical location. To evaluate 23 these, the net radiation is evaluated using the Forest Radiation Model at six different locations in 24 predominantly white spruce (Picea glauca) canopy cover across North America, ranging from 45 to 25 66°N latitudes. Results show that the variation of net radiation with vegetation density considerably 26 varies with latitude. In higher latitude forests, the magnitude of net radiation is generally smaller, 27 and the minimum radiation is exhibited at relatively sparser vegetation densities, under clear sky 28 conditions. For interspersed cloudy sky conditions, net radiation non-monotonically varies with 29 latitude across the sites, depending on the seasonal sky cloudiness and air temperature. Latitudinal 30 sensitivity of net radiation is lower on north-facing hillslopes than on south-facing sites. 31