Horizontal sensible heat advection increases snow melt rates: Beyond point measurements

Abstract. In the atmospheric surface layer, it is generally assumed that vertical fluxes dominate near-surface heat transport and heat exchange between the atmosphere and the earth's surface. However, when the earth's surface is covered in patchy snow, near-surface horizontal advection of heat may be significant, and it's impact on snow melt rates is disputed. We estimated the contribution of horizontal sensible heat advection (Q_H) to snow melt using 8 days of measurements collected in May 2023 in the alpine East River basin in Colorado, Rocky Mountains, USA. We used an infrared video camera and polyester sheet to estimate Q_H, 3-meter resolution satellite imagery to track fractional snow covered area (fSCA), a scanning lidar to estimate snow melt rates, and micrometeorological and eddy covariance measurements to characterize the surface energy balance. Infrared camera measurements of Q_H over a single snow patch show that Q_H contributes to snow melt and is largest at the patch's upwind edge. Lidar-based snow melt measurements show that melt rates are highest at the upwind edge, further suggesting that Q_H causes snow melt. Over four days when fSCA was less than 62%, Q_H estimates ranged from  234–584 W m^-2 and mid-day net radiation ranged from 331–508 W m^-2. When we considered only vertical heat fluxes, radiative fluxes, and snow melt in a snow patch surface energy balance equation, we found a mid-day residual between 267–668  W m^-2, and the residual increased as fSCA decreased. The approximate match between the energy balance residual and estimated Q_H suggests that advection is an important factor to consider when predicting snow melt. On days with low fSCA (< 62%), horizontal advection may contribute as much to snow melt as net radiation.