1:45 PM - 2:00 PM
[ACG46-06] Surface energy balance variation depends on surface temperature forced by absorbed radiant fluxes
Keywords:radiant flux, surface energy balance, Arctic region, Greenland
Mass loss in low-elevation Greenland coastal ice caps is significantly higher than in the Greenland Ice Sheet in the recent warming climate. It is especially important to quantitatively verify a long-term variation of climate conditions and surface mass balance near the equilibrium line. Here, we investigated the interannual variation of surface energy balance and the surface melting rate at SIGMA-B site (77.518° N, 69.062° W; 944 m a.s.l.) estimated as being located near the equilibrium line of Qaanaaq Ice Cap. This investigation aims to understand the current condition of the snow/ice surface and its accumulation/ablation process based on the ground meteorological observation data obtained since 2012 at the site.
The surface energy balance was calculated using Eq. (1), which includes the following energy components: net shortwave radiation (SWnet=(1−α)SWd), net longwave radiation (LWnet=εLWd−εσTs4), sensible heat flux (H), latent heat flux (ιE), sensible heat flux due to raindrops (QR), subsurface heat flux (QS), and energy used for heating/cooling or melting (Qnet). Radiant flux absorbed by the snow surface (Rabs) is the sum of SWnet and downward longwave radiation (εLWd) absorbed by the surface. In this study, the direction of energy transport to the snow/ice surface is defined as positive.
Qnet=(1−α)SWd+εLWd−εσTs4+H+ιE+QR+QS. (1)
The energy balance analysis showed that the largest surface melting (1083 mm w.e.) occurred in 2018/19. The mean values of Rabs and snow surface temperature (Ts) in spring (March–May) and summer (June–August) of the same year (Rabs: spring; 271.4 W m−2, summer; 349.1 W m−2, Ts: spring; −16.4°C, summer; −0.9°C) were significantly higher than in the same seasons of other years. This result indicates that heating by Rabs was dominant during this period in 2018/19, resulting in an increase in Ts. The mean Qnet for the same seasons in the same year were 3.4 W m−2 (spring) and 42.9 W m–2 (summer), respectively. The spring mean Qnet was the smallest during the observation period, while the summer mean Qnet was the largest.
Despite the predominance of snow surface heating by Rabs in 2018/19 spring, the Qnet was smaller than it in other springs. This phenomenon can be attributed to the increased heat loss from εσTs4, H, ιE, and Qs caused by the increase in Ts. On the other hand, in summer, when Ts is considered to be close to 0°C, Qnet was larger. This means that when Rabs surface heating occurs due to SWnet and εLWd that directly affect the surface energy balance (regarded as forcing terms), the variation of Qnet (increase or decrease) depends on εσTs4, H, ιE, and QS that related to Ts (regarded as passive terms). That is, in spring, when Ts is low, Rabs heating causes the Ts to increase, resulting in the larger heat loss due to the passive terms and decreasing the amount of increase in Qnet finally. While in summer, when Ts is close to 0°C , it cannot increase above 0°C even with Rabs heating, resulting in less energy loss due to the passive terms and increasing Qnet increase consequently.
The surface energy balance was calculated using Eq. (1), which includes the following energy components: net shortwave radiation (SWnet=(1−α)SWd), net longwave radiation (LWnet=εLWd−εσTs4), sensible heat flux (H), latent heat flux (ιE), sensible heat flux due to raindrops (QR), subsurface heat flux (QS), and energy used for heating/cooling or melting (Qnet). Radiant flux absorbed by the snow surface (Rabs) is the sum of SWnet and downward longwave radiation (εLWd) absorbed by the surface. In this study, the direction of energy transport to the snow/ice surface is defined as positive.
Qnet=(1−α)SWd+εLWd−εσTs4+H+ιE+QR+QS. (1)
The energy balance analysis showed that the largest surface melting (1083 mm w.e.) occurred in 2018/19. The mean values of Rabs and snow surface temperature (Ts) in spring (March–May) and summer (June–August) of the same year (Rabs: spring; 271.4 W m−2, summer; 349.1 W m−2, Ts: spring; −16.4°C, summer; −0.9°C) were significantly higher than in the same seasons of other years. This result indicates that heating by Rabs was dominant during this period in 2018/19, resulting in an increase in Ts. The mean Qnet for the same seasons in the same year were 3.4 W m−2 (spring) and 42.9 W m–2 (summer), respectively. The spring mean Qnet was the smallest during the observation period, while the summer mean Qnet was the largest.
Despite the predominance of snow surface heating by Rabs in 2018/19 spring, the Qnet was smaller than it in other springs. This phenomenon can be attributed to the increased heat loss from εσTs4, H, ιE, and Qs caused by the increase in Ts. On the other hand, in summer, when Ts is considered to be close to 0°C, Qnet was larger. This means that when Rabs surface heating occurs due to SWnet and εLWd that directly affect the surface energy balance (regarded as forcing terms), the variation of Qnet (increase or decrease) depends on εσTs4, H, ιE, and QS that related to Ts (regarded as passive terms). That is, in spring, when Ts is low, Rabs heating causes the Ts to increase, resulting in the larger heat loss due to the passive terms and decreasing the amount of increase in Qnet finally. While in summer, when Ts is close to 0°C , it cannot increase above 0°C even with Rabs heating, resulting in less energy loss due to the passive terms and increasing Qnet increase consequently.