Abstract
Abstract. Over glaciers in the outer tropics, during the dry winter season, turbulent fluxes are an important sink of melt energy due to high sublimation rates, but measurements in stable surface layers in remote and complex terrains remain challenging. Eddy-covariance (EC) and bulk-aerodynamic (BA) methods were used to estimate surface turbulent heat fluxes of sensible (H) and latent heat (LE) in the ablation zone of the tropical Zongo Glacier, Bolivia (16° S, 5080 m a.s.l.), from 22 July to 1 September 2007. We studied the turbulent fluxes and their associated random and systematic measurement errors under the three most frequent wind regimes. For nightly, density-driven katabatic flows, and for strong downslope flows related to large-scale forcing, H generally heats the surface (i.e. is positive), while LE cools it down (i.e. is negative). On average, both fluxes exhibit similar magnitudes and cancel each other out. Most energy losses through turbulence occur for daytime upslope flows, when H is weak due to small temperature gradients and LE is strongly negative due to very dry air. Mean random errors of the BA method (6 % on net H + LE fluxes) originated mainly from large uncertainties in roughness lengths. For EC fluxes, mean random errors were due mainly to poor statistical sampling of large-scale outer-layer eddies (12 %). The BA method is highly sensitive to the method used to derive surface temperature from longwave radiation measurements and underestimates fluxes due to vertical flux divergence at low heights and nonstationarity of turbulent flow. The EC method also probably underestimates the fluxes, albeit to a lesser extent, due to underestimation of vertical wind speed and to vertical flux divergence. For both methods, when H and LE compensate each other in downslope fluxes, biases tend to cancel each other out or remain small. When the net turbulent fluxes (H + LE) are the largest in upslope flows, nonstationarity effects and underestimations of the vertical wind speed do not compensate, and surface temperature errors are important, so that large biases on H + LE are expected when using both the EC and the BA method.
Highlights
Surface turbulent heat fluxes play a significant role in the energy balance of mountain glaciers (Sicart et al, 2008; Gillett and Cullen, 2011), a crucial calculation for assessing melt and mass balance using meteorological variables
The random error in Hec and LEec increased with wind speed for the Mann and Lenschow (ML) method and did not change significantly with wind speed for the Hollinger and Richardson (HR) method (Fig. 3a and b)
The random error predicted with the ML method increased and was larger than that of the HR method, which remained constant
Summary
Surface turbulent heat fluxes play a significant role in the energy balance of mountain glaciers (Sicart et al, 2008; Gillett and Cullen, 2011), a crucial calculation for assessing melt and mass balance using meteorological variables. Several characteristics of turbulent flows challenge the measurement of turbulent fluxes, such as flux divergence between the surface and the instruments under frequent katabatic winds (Grisogono and Oerlemans, 2002), non-equilibrium surface layers related to largescale orographic disturbances (Smeets et al, 1999), temperature overestimates due to strong shortwave radiation and large surface albedo (Huwald et al, 2009), or intermittency in weak wind and strong stable stratification (Mahrt, 2007) These characteristics can cause significant errors in estimates of turbulent fluxes and in the energy balance
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