Analysis of Energy Balance Closure at Different Temporal Scales

Measurements of the surface energy fluxes (turbulent and radiative) and other ancillary atmospheric/soil parameters made at the Columbia River Gorge (Oregon) in areas of complex terrain during the 10-month long field campaign (Wind Forecast Improvement Project 2, WFIP 2) are used to study the surface energy budget (SEB) and surface fluxes over different temporal scales. This study analyzes and discusses SEB closure based on the half-hourly, daily, monthly, and seasonal (10-month) averaged data collected over different underlying soil surfaces (dry, wet, and frozen) using the same instruments, experimental setup, location, and data processing procedure. Our direct measurements of energy balance show that the sum of the turbulent sensible and latent heat fluxes systematically underestimate the available energy at half-hourly time scales by around 20-30%. This imbalance of the surface energy budget is comparable to other terrestrial sites. However, the residual energy imbalance is significantly reduced at daily, weekly, and monthly averaging timescales, and, moreover, the SEB can be closed within reasonable limits on seasonal and sub-annual timescales (311-day averaging for the entire field campaign dataset). Increasing the averaging time to daily and longer intervals substantially reduces the ground heat flux and the storage terms, because energy locally entering the soil, air column, and vegetation in the morning is released in the afternoon and evening.The study shows that SEB closure is low for the wet soils as compared to dry soils. In the same time, the statistical dependence of the turbulent fluxes and the net radiation for freezing soil surfaces appears weak, if not non-existent, apparently due to lack of the latent heat of fusion term in the traditional SEB equation.