Abstract
This study investigated the feasibility of partitioning the available energy between sensible (H) and latent (LE) heat fluxes via variational assimilation of reference-level air temperature and specific humidity. For this purpose, sequences of reference-level air temperature and specific humidity were assimilated into an atmospheric boundary layer model (ABL) within a variational data assimilation (VDA) framework to estimate H and LE. The VDA approach was tested at six sites (namely, Arou, Audubon, Bondville, Brookings, Desert, and Willow Creek) with contrasting climatic and vegetative conditions. The unknowns of the VDA system were the neutral bulk heat transfer coefficient (CHN) and evaporative fraction (EF). EF estimates were found to agree well with observations in terms of magnitude and day-to-day fluctuations in wet/densely vegetated sites but degraded in dry/sparsely vegetated sites. Similarly, in wet/densely vegetated sites, the variations in the CHN estimates were found to be consistent with those of the leaf area index (LAI) while this consistency deteriorated in dry/sparely vegetated sites. The root mean square errors (RMSEs) of daily H and LE estimates at the Arou site (wet) were 25.43 (Wm−2) and 55.81 (Wm−2), which are respectively 57.6% and 45.4% smaller than those of 60.00 (Wm−2) and 102.21 (Wm−2) at the Desert site (dry). Overall, the results show that the VDA system performs well at wet/densely vegetated sites (e.g., Arou and Willow Creek), but its performance degrades at dry/slightly vegetated sites (e.g., Desert and Audubon). These outcomes show that the sequences of reference-level air temperature and specific humidity have more information on the partitioning of available energy between the sensible and latent heat fluxes in wet/densely vegetated sites than dry/slightly vegetated sites.
Highlights
The accurate estimation of sensible (H) and latent (LE) heat fluxes is of vital importance in different disciplines, such as meteorology, ecology, agronomy, and hydrology [1,2]
The main unknowns of the Tajfar et al [81] variational data assimilation (VDA) approach are the neutral bulk heat transfer coefficient (CHN) and evaporative fraction (EF)
The results indicate that the sequences of air temperature and specific humidity have a significant amount of information for partitioning the available energy between turbulent heat fluxes at sites with high Soil moisture (SM) and/or leaf area index (LAI) values, but their information content significantly reduces at sites with low SM and/or LAI
Summary
The accurate estimation of sensible (H) and latent (LE) heat fluxes is of vital importance in different disciplines, such as meteorology, ecology, agronomy, and hydrology [1,2]. Different approaches have been developed to estimate turbulent heat fluxes [12,13,14,15,16]. The first group, known as the triangle method, estimates latent heat flux by using empirical relations between land surface temperature (LST) and vegetation indices (VIs) [17,18,19,20,21,22,23,24,25,26,27]. The fourth group, the land data assimilation system (LDAS), estimates turbulent heat fluxes by the ensemble Kalman filter (EnKF) approach [39,40,41,42,43,44]
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