Combining the bulk transfer formulation and surface renewal analysis for estimating the sensible heat flux without involving the parameter KB-1

The single‐source bulk transfer formulation (based on the Monin‐Obukhov Similarity Theory, MOST) has been used to estimate the sensible heat flux, H, in the framework of remote sensing over homogeneous surfaces (HMOST). The latter involves the canopy parameter, , which is difficult to parameterize. Over short and dense grass at a site influenced by regional advection of sensible heat flux, HMOST with  = 2 (i.e., the value recommended) correlated strongly with the H measured using the Eddy Covariance, EC, method, HEC. However, it overestimated HEC by 50% under stable conditions for samples showing a local air temperature gradient larger than the measurement error, 0.4 km−1. Combining MOST and Surface Renewal analysis, three methods of estimating H that avoid dependency have been derived. These new expressions explain the variability of H versus , where is the friction velocity, is the radiometric surface temperature, and is the air temperature at height, z. At two measurement heights, the three methods performed excellently. One of the methods developed required the same readily/commonly available inputs as HMOST due to the fact that the ratio between and the ramp amplitude was found fairly constant under stable and unstable cases. Over homogeneous canopies, at a site influenced by regional advection of sensible heat flux, the methods proposed are an alternative to the traditional bulk transfer method because they are reliable, exempt of calibration against the EC method, and are comparable or identical in cost of application. It is suggested that the methodology may be useful over bare soil and sparse vegetation.This research was funded by CERESS project AGL2011–30498 (Ministerio de Economía y Competitividad of Spain, cofunded FEDER), CGL2012–37416‐C04‐01 (Ministerio de Ciencia y Innovación of Spain), and CEI Iberus, 2014 (Proyecto financiado por el Ministerio de Educación en el marco del Programa Campus de Excelencia Internacional of Spain)

Reintroducing radiometric surface temperature into the Penman-Monteith formulation

Here we demonstrate a novel method to physically integrate radiometric surface temperature (TR) into the Penman-Monteith (PM) formulation for estimating the terrestrial sensible and latent heat fluxes (H and λE) in the framework of a modified Surface Temperature Initiated Closure (STIC). It combines TR data with standard energy balance closure models for deriving a hybrid scheme that does not require parameterization of the surface (or stomatal) and aerodynamic conductances (gS and gB). STIC is formed by the simultaneous solution of four state equations and it uses TR as an additional data source for retrieving the “near surface” moisture availability (M) and the Priestley-Taylor coefficient (α). The performance of STIC is tested using high-temporal resolution TR observations collected from different international surface energy flux experiments in conjunction with corresponding net radiation (RN), ground heat flux (G), air temperature (TA), and relative humidity (RH) measurements. A comparison of the STIC outputs with the eddy covariance measurements of λE and H revealed RMSDs of 7–16% and 40–74% in half-hourly λE and H estimates. These statistics were 5–13% and 10–44% in daily λE and H. The errors and uncertainties in both surface fluxes are comparable to the models that typically use land surface parameterizations for determining the unobserved components (gS and gB) of the surface energy balance models. However, the scheme is simpler, has the capabilities for generating spatially explicit surface energy fluxes and independent of submodels for boundary layer developments

Interactions surface continentale/atmosphère : l'expérience HAPEX-Sahel

L'un des objectifs d'Hapex-Sahel est l'étude des interactions entre la surface continentale et l'atmosphère pour caractériser la climatologie à la méso-échelle du milieu sahélien. Afin de mieux paramétriser ces interactions à cette échelle, il est nécessaire d'appréhender : l'effet de la biosphère sur les caractéristiques climatiques ; l'impact des conditions climatiques sur le fonctionnement de la biosphère. Les résultats stationnels des mesures de flux de vapeur d'eau, de chaleur sensible et de CO2 montrent qu'en fonction de la distribution des précipitations donc des stocks d'eau disponible, la végétation se développe et accroît ses surfaces d'échange. La quantité de matière organique produite provient d'un prélèvement du CO2 atmosphérique, parallèlement à un transfert de vapeur d'eau de l'écosystème vers l'atmosphère. Ces échanges conduisent à une augmentation de la quantité de vapeur atmosphérique et à une réduction de la concentration de CO2 dans l'atmosphère. Avec l'épuisement du stock hydrique du sol, l'activité physiologique du système végétal se réduit, se traduisant par une réduction des échanges de vapeur d'eau et de CO2 et une augmentation des transferts de chaleur sensible vers l'atmosphère. Les variations spatiales pour les divers échanges des systèmes biologiques composant la biosphère sahélienne sont intégrées dans les paramètres physiques de la couche limite de surface régionale (C.L.S.). En effet, les résultats montrent que la masse d'air située au-dessus des surfaces d'échange somme, dans ses caractéristiques physiques, l'apport des différents flux et donne une valeur surfacique pondérée, définissant le climat à la mésoéchelle. Le suivi des paramètres physiques de cette couche limite de surface régionale permet d'évaluer l'importance des échanges entre la biosphère sahélienne et l'atmosphère par intégration spatio-temporelle des flux. (Résumé d'auteur

Revisiting the paper “Using radiometric surface temperature for surface energy flux estimation in Mediterranean drylands from a two-source perspective”

The recent paper by Morillas et al. [Morillas, L. et al. Using radiometric surface temperature for surface energy flux estimation in Mediterranean drylands from a two-source perspective, Remote Sens. Environ. 136, 234–246, 2013] evaluates the two-source model (TSM) of Norman et al. (1995) with revisions by Kustas and Norman (1999) over a semiarid tussock grassland site in southeastern Spain. The TSM - in its current incarnation, the two-source energy balance model (TSEB) - was applied to this landscape using ground-based infrared radiometer sensors to estimate both the composite surface radiometric temperature and component soil and canopy temperatures. Morillas et al. (2013) found the TSEB model substantially underestimated the sensible H (and overestimated the latent heat LE) fluxes. Using the same data set from Morillas et al. (2013), we were able to confirm their results. We also found energy transport and exchange behavior derived from primarily the observations themselves to differ significantly from a number of prior studies using land surface temperature for estimating heat fluxes with one-source modeling approaches in semi-arid landscapes. However, revisions to key vegetation inputs to TSEB and the soil resistance formulation resulted in a significant reduction in the bias and root mean square error (RMSE) between model output of H and LE and the measurements compared to the prior results from Morillas et al. (2013). These included more representative ground-based vegetation greenness and local leaf area index values as well as modifications to the coefficients of the soil resistance formulation to account for the very rough (rocky) soil surface conditions with a clumped canopy. This indicates that both limitations in remote estimates of biophysical indicators of the canopy at the site and the lack of adjustment in soil resistance formulation to account for site specific characteristics, contributed to the earlier findings of Morillas et al. (2013). This suggests further studies need to be conducted to reduce the uncertainties in the vegetation and land surface temperature input data in order to more accurately assess the effects of the transport exchange processes of this Mediterranean landscape on TSEB formulations.Dr. Laura Morillas, Professor Monica Garcia, Dr. Luis Villagarcía and Dr. Francisco Domingo would like to thank funding by Andalusia Regional Government projects (P06-RNM-01732, P08-RNM-3721), European Union (ERDF funds) with support from the Spanish Ministry of Science and Innovation (CGL2011-27493) and the Danish Council for Independent Research | Technology and Production Sciences (Grant 09-070382) USDA is an equal opportunity provider and employer.Peer reviewe

Spatial and Temporal Variations in Fluxes of Energy, Water Vapour and Carbon Dioxide during OASIS 1994 & 1995

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