Observations of Near-Surface Heat-Flux and Temperature Profiles Through the Early Evening Transition over Contrasting Surfaces

Abstract

Near-surface turbulence data from the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program are used to study countergradient heat fluxes through the early evening transition. Two sites, subjected to similar large-scale forcing, but with vastly different surface and sub-surface characteristics, are considered. The Playa site is situated at the interior of a large dry lakebed desert with high sub-surface soil moisture, shallow water table, and devoid of vegetation. The Sagebrush site is located in a desert steppe region with sparse vegetation and little soil moisture. Countergradient sensible heat fluxes are observed during the transition at both sites. The transition process is both site and height dependent. At the Sagebrush site, the countergradient flux at 5 m and below occurs when the sign change of the sensible heat flux precedes the local temperature gradient sign change. For 10 m and above, the countergradient flux occurs when the sign change of the sensible heat flux follows the local temperature gradient sign change. At the Playa site, the countergradient flux at all tower levels occurs when the sign change of the sensible heat flux follows the local temperature gradient sign change. The phenomenon is explained in terms of the mean temperature and heat-flux evolution. The temperature gradient sign reversal is a top-down process while the flux reversal occurs nearly simultaneously at all heights. The differing countergradient behaviour is primarily due to the different subsurface thermal characteristics at the two sites. The combined high volumetric heat capacity and high thermal conductivity at the Playa site lead to small vertical temperature gradients that affect the relative magnitude of terms in the heat-flux tendency equation. A critical ratio of the gradient production to buoyant production of sensible heat flux is suggested so as to predict the countergradient behaviour.B Eric R. Pardyjak

A methodology for computing spatially and temporally varying surface sensible heat flux from thermal imagery

“…Due to the arid climate of the Dugway Proving Grounds (Hang et al , 2016), latent heat fluxes are very weak. Measurements of water vapour fluxes were carried out at the MATERHORN Playa site using a Campbell Scientific (Logan, Utah) EC150 (infrared gas analyser) and CSAT3 (sonic anemometer‐thermometer) at a height of 10.4 m (Jensen et al , 2016) give a detailed methodology). During the period of interest, the observed latent heat fluxes (computed using the eddy‐covariance method) ranged from a minimum of ≈13 W m −2 to a maximum of ≈ 30 W m −2 , with a 1 h mean value of 20.4,W m −2 .…”

“…For this experiment, the soil near the Playa surface was mostly clay, silty clay or silty clay loam. The density, thermal conductivity, and specific heat were determined in Hang et al (2016) and Jensen et al (2016) during the MATERHORN spring field campaign and were 1426.2 kg m −3 , 0.77 W m −1 K −1 , and 1.472×10 3 J kg −1 K −1 respectively. Note that these values lead to a thermal admittance () of 1270 J m −2 K −1 s −1/2 .…”

“…The annual temperature of the site is ∼13.7 °C, with an annual rainfall and evaporation of ∼100 and ∼170 mm respectively (Malek, 2003; Hang et al , 2016). More details of the experimental site can be found in Hang et al (2016) and Jensen et al (2016).…”

“…The annual temperature of the site is ∼13.7 • C, with an annual rainfall and evaporation of ∼100 and ∼170 mm respectively (Malek, 2003;Hang et al, 2016). More details of the experimental site can be found in Hang et al (2016) and Jensen et al (2016). For this case-study, we primarily focus on Intensive Observation Period 9 (IOP9) due to the period's characteristics (mid-afternoon, with strong winds), validating the use of and comparison to the eddy-covariance method.…”

Turbulence kinetic energy budget during the afternoon transition – Part 1: Observed surface TKE budget and boundary layer description for 10 intensive observation period days

“…This result may, however, not be a general feature of the afternoon and evening transition as discussed by Jensen et al (2014Jensen et al ( , 2015, who obtained different results with other data sets. It is interesting to note, however, that for this data set, when the Ri number is close to zero and the buoyancy flux is close to zero, such as for the data encircled with the smaller vertical ellipses in Fig.…”