Underestimation Of Sensible Heat Flux Research Articles

Atmospheric boundary layer height over a rain-shadow region: An intercomparison of multi-observations and model simulations

The lack of reliable and continuous measurements of the atmospheric boundary layer (ABL) height poses a significant problem for accurate weather forecasting and examining the intricate dynamics between the ABL and the free troposphere (FT). To address this crucial scientific problem, the present study investigates the ABL height from various simultaneous observations of multi-remote sensing instruments Micropulse lidar (MPL), Ceilometer (CL), Wind profiler radar (WPR) and Radiosonde (RS) and four different PBL schemes (Asymmetrical Convective Model version 2 (ACM2), Yonsei University (YSU), Mellor-Yamada Nakanishi and Nino level 2.5 (MYNN2), and Bougeault–Lacarrere (BouLac) during different sky and surface conditions. The diurnal variation of ABL heights between observation and model differs by ∼230 m. The ACM2 and MYNN2 show better performance (R > 0.85) of ABL height than other numerical schemes. The ABL height from different observations such as CL, WPR, MPL, RS and simulated ACM2 and MYNN2 is about 1098 ± 196 m, 1303 ± 217 m, 1461 ± 391 m, and 1716 ± 639 m, 1808 ± 407 m, and 1585 ± 393 m respectively, during the radiosonde launching time (∼11:00–12:00 IST). The difference in mean ABL height is observed due to their different measurement techniques and tracer identity between observation and model simulations. The study underscores a consistent and abrupt reduction in ABL height between observations (<480 m), model simulations (∼700 m), and re-analysis data sets (∼550 m) during wet-surface conditions, highlighting the model's ability to replicate consistent ABL behaviour in different surface conditions. While in cloudy conditions, the WRF-model underestimates ∼50–200 m than the observed ABL. This discrepancy is mainly observed due to the underestimation of sensible heat flux and downward shortwave radiation in model simulations. Due to the distinct ABL growth rate difference between the model (∼100–200 m/h) and observations (∼105–130 m/h), the time of attaining peak ABL height differs by one hour among them during different sky conditions. This is attributed to the large deviation in surface temperature, the incoming solar radiation, and the sensible heat fluxes.

Accounting for Turbulence-Induced Canopy Heat Transfer in the Simulation of Sensible Heat Flux in SEBS Model

Surface turbulent heat fluxes are crucial for monitoring drought, heat waves, urban heat islands, agricultural water management, and other hydrological applications. Energy Balance Models (EBMs) are widely used to simulate surface heat fluxes from a combination of remote sensing-derived variables and meteorological data. Single-source EBMs, in particular, are preferred in mapping surface turbulent heat fluxes due to their relative simplicity. However, most single-source EBMs suffer from uncertainties inherent to the parameter kB−1, which is used to account for differences in the source of heat and the sink of momentum when representing aerodynamic resistance in single-source EBMs. For instance, the parameterization of kB−1 in the commonly used single-source Surface Energy Balance System (SEBS) model uses a constant value of the foliage heat transfer coefficient (Ct), in the parameterization of the vegetation component of kB−1 (kBv−1). Thus, SEBS ignores the effect of turbulence on canopy heat transfer. As a result, SEBS has been found to greatly underestimate sensible heat flux in tall forest canopies, where turbulence is a key contributor to canopy heat transfer. This study presents a revised parameterization of kBv−1 for the SEBS model. A physically based formulation of Ct, which considers the effect of turbulence on Ct, is used in deriving the revised parameterization. Simulation results across 15 eddy covariance (EC) flux tower sites show that the revised parameterization significantly reduces the underestimation of sensible heat flux compared to the original parameterization under tall forest canopies. The revised parameterization is relatively simple and does not require additional information on canopy structure compared to some more complex parameterizations proposed in the literature. As such, the revised parameterization is suitable for mapping surface turbulent heat fluxes, especially under tall forest canopies.

A Sensitivity Study of an Effective Aerodynamic Parameter Scheme in Simulating Land–Atmosphere Interaction for a Sea–Land Breeze Case Around the Bohai Gulf of China

Abstract Numerical simulations of the atmospheric boundary layer require careful representation of the surface heterogeneity, which involves the upscaling parameterization scheme for the heterogeneous surface parameters. In this study, the sensitivity comparisons of an effective aerodynamic parameter scheme against the area-weighted average scheme in simulating the land–atmosphere interaction over heterogeneous terrain were carried out by conducting multinested simulations with the Weather Research and Forecasting (WRF) Model at coarse and fine resolutions, for a typical sea–land breeze case in the Bohai Gulf of China. The results show that the limited-area model is sensitive to the aerodynamic parameter scheme and the effective aerodynamic parameter scheme exhibits a better performance in simulating the variables and parameters in the land–atmosphere interaction process, such as surface wind speed, sensible heat flux, latent heat flux, friction velocity, and surface air temperature, among others, for short-term simulations. Particularly, the underestimation of sensible heat flux and overestimation of latent heat flux over heterogeneous terrain with area-weighted average scheme for aerodynamic parameters can be improved with the effective parameter scheme in the coastal regions, where the mean simulation error with the effective parameter scheme is about one-half of that with the average scheme for sensible heat flux and one-third for latent heat flux.

Validation of temperature-perturbation and CFD-based modelling for the prediction of the thermal urban environment: the Lecce (IT) case study

This paper discusses the performance of the temperature perturbation-type ADMS-Temperature and Humidity Model (ADMS-TH) and the Computational Fluid Dynamics (CFD)-based model ENVI-met for the prediction of urban air temperature using measurements collected in the city of Lecce (IT) in summer 2012. The goal is to identify the most important factors influencing numerical predictions. Direct comparisons with measured data and statistical indices show that modelled results are within the range of acceptance. Daily trends are well captured although an underestimation of maximum temperature is observed. In ADMS-TH this is due to an underestimation of sensible heat fluxes during daytime, while in ENVI-met it can be attributed to an underestimation of turbulent momentum and thermal diffusivity. Overall, ADMS-TH did predict the temperature cycle with higher accuracy than ENVI-met and its performance was particularly good during the night. ENVI-met required an ad-hoc tuning of surface boundary conditions to predict nocturnal cooling, satisfactorily.

Diagnostic value of magnetic resonance cholangiopancreatography to detect bile duct stones in acute biliary pancreatitis

This paper discusses the performance of the temperature perturbation-type ADMS-Temperature and Humidity Model (ADMS-TH) and the Computational Fluid Dynamics (CFD)-based model ENVI-met for the prediction of urban air temperature using measurements collected in the city of Lecce (IT) in summer 2012. The goal is to identify the most important factors influencing numerical predictions. Direct comparisons with measured data and statistical indices show that modelled results are within the range of acceptance. Daily trends are well captured although an underestimation of maximum temperature is observed. In ADMS-TH this is due to an underestimation of sensible heat fluxes during daytime, while in ENVI-met it can be attributed to an underestimation of turbulent momentum and thermal diffusivity. Overall, ADMS-TH did predict the temperature cycle with higher accuracy than ENVI-met and its performance was particularly good during the night. ENVI-met required an ad-hoc tuning of surface boundary conditions to predict nocturnal cooling, satisfactorily.

The impact of two land‐surface schemes on the characteristics of summer precipitation over East Asia from the RegCM4 simulations

This study evaluates the performance of the regional climate model RegCM4, which incorporates the Biosphere–Atmosphere Transfer Scheme (BATS) and Community Land Model (CLM3) land-surface schemes, in simulating the summer precipitation over East Asia. The characteristics of summer precipitation are analysed in terms of mean amount, frequency and intensity of daily precipitation. The results show that the simulation of the summer precipitation is significantly sensitive to the choices of the land-surface schemes. Despite several deficiencies, the simulation of daily precipitation with CLM3 exhibits superior performance to that with BATS. The BATS simulation tends to systematically overestimate both precipitation frequency and intensity, and hence total precipitation across the whole domain. On the other hand, the CLM3 simulation substantially reduces the wet biases produced in the BATS simulation. The difference in performance between the two simulations mainly results from convective precipitation rather than large-scale precipitation. Since excessive convective precipitation tends to suppress large-scale precipitation, the BATS simulation also exhibits a limitation in properly simulating the ratio of convective and large-scale precipitation. Such behaviour can be explained by the influence of soil moisture on convective precipitation. Persistently wetter soil moisture in the BATS land-surface scheme can modulate the partitioning of surface heat fluxes inadequately, leading to overestimation of latent heat flux and underestimation of sensible heat flux over South China, in particular. Consequently, it affects the thermodynamic structure (as described by the stability indices), which in turn affects the atmospheric stability to determine the convective activity. The CLM3 simulation generates a more realistic representation of equivalent potential temperature, convective available potential energy and convective inhibition, and thus improves the characteristics of daily precipitation.

Effects of different gap filling methods and land surface energy balance closure on annual net ecosystem exchange in a semiarid area of China

Based on eddy covariance measurements over two kinds of land surfaces (a degraded grassland and a maize cropland) in a semiarid area of China in 2005 and 2008, the effects of different gap filling methods, energy balance closure and friction velocity threshold (u*) on annual net ecosystem exchange (NEE) were analyzed. Six gap filling methods, including mean diurnal variation (MDV), marginal distribution sampling (MDS), and nonlinear regressions method, were investigated by generating secondary datasets with four different artificial gap lengths (ranging in length from single half-hours to 12 consecutive days). The MDS generally showed a good overall performance especially for long gaps, with an annual sum bias error less than 5 g C m−2 yr−1. There was a large positive annual sum bias error for nonlinear regressions, indicating an overestimate on net ecosystem respiration. The offset in the annual sum NEE for four nonlinear regressions was from 8.0 to 30.8 g C m−2 yr−1. As soil water content was a limiting factor in the semiarid area, the nonlinear regressions considering both soil temperature and soil water content as controlling variables had a better performance than others. The performance of MDV was better in daytime than in nighttime, with an annual sum bias error falling between −2.6 and −13.4 g C m−2 yr−1. Overall, the accuracy of the gap filling method was dependent on the type of the land surface, gap length, and the time of day when the data gap occurred. The energy balance ratio for the two ecosystems was nearly 80%. Turbulent intensity had a large impact on energy balance ratio. Low energy balance ratio was observed under low friction velocity during the night. When there was a large fetch distance in a wind direction, a low energy balance ratio was caused by mismatch of the footprints between the available energy and turbulent fluxes. The effect of energy balance correction on CO2 flux was evaluated by assuming the imbalance caused by the underestimation of sensible heat flux and latent heat flux. The results showed an average increase of 10 g C m−2 yr−1 for annual NEE in both ecosystems with an energy balance correction. On the other hand, the u* threshold also have a large impact on annual sum NEE. Net carbon emission increased 37.5 g C m−2 yr−1 as u* threshold increased from 0.1 to 0.2 m s−1, indicating a large impact of imposing u* threshold on net ecosystem carbon exchange.

Integration of soil moisture in SEBS for improving evapotranspiration estimation under water stress conditions

In this paper we integrate the information about water stress into SEBS, one of the surface energy balance models that employ remote sensing (RS) data. The level of water stress is taken into account in the calculation of sensible heat, through a modified definition of kB−1, the parameter that summarizes the excess aerodynamic resistance to heat transfer compared to momentum transfer. Surface energy balance models are employed to obtain evapotranspiration as the remainder of available energy minus sensible heat flux (H). These models assume that information on the ratio of actual to potential evaporation is implicitly embedded in the land surface temperature. This assumption is usually adequate where available energy is the limiting factor for evapotranspiration (ET), but there is a problem when water availability becomes limiting for ET. In this case, the daily evapotranspiration is often overestimated, in particular for sparsely vegetated semi-arid regions, because of an underestimation of sensible heat flux for these areas. Our method remedies this shortcoming by progressively decreasing kB−1 with increasing levels of water stress. This decreases aerodynamic resistance, and hence increases H, leading to lower estimates of ET. The decrease of kB−1 with a rise in plant water stress is based on general plant physiological observations related to vertical canopy stomatal conductance profiles, which affects the exchange of sensible and latent heat between the canopy and the atmosphere.The new approach was tested by comparing SEBS H outputs with field observations from Bowen ratio stations distributed over the Konya basin in Turkey, and the results indicate a large improvement when the soil moisture is integrated explicitly in the calculation of sensible heat flux by SEBS. More importantly, the new approach provides a considerable operational improvement for regional ET mapping through integrating microwave soil moisture measurements into SEBS, as illustrated by our findings for this semi-arid region. The daily ET map generated by the new SEBS model captured better the low ET values observed in the drylands, and also reflected better the overall contrast between irrigated fields and the extremely dry surrounding areas. Improved mapping of regional ET by soil moisture integrated SEBS offers the opportunity to provide more accurate estimation of energy and water fluxes in regions where plant water stress is a recurrent feature.

Validation of a numerical model for urban energy‐exchange using outdoor scale‐model measurements

AbstractThe objectives of our study are (1) to evaluate the simple urban energy balance model for mesoscale simulation (SUMM) using such data that are free from many real world uncertainties in respect to spatial variability in material, geometry, and land use, and (2) to analyse the sensitivity of land surface parameters (LSP), which are used in the model. The model was evaluated using the data obtained from comprehensive outdoor scale‐model (COSMO) experiments during a period, which covers roughly half of a year (winter and spring‐early summer) including various wind conditions. SUMM simulated surface layer energy fluxes, surface temperature, and interior temperature fairly well under windy conditions while it underestimated sensible heat flux under calm conditions. On average, simulated sensible heat flux underestimated observed value by 30% (0.73 MJ m−2 d−1) in daytime. Errors of net radiation (4%; 0.40 MJ m−2 d−1) and heat storage (5%; 0.33 MJ m−2 d−1) were smaller than that of sensible heat flux in daytime. This underestimation of sensible heat flux can be attributed to the inadequate parameterization of the surface layer bulk transfer coefficient used in SUMM under calm conditions. On the basis of the sensitivity analyses, parameterization of the surface layer bulk transfer coefficient using Monin‐Obukhov similarity theory (MOST) shows that the model performance is very sensitive to this coefficient, while it is less sensitive to the relative values of the bulk transfer coefficients of local faces. Copyright © 2007 Royal Meteorological Society

Impact of leaf physiology on gas exchange in a Japanese evergreen broad-leaved forest

We used a multi-layer model to analyse the impact of leaf physiology on the diurnal, seasonal, and inter-annual fluctuations in gas exchange in a warm-temperate evergreen broad-leaved forest in Japan. The influences of physiological parameters at the single leaf scale on the canopy scale gas exchange were investigated, including normalised dark respiration rate, R nleaf25, normalised maximum carboxylation rate, V cmax25, and the stomatal coefficient, m, of an improved ball-type stomatal conductance model. Simulated sensible and latent heat fluxes and CO 2 flux at the canopy roughly reproduced the amplitude and diurnal and seasonal fluctuations in the observed fluxes, with the constant m and one set of reference V cmax and R nleaf with their temperature dependences. Overestimations of latent heat flux and thus underestimation of sensible heat flux with a constant m demonstrated that additional stomatal closure should be expected during a drought period. Overestimation of CO 2 flux during the leaf expansion period and the severe drought period with changing m values related to soil moisture conditions demonstrated that the decline in canopy scale CO 2 uptake during these periods was related to some physiological restraints, other than simple uniform stomatal closure, at the single leaf scale.

Relating eddy correlation sensible heat flux to horizontal sensor separation in the unstable atmospheric surface layer

Measurements of a scalar flux from an extended surface are frequently made with the eddy correlation technique consisting of a vertical velocity sensor and a sensor for the scalar of interest. In many cases the two sensors have to be mounted with a significant horizontal separation to avoid flow interference. Consequently, the technique will underestimate the scalar flux. This paper addresses the issue of flux underestimation due to this separation. A model is developed in the framework of Monin‐Obukhov similarity for the spatial covariance of vertical velocity and air temperature in the unstable surface layer. It allows the underestimation of sensible heat flux to be assessed using information on separation orientation relative to wind direction, atmospheric stability, measurement height, and separation distance. The coefficient in the model is evaluated with observations made over a potato field and a clover field. The principles established here should also be applicable to fluxes of scalars other than sensible heat.