Evaluating the response of the diurnal range of surface and air temperature to evaporative conditions across vegetation types in ERA5 and FLUXNET data

2021 ◽  
Author(s):  
Annu Panwar ◽  
Axel Kleidon
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Air Temperature ◽  
Extreme Events ◽  
Heat Storage ◽  
Vegetation Type ◽  
Diurnal Variations ◽  
Vegetation Types ◽  
Evaporative Fraction ◽  
Simple Boundary

<p>The diurnal variations of surface and air temperature are related but their different responses to evaporative conditions can inform us about land-atmosphere interactions, extreme events, and their response to global change. Here, we evaluate the sensitivity of the diurnal ranges of surface (DT<sub>s</sub>R) and air (DT<sub>a</sub>R) temperature to evaporative fraction, across short vegetation, savanna, and forests at 106 Fluxnet observational sites and in the ERA5 global reanalysis. We show that the sensitivity of DT<sub>s</sub>R to evaporative fraction depends on vegetation type, whereas for DT<sub>a</sub>R it does not. Using FLUXNET data we found that on days with low evaporative fraction, DT<sub>s</sub>R is enhanced by up to 20 °C (30 °C in ERA5) in short vegetation, whereas only by 8 °C (10 °C in ERA5) in forests. Particularly, in short vegetation, ERA5 shows stronger responses, which is attributable to a negative bias on days with the high evaporative fraction. ERA5 also tends to have lower shortwave and longwave radiation input when compared to FLUXNET data. Contrary to DT<sub>s</sub>R, DT<sub>a</sub>R responds rather similarly to evaporative fraction irrespective of vegetation type (8 °C in FLUXNET, 10 °C in ERA5). To explain this, we show that the DT<sub>a</sub>R response to the evaporative fraction is compensated for differences in atmospheric boundary layer height by up to 2000 m, which is similar across vegetation types. We demonstrate this with a simple boundary layer heat storage calculation, indicating that DT<sub>a</sub>R is primarily shaped by changes in boundary layer heat storage whereas DT<sub>s</sub>R mainly responds to solar radiation, evaporation, and vegetation.  Our study reveals some systematic biases in ERA5 that need to be considered when using its temperature products for understanding land-atmosphere interactions or extreme events. To conclude, this study demonstrates the importance of vegetation and the dynamics of the atmospheric boundary layer in regulating diurnal variations in surface and air temperature under different evaporative conditions.</p>

scholarly journals Solutions to the 3D Transport Equation and 1D Diffusion Equation for Passive Tracers in the Atmospheric Boundary Layer and Their Applications

2019 ◽  
Vol 2019 (1) ◽  
pp. 21-47 ◽  
Author(s):  
Shuzhan Ren
Keyword(s):  
Boundary Layer ◽  
Diffusion Equation ◽  
Atmospheric Boundary Layer ◽  
Transport Equation ◽  
Analytical Solutions ◽  
Inverse Modeling ◽  
Surface Flux ◽  
Diurnal Variations ◽  
Surface Fluxes ◽  
Passive Tracers

Abstract A solution to the 3D transport equation for passive tracers in the atmospheric boundary layer (ABL), formulated in terms of Green’s function (GF), is derived to show the connection between the concentration and surface fluxes of passive tracers through GF. Analytical solutions to the 1D vertical diffusion equation are derived to reveal the nonlinear dependence of the concentration and flux on the diffusivity, time, and height, and are employed to examine the impact of the diffusivity on the diurnal variations of CO2 in the ABL. The properties of transport operator H and their implications in inverse modeling are discussed. It is found that H has a significant contribution to the rectifier effect in the diurnal variations of CO2. Since H is the integral of GF in time, the narrow distribution of GF in time justifies the reduction of the size of H in inverse modeling. The exponential decay of GF with height suggests that the estimated surface fluxes in inverse modeling are more sensitive to the observations in the lower ABL. The solutions and first mean value theorem are employed to discuss the uncertainties associated with the concentration–mean surface flux equation used to link the concentrations and mean surface flux. Both analytical and numerical results show that the equation can introduce big errors, particularly when surface flux is sign indefinite. Numerical results show that the conclusions about the evolution properties of passive tracers based on the analytical solutions also hold in the cases with a more complicated diffusion coefficient and time-varying ABL height.

scholarly journals Quantifying the coupling degree between land surface and the atmospheric boundary layer with the coupled vegetation-atmosphere model HIRVAC

2001 ◽  
Vol 19 (5) ◽  
pp. 581-587 ◽  
Author(s):  
V. Goldberg ◽  
Ch. Bernhofer
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Land Surface ◽  
Atmospheric Composition ◽  
Vegetation Canopy ◽  
Evaporative Fraction ◽  
Composition And Structure ◽  
Dynamic Forcing ◽  
Strong Dynamic ◽  
Good Agreement

Abstract. In the present study, the ability of different indices to quantify the coupling degree between a vegetated surface and the atmospheric boundary layer is tested. For this purpose, a one-and-a-half dimensional atmospheric boundary layer model, including a high resolved vegetation canopy, was applied (HIRVAC) and indices, such as the decoupling factor Ω, as well as other measures derived from model out-put were used. The aim of the study was to show that the quite complex coupling and feedback mechanisms can be described with these relatively simple measures. Model results illustrate that the vegetation and the atmosphere are well coupled (expressed by a lower Ω) under conditions of a tall and dense canopy, as well as under strong dynamic forcing. This better aerodynamic coupling leads to an increase in evapotranspiration, as well as an increase in the evaporative fraction. This fact was also shown by the second coupling measure: the relative changes in daily model evapotranspiration. This measure was inspired by the assumption that these changes are primarily dependent on the coupling degree between the surface and the atmosphere, if the other boundary conditions in the model are fixed. A third sensitivity measure was used according to Jacobs and de Bruin (1992). It shows that the sensitivity of evaporative fraction to stomata resistance is much higher with a better aerodynamic coupling. The results of the factor Ω; are in a good agreement with the findings of Jacobs and de Bruin: they stress that it is a valuable strategy to group vegetation into two simple categories (smooth and rough) for the understanding of vegetation-atmosphere coupling.Key words. Atmospheric composition and structure (biosphere- atmosphere interactions) – Hydrology (evapotranspiration; hydroclimatology)

scholarly journals Imprints of evaporation and vegetation type in diurnal temperature variations

2020 ◽  
Author(s):  
Annu Panwar ◽  
Maik Renner ◽  
Axel Kleidon
Keyword(s):  
Solar Radiation ◽  
Vegetation Type ◽  
Balance Model ◽  
Vegetation Types ◽  
Evaporative Fraction ◽  
Temperature Variations ◽  
Diurnal Temperature ◽  
Air Temperatures ◽  
Aerodynamic Properties ◽  
Dry Conditions

Abstract. Diurnal temperature variations are strongly shaped by the absorption of solar radiation, but evaporation, or the latent heat flux, also plays an important role. Generally, evaporation cools. Its relation to diurnal temperature variations, however, is unclear. This study investigates the diurnal response of surface and air temperatures to evaporation for different vegetation types. We used the warming rate of temperature to absorbed solar radiation in the morning under clear-sky conditions and evaluated how the warming rates change for different evaporative fractions. Results for 51 FLUXNET sites show that the diurnal variation of air temperature carries very weak imprints of evaporation across all vegetation types. However, surface temperature warming rates of short vegetation decrease significantly by ~ 23 × 10−3 K/W m−2 from dry to wet conditions. Contrarily, warming rates of surface and air temperatures are similar at forest sites and carry literally no imprints of evaporation. We explain these contrasting patterns with a surface energy balance model. The model reveals a strong sensitivity of the warming rates to evaporative fraction and aerodynamic conductance. However, for forests the sensitivity to evaporative fraction is strongly reduced by 74 % due to their large aerodynamic conductance. The remaining imprint is reduced further by ~ 50 % through their enhanced aerodynamic conductance under dry conditions. Our model then compares the individual contributions of solar radiation, evaporation and vegetation types in shaping the diurnal temperature range. These findings have implications for the interpretation of land-atmosphere interactions and the influences of water limitation and vegetation on diurnal temperatures, which is of key importance for ecological functioning. We conclude that diurnal temperature variations may be useful to predict evaporation for short vegetation. In forests, however, the diurnal variations in temperatures are mainly governed by their aerodynamic properties resulting in no imprint of evaporation in diurnal temperature variations.

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