Turbulent diffusivity and diurnal variations in the atmospheric boundary layer

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.

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Doppler Sodar Observations of the Diurnal Variations of the Atmospheric Boundary Layer over Gadanki, a Tropical Rural Station

International Journal of Trend in Scientific Research and Development ◽

10.31142/ijtsrd5974 ◽

2017 ◽

Vol Volume-2 (Issue-1) ◽

pp. 517-522

Author(s):

D. Sharmila ◽

Prof. M. P. Rao ◽

Keyword(s):

Boundary Layer ◽

Atmospheric Boundary Layer ◽

Diurnal Variations ◽

Rural Station ◽

Doppler Sodar

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

10.5194/egusphere-egu21-2603 ◽

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

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 (DTsR) and air (DTaR) 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 DTsR to evaporative fraction depends on vegetation type, whereas for DTaR it does not. Using FLUXNET data we found that on days with low evaporative fraction, DTsR is enhanced by up to 20 &#176;C (30 &#176;C in ERA5) in short vegetation, whereas only by 8 &#176;C (10 &#176;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 DTsR, DTaR responds rather similarly to evaporative fraction irrespective of vegetation type (8 &#176;C in FLUXNET, 10 &#176;C in ERA5). To explain this, we show that the DTaR 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 DTaR is primarily shaped by changes in boundary layer heat storage whereas DTsR mainly responds to solar radiation, evaporation, and vegetation. &#160;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.

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