Daily cycle simulations of thermally stratified flows over forests

Matías Ávila ,Hugo Olivares-Espinosa ,Roberto Chávez‐Arroyo ,Johan Arnqvist ,Ebba Dellwik

doi:10.1088/1742-6596/1256/1/012003

Abstract

The aim of the present work is to obtain a better understanding of how to model the thermally stratified wind field over a forest during full diurnal cycles. The setup of the study assumes a horizontally homogeneous forest, with the objective of finding a simple and efficient way to model the canopy flow using time-dependent input data, obtained from measurements and mesoscale simulations. With this, new insights can be gained for future microscale modelling of complex forested terrains using mesoscale input data. In terrain without forest a diurnal cycle is commonly simulated by imposing time-dependent ground temperature. However, the presence of forests partially isolates the temperature at ground level from the flow above the canopy, making this common approach ineffective. This work proposes imposing the time-dependent net radiation at the forest canopy top to drive the thermal stratification changes along the diurnal cycle. To this end, several full days of simulation are driven by prescribing the net radiative heat flux balance measured on top of the canopy, together with a geostrophic pressure gradient. The advantage of the method is its simplicity and that the input data can be easily obtained from mesoscale modelling.When compared to the observations at the Swedish site Ryningsnas, the new method dramatically improves estimations of wind speed, wind direction and turbulent kinetic energy compared to simulations that only assume neutral stratification. Out of the variables studied, temperature and turbulent heat flux profiles were the ones that qualitatively followed the measurements the best, while wind speed and turbulent kinetic energy showed a larger disagreement.

Highlights

A key point for the assessment of wind energy efficiency is the accurate characterization of the resource in the planning of wind farms, requiring a careful determination of the micrometeorological processes
The simple method of driving the model with net radiative flux on the top of the canopy, while omitting any heat storage by the canopy itself, was shown to provide diurnal cycles of wind, temperature and turbulence, that qualitatively agreed with measured equivalents
The nighttime turbulent heat flux was underestimated by the model, as was the nocturnal velocity gradient

Summary

Introduction

A key point for the assessment of wind energy efficiency is the accurate characterization of the resource in the planning of wind farms, requiring a careful determination of the micrometeorological processes. Wind farms continue to be erected in high-latitude forested areas, where transient effects of thermal stratification strongly affect the turbulence levels, as well as wind profile and wind direction. The aim of the present work is to obtain a better understanding of how to simulate full diurnal cycles of thermally stratified wind field over forested sites in microscale models, for wind energy assessment.

Objectives

Findings

Conclusion