Assessment of Wind and Temperature in the Roughness Sub-Layer using ARPS-CANOPY

The need for a multi-scale modeling system capable of simulating mean and turbulent components of flow through a forest canopy while assessing the impact of vegetation on the surface energy budget motivated a series of modifications to the Advanced Regional Prediction System (ARPS) atmospheric model. Modifications to the momentum and turbulent kinetic energy equations account for the effects of unresolved vegetation elements (e.g., branches, leaves) on simulated wind flow through a forest canopy. The impact of the canopy on the surface energy budget has been implemented by computing net radiation flux at canopy top and prescribing an exponential decay of net radiation inside the canopy to emulate attenuation of net radiation by vegetation elements. This new version of ARPS is referred to herein as ARPS-CANOPY.

In this study, ARPS-CANOPY simulations of the roughness sub-layer are compared to data from the CHATS (Canopy Horizontal Array Turbulence Study) experiment. Two grid nests are utilized, one with relatively coarse 90-m horizontal grid spacing and one with 30-m horizontal grid spacing, to compare the effect of grid resolution and aspect ratio on simulated mean profiles. A series of numerical experiments have been conducted to simulate mean and turbulent momentum and scalar fields in and above the orchard canopy, both before and after leaf-out. Experiment results show that ARPS-CANOPY is able to reproduce the general shape and diurnal trends of the mean wind and temperature profiles, as well as the mean turbulent kinetic energy profiles.

This work is part of a Joint Fire Science Program (JFSP) funded project to develop and validate modeling tools for predicting smoke dispersion from low-intensity fires. An important objective of the CHATS assessment work is to better understand the potential limitations of performing simulations at relatively coarse horizontal resolution, a necessity for real or near-real time smoke dispersion prediction. The CHATS experiment results confirm the validity of ARPS-CANOPY and suggest that use of relatively coarse horizontal resolution reproduces overall profile shapes and diurnal trends, although the finer resolution simulated wind and temperature profiles more closely fit the observations.