Examining the influence of heterogeneous forest canopy on shallow convection at the third ARM Mobile Facility (AMF3) site

Examining the influence of heterogeneous forest canopy on shallow convection

convection

The rising of warm air and the sinking of cool air. Convection can cause local breezes, winds, and thunderstorms.

Source: NASA earth observatory Source

at the third ARM Mobile Facility (AMF3) site

Timothy W. Juliano, National Center for Atmospheric Research, Principal Investigator
Heping Liu, Washington State University, Co-Principal Investigator
Jeremy Sauer, National Center for Atmospheric Research, Co-Principal Investigator

This proposal focuses on the linkage between turbulent forest canopy exchanges and the overlying atmosphere under convective conditions. Forests cover a large fraction of the Earth’s land surface (~30%), and they are important for exchanges of momentum, heat, moisture, and other constituents (e.g., aerosols)

aerosols

Small particles or droplets, such as wildfire smoke, volcanic gases, or salty sea spray, that float in the air. They are emitted by both natural events and human activities.

Source: NASA Global Climate Change Source

into the lower atmosphere. The southeastern United States is no exception, with forests playing a vital role in the regional ecosystem.

ecosystem

A natural community of plants, animals, and other living organisms and the physical environment in which they live and interact.

Source: A student's guide to Global Climate Change Source

During the spring and summer months in this region, locally-forced, shallow convection (LFSC) – that is, convective elements that extend ~1-3 km deep and typically develop under relatively weak and steady synoptic-scale forcing conditions – is a frequent occurrence. Previous studies have shown that land heterogeneity is an important factor in determining the properties of shallow convection. Therefore, to improve process-level understanding and model representations of land-atmosphere interactions, the Department of Energy Atmospheric Radiation Measurement (ARM) program is deploying the third ARM Mobile Facility (AMF3) to northern Alabama in the Bankhead National Forest (BNF).

LFSC is often tightly coupled to land-atmosphere exchanges, as turbulent structures in the atmospheric boundary layer form the building blocks for thermals to spawn and grow throughout the morning transition period given supportive environmental conditions, often developing into shallow cumulus clouds. Despite the extensive number of studies that have examined LFSC throughout the world, relatively little is known about the role of horizontally heterogeneous turbulent canopy exchanges on the formation and growth of moist, convective atmospheric boundary layer structures under different SSF regimes.

We are afforded an excellent opportunity to deepen our understanding of the role that forests play in the modulation of convective atmospheric processes. Thus, we propose to utilize BNF tower measurements to quantify turbulent exchanges within and above the horizontally heterogeneous forest canopy. During this pilot study, we will take a three-pronged approach, combining the AMF3 measurements with novel numerical simulations. More specifically, we propose to:
Apply the self-organizing map (SOM) technique to identify and characterize the synoptic-scale forcing regimes that dominate at the BNF site;
Perform an AMF3 observational synthesis

synthesis

The process of combining a number of things into a coherent whole to form a theory or system.

Source: National Library of Medicine Source

for the time period that climatologically experiences the most LFSC (approximately 1 April – 31 August); and
Conduct high-resolution, canopy-resolved numerical simulations of LFSC based upon the SOM synoptic-scale forcing regime classification and AMF3 measurements.

This proposal focuses on the research topic Southeast U.S. atmospheric processes through early use of observations from the third ARM Mobile Facility (AMF3) (Topic 4 under this FOA) – specifically the use of BER-funded field facilities and novel high-resolution simulations – to study land-atmosphere interactions in the BNF, namely the turbulent transfer of momentum, heat, moisture, and aerosols between the forest canopy and overlying atmosphere under conditions supportive of LFSC.

Our observational approach will rely mainly on the comprehensive flux

flux

The amount of some type of particle or energy crossing a unit area per unit time.

Source: NRC Library Source

tower measurements to enable exploration of turbulent transport in the canopy sublayer. We will apply spectral techniques to better understand the dominant spatial and temporal scales of turbulence structures. Moreover, quadrant and octant analysis can quantify the frequency and intensity of ejection/sweep events that are responsible for the transport of momentum and scalars between the canopy layer and overlying surface layer. Coupling these findings with a radar-based shallow cumulus cloud identification method and the SOM approach will better elucidate turbulent canopy exchange processes during LFSC conditions.

To extend our fundamental understanding of the impact of heterogeneous canopy processes on LFSC near the BNF site, we will conduct microscale simulations using two novel large-eddy simulation modeling frameworks, one with a multi-layer canopy model (WRF-MCANOPY) and the other a novel graphics processing unit-based model called FastEddy®. The simulations will use a detailed 3D fuels dataset for accurate characterization of the canopy at the AMF3 site. Combined with the BNF measurements, results from our canopy-scale simulations will allow us to better understand the linkage between heterogeneous forest canopy and moist, convective structures leading to LFSC near AMF3.

In summary, we anticipate that our research will improve our fundamental understanding of the connection between synoptic-scale forcing regimes, the canopy structure, and characteristics of LFSC. Results may inform future instrument deployments at AMF3, as well as model development efforts.