Decadal Near-Surface Turbulent Flux Measurements from ARM¶

Overview¶

Within this notebook, we will cover the methodology and analysis within Sullivan et al. (2025) :

1. History of near 3 decades of ARM’s near-surface turbulent flux observations

2. Introduction to the Energy Balance Bowen Ratio (EBBR) and Eddy Covariance (EC) methods

3. Comparison of Sensible and Latent Heat Fluxes from these methods for 2015 to 2023 period at SGP

Prerequisites¶

• Time to learn: 15 minutes

History of ARM’s Near-Surface Turbulent Flux Observations at SGP¶

Sensible and latent heat fluxes (H and LE) are critical to modulating the sources and sinks of the energy and water budgets across the land-atmosphere-biosphere interface.

In-situ measurements of H and LE are critical for understanding and predicting processes relevant for heat waves, droughts, argiculture and irrigation scheduling, and are necessary to capture the fine scale scales at which these processes occur.

The U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) user facility has measured fluxes primarily using in-situ meteorologically driven energy balance flux gradient method with energy balance Bowen ratio (EBBR) system, the eddy covariance method (EC) with the carbon dioxide flux (CO2FLX), and the eddy correlation (ECOR) flux measurement system.

ARM began measuring fluxes in 1992 ^[1] using the EBBR system at the Southern Great Plains (SGP) atmospheric observatory, at ten grassland extended facilities (EF) within Oklahoma and Kansas and one EF on the northern edge of cropland. THe intention was that these facilities would be representative of a typical global climate model (GCM) grid cell.

The Energy Balence Bowen Ratio (EBBR) Method¶

The EBBR measures near-surface gradients of temperature and humidty^[2] to approximate the Bowen Ratio:

where:

• $\beta$ is the Bowen ratio

• $H$ is the sensible heat flux

• $LE$ is the latent heat flux

• $C_p$ is the specific heat of air ($\mathrm{J\ kg^{-1}\ K^{-1}}$)

• $\rho$ is the density of air ($\mathrm{kg\ m^{-3}}$)

• $\lambda$ is the latent heat of vaporization of water (or the latent heat of sublimation for frozen conditions) ($\mathrm{J\ kg^{-1}}$)

• $\overline{\Delta T}$ is the mean temperature difference between the upper and lower sensors ($\mathrm{K}$)

• $\overline{\Delta \rho_v}$ is the mean difference in water vapor density between the upper and lower sensor ($\mathrm{kg\ m^{-3}}$)

EBBR also measures net radiation, soil heat flow, soil temperature, and soil moisture^[3].

Combining an equational form of a closed surface energy budget, where the sum of sensible and latent heat fluxes equals the net radiation less energy consumed as the ground heat flux.

The definition of the Bowen ratio as the ratio of sensible to latent heat flux gives equations for the sensible and latent heat fluxes as:

where

• $R$ is radiation

• $G$ is surface soil heat flux

• $R$, $G$, $H$, and $LE$ are in $\mathrm{W\ m^{-2}}$

• “other components” are assumed to be null

The Eddy Covariance (EC) Method¶

The EC method estimates fluxes from the covariance of the vertical wind speed and the quantity of interest:

horizontal wind speed for momentum flux ($\tau$)

temperature for $H$

water vapor concentration for $LE$

another scalar (for example, CO$_2$ or CH$_4$ concentration)

where:

• $w'$ is the instantaneous perturbation, used herein as the departure of a given variable from its mean, of the vertical wind speed component ($\mathrm{m\ s^{-1}}$)

• $u'$ is the instantaneous perturbation of the horizontal wind speed component ($\mathrm{m\ s^{-1}}$)

• $T'$ is the instantaneous perturbation of temperature ($\mathrm{K}$)

• $X_v'$ is the instantaneous perturbation of the mixing ratio of water vapor in air ($\mathrm{kg\ kg^{-1}}$)

• $X_c'$ is the instantaneous perturbation of the mixing ratio of scalar $c$ in air ($\mathrm{kg\ kg^{-1}}$)

• the overbar denotes a time-average operator

Accounting for the thermodynamic contribution of temperature fluctuations, $LE$ can be computed as:

where:

• $\mu$ is the ratio of molar masses of dry air and water vapor

• $\sigma$ is the ratio of the densities of water vapor and dry air

• and T is the air temperature.

EBBR and EC Data Ingest¶

From Sullivan et al. (2025), the timelines for the EBBR and EC products:

Sensible and Latent Heat Flux Diel Cycles¶

Diel Cycles per Crop Type¶

From Sullivan et al. (2025), at Southern Great Plains Extended Site (E39):

• southerly (100-260°) fetch contains the cropland footprint

• northerly (0-80 and 280-360°) fetch contains the ungrazed grass footprint