Characterizing Land-Atmosphere Interactions During the Afternoon-to-Evening Transition Using ARM SGP Observations

The land-atmosphere system consists of the soil, vegetation, and lowest layer of the atmosphere. The energy and water transport processes in this system contribute to the structure and the daily cycle of the atmospheric boundary layer, which is the layer between the land surface and the free troposphere that controls the exchange of momentum, energy, and water. The state of the atmospheric boundary layer is the result of complex two-way interactions between local processes such as surface fluxes

flux

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

Source: NRC Library Source

and entrainment at the atmospheric boundary layer top as well as large-scale and mesoscale

mesoscale

Size scale referring to weather systems smaller than synoptic-scale systems but larger than storm-scale systems. Horizontal dimensions generally range from around 50 miles to several hundred miles. Squall lines, MCCs, and MCSs are examples of mesoscale weather systems

Source: NOAA Source

processes such as advection and subsidence.
The diurnal thermodynamic evolution of the atmospheric boundary layer, moisture advection, and mesoscale circulations, as well as the turbulent flux exchange at both the surface and at the top of the atmospheric boundary layer, are all important aspects of the land-atmosphere challenge. A key component in land-atmosphere feedback is the turbulent mixing of momentum, heat, and moisture in the convective boundary layer and its changes during the afternoon to evening transition. During this transition, the length scales of turbulent motions change dramatically, and forces other than surface heating by solar radiation become important for dictating how the boundary layer evolves. An accurate understanding of the afternoon to evening transition is critical for a range of phenomena, including the stability of the nocturnal boundary layer, nighttime 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

the low-level jet, and the transport of 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

and tracers from the boundary layer to the free troposphere.



This proposal aims to improve our understanding of the complex land-atmosphere processes and their interaction during the afternoon to evening transition. In particular, we propose to use a combination of land-surface and profiling observations together with large eddy simulations (LES) and mesoscale model simulations to better understand how the surface energy balance and lower atmosphere evolve during the afternoon to evening transition. While there are many questions that need to be addressed to better understand land-atmosphere interactions during the afternoon to evening transition, we will focus on these three:
1. What are the relative magnitudes of the “residual forcings” (e.g., radiative processes, entrainment, shear, advection, and clouds) that become apparent when the solar heating forcing decreases during the afternoon to evening transition on the evolution of the atmospheric boundary layer?
2. What is the role of land-use heterogeneity, and the associated differences in how different components of the surface energy budget change as the sun sets, on the evolution of the atmospheric boundary layer during afternoon to evening transition?
3. What are the relative contributions to the surface moisture increase from advection, latent heating from the surface, and entrainment during the afternoon to evening transition, and how does this affect the stability of the atmospheric boundary layer?
We will analyze the observations from all five of the profiling extended facilities at the Department of Energy Atmospheric Radiation Measurement (ARM) facility Southern Great Plain (SGP) site. These sites are located in different regions relative to the winter wheat belt in northern Oklahoma, and thus the evapotranspiration

evapotranspiration

The process by which water moves from the land or ocean surface to the atmosphere (evaporation) and transpiration (evaporation from plants).

Source: USGS Source

is markedly different among the sites, including in the seasonal variability at each. To separate out the different time-scales of the processes at work during the afternoon to evening transition, we will study the “rapid sunset” event that occurred during the 2017 Solar Eclipse in conjunction with normal afternoon to evening transition events. To complement the observational analysis, we will use LES and mesoscale models and explore a wide range of sensitivities to boundary parameters of the land-atmosphere system, such as incoming radiation, the degree of heterogeneity, and land surface properties.
We expect our results to culminate in improved understanding of the interactions between the land and atmosphere during afternoon to evening transition, and improvements to the MYNN-EDMF boundary layer parameterization scheme used within the WRF model.