Land-Atmosphere Interactions

Abstract

Land-atmosphere interactions encompass a multitude of processes that link the land surface with the atmospheric boundary layer. Interactions are bidirectional, include energy and material exchanges, and can include feedbacks that can amplify or attenuate coupled processes. Shortwave radiation drives most of the biogeophysical processes at the land surface. Photosynthetically active radiation (PAR) is the subset of shortwave radiation (400–700 nanometers) and is critical for most life on the planet. Thermal infrared is the more energetic subset of terrestrial radiation that results primarily from interactions of solar radiation with the land surface. Microwaves are an important subset of terrestrial radiation that facilitate monitoring both atmosphere and land surface. Net radiation is the energy left over after accounting for incoming direct and indirect solar radiation less outgoing solar radiation reflected by the surface, plus incoming longwave radiation (from water vapor and other gases in the atmosphere and terrestrial materials within view of the surface), less outgoing longwave radiation from the land surface. This radiation remaining at an “ideal surface” can be simply partitioned into energy transferred into the surface (ground heat flux) plus energy transferred to heat the atmosphere above the surface (sensible heat flux) plus energy transferred via evapotranspiration (latent heat flux) to moisten the atmosphere. Additionally, objects on the surface can absorb radiation and later radiate this stored heat. Photosynthesis uses only a small portion of incident energy. Precipitation on the surface may (1) return to the atmosphere as water vapor (latent heat flux), (2) move as liquid laterally to another surface point (runoff), (3) move as liquid below the surface (drainage), (4) be retained at or below the surface, including in the soil (storage), (5) be transported away, if frozen, from the surface by wind (advection), or combinations of these. Material exchanges between surface and atmosphere include mineral dust, organic particles, biota, and biological materials such as pollen, seeds, combustion products, volcanic ash and ejecta, trace gas emissions, and anthropogenic emissions from stationary and mobile sources. Interactions between the land surface and lower portion of the atmosphere at various time scales from seconds to centuries are influenced by the amount and type of incident sunlight, radiative characteristics of the materials at the surface, amount of moisture at and below the surface, vegetation amount and type, soils and substrate, vertical structures at the surface that affect wind, land cover type and arrangement, atmospheric constituents, and recent weather. Here we focus on interactions moving from land to the atmosphere.