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Land–Atmosphere Interactions

Land-Atmosphere

Interactions

Introduction...................................................................................................115

Processes.........................................................................................................115

Natural Phenomena......................................................................................116

Somnath Baidya Slope and Valley Winds • Sea Breezes • Asian Monsoon

Roy and Deeksha Manmade Activities......................................................................................118

RastOgi Tropical Deforestation • Agriculture • Urbanization

University of Illinois Bibliography..................................................................................................120

Introduction

Land and atmosphere are integral components of our natural environment. Interactions between land and atmosphere occur through continuous exchange of heat, moisture, momentum, and various gases (Figure 13.1). These exchanges result in various natural phenomena, such as sea breezes and monsoons, at wide ranges of spatial and temporal scales. Human activity such as deforestation, agriculture, and urbanization can modify these exchanges and thus affect weather, climate, water cycle, and other aspects of the natural environment. Hence, land-atmosphere interaction is a topic of interest for scientists and policymakers worldwide.

Processes

The land surface is a major source of energy for the atmosphere. Even though all energy originally comes from the sun, this energy is in the form of shortwaves that cannot be readily absorbed by the atmospheric gases. Some of these solar shortwaves are reflected back but a major part is absorbed by the surface. Some of this energy is used by plants for photosynthesis but the rest heats up the surface. The surface then tries to cool down by transferring energy to the atmosphere by three processes: longwave radiation, conduction, and latent heating. Most of the energy transfer occurs through longwave radiation emitted by the surface. Unlike solar shortwaves, these longwaves are easily absorbed by the atmosphere, especially by clouds and water vapor. The second process is conduction where heat exchange occurs between the surface and the lowest layer of the atmosphere in direct contact with the surface. During the daytime heat flows from the warm surface into the lower atmosphere and is then carried upward by convection. At night, the surface is cooler than the air above and consequently heat flows downward from the atmosphere into the ground. Finally, energy is also transferred in the form of latent heating by evaporation and transpiration from the ground and vegetation. During evaporation and transpiration, liquid water in the soil and vegetation absorbs heat and is released into the atmosphere in the form of water vapor. The heat carried by the vapors is released into the atmosphere when the vapors condense to form cloud droplets.

Schematic diagram representing various processes of energy and water exchange between the land surface and the atmosphere

FIGURE 13.1 Schematic diagram representing various processes of energy and water exchange between the land surface and the atmosphere.

The Earth’s surface acts as both a source and a sink for moisture in the atmosphere. Moisture is transported as water vapor from the land surface to the atmosphere by evaporation and transpiration. Most of this moisture is returned back to the surface in the form of rain, snow, hail, and freezing rain. A tiny amount of moisture is also transferred back to the surface when some of the water vapor in the lower atmosphere condenses directly onto the surface as dew and frost on cold nights. Some of this moisture seeps into the ground and becomes part of the groundwater system, whereas the rest flow downslope as surface runoff into nearby waterbodies. Even though land covers less than 30% of the Earth’s surface, the continuous and vigorous back- and-forth exchange of moisture between land and the atmosphere plays a significant role in the overall water cycle.

The land surface is the primary sink of atmospheric momentum. Friction due to natural and manmade obstacles such as hills, trees, and buildings slow down the wind. During this process kinetic energy in the wind is converted to heat. Friction due to land surface is typically expressed as a function of the surface roughness. Surfaces with tall, rigid densely packed obstacles, such as tropical forests and urban areas, have higher roughness and hence generate stronger friction than relatively smooth surfaces such as sandy deserts.

The Earth’s surface is also a source and a sink of atmospheric gases such as carbon dioxide, oxygen, and nitrogen. Carbon dioxide is absorbed by plants during photosynthesis and released from the surface back into the atmosphere during respiration by plants and animals. This natural carbon cycle has been significantly modified by humans since the Industrial Revolution due to the large amount of carbon dioxide released by fossil fuel burning without a corresponding natural sink. Oxygen is absorbed by plants and animals during respiration and released back into the atmosphere by plants as a byproduct of photosynthesis. Atmospheric nitrogen is absorbed by bacteria in the soil and nitrogen-fixing plants and released back into the atmosphere during decomposition of biomass.

 
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