An explanation of how forests create rainfall

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An explanation of how forests create rainfall

Post by Rand » Fri Feb 09, 2018 4:25 pm

If you cut your forest, the winds will not blow from the ocean and will not bring you rain. Natural forests draw atmospheric moisture inland from the ocean in a positive feedback loop. This builds up precipitation inland, compensating for water lost through river flow and ultimately increasing river runoff due to the sustained low pressure area inland. Forests make rivers.

Biotic Pump of Atmospheric Moisture
Due to their high leaf area index, natural forests maintain high transpiration fluxes (thick dark blue arrow), which exceed the evaporation fluxes over the ocean (thin dark blue arrow). The evaporated moisture undergoes condensation and disappears from the gas phase. Air in the atmospheric column above the forest rarifies. As a result, ascending air motion develops over the forest canopy, which, in turn, "sucks in" moist air from the ocean (light blue arrow). It then returns to the ocean in the upper atmosphere (dotted arrow) after precipitation of moisture over the continent.

Main Findings
The-chicken-or-the-egg problem of whether forests grow where it is wet, or it is wet where the forests grow, solves unambiguously in favor of the forests' priority. Physical foundations for this conclusion (the non-equilibrium vertical distribution of atmospheric water vapor and the associated upward-directed force of osmotic nature, termed the evaporative force), as well as the empirical evidence (precipitation dependence on distance from the ocean in forested versus non-forested areas) illustrating the action of forest moisture pump and its decisive role in the maintenance of water cycle on land, are described.
Boreal biotic pump in action: in winter, when the forest is dormant, precipitation is concentrated over the ocean; over land it markedly declines from west to east. In summer, when the forest is active and draws moisture from the ocean, rainfall predominantly occurs over land. Now it does not decline with distance from the ocean, but remains approximately constant over seven thousand kilometers! (Figure 5 from Theoretical and Applied Climatology 2013)

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