There is a
method to increase precipitation called "Cloud seeding". In this post
another approach will be described – "Artificial rain by utilizing
sunlight – Hadley microcell".
Hadley cell
is a natural phenomenon (see Hadley cell). Part
of Hadley cell close to the equator receives rich precipitation (see Intertropical Convergence Zone).
Why rain happens
There are
several mechanisms of creating precipitations in nature (see Precipitation types). One of them is convectional precipitation. The stages of convectional
precipitation are:
1. Air is heated from solar energy.
1. Air is heated from solar energy.
2. Warm air
is lighter than surrounding air and lifts up.
3. At
altitude the air expands adiabatically. Its temperature decreases due to
adiabatic expansion.
4. Moisture
in the air condenses. This process releases latent heat and additionally
decreases air density that gives additional lifting force.
5.
Condensed water falls to ground. When condensed water leaves air, the air
becomes more lighter and receive additional lifting force.
How convectional precipitation
mechanism works in subtropical deserts
Saying
shortly it doesn't work. Convection needs to receive energy to work.
1.
Significant part of sunlight is reflected due to relatively high albedo.
2. In order convection occurs heated air
needs to be replaced by new volume of air. But airflow is restricted due to
ground surface geometry. So, heated air gets stuck to the ground. Considering
low heat conductivity of air it leads to overheating of ground surface and
higher thermal radiation.
3. Heat
conductivity of air is much lower than one of ground. It leads ground absorb
sun energy much readily compared to air.
As a result
the air receives very small amount of sun energy (maybe 3% or less). Most of
the sun energy is reflected, is reemitted as thermal radiation or goes into the
ground, and convection receive almost no energy. So convection almost doesn't
work. Additionally Hadley cell creates descending air current in subtropics (see Horse latitudes) that suppresses this weak convection.
For
comparison let's consider the processes at equatorial forests.
Albedo is
relatively low, so sun energy is better absorbed. Trees absorb sunlight and
heat the air. Heated air is readily replaced by air from below. Ground receives
and absorbs insignificant amount of sunlight. As a result the air receive high
amount of sun energy and convection works well.
Hadley microcell
Hadley
microcell is artificial ground surface modification. It is several kilometers
in diameter. A land is covered by special heating elements. Each heating
element is elevated several meters above the surface. A heating element is
permeable for air to allow free airflow. So Hadley microcell mimics equatorial
forests.
Building a
Hadley microcell can probably increase precipitation.
P.S. Trees
make convection. So trees and forests are important for precipitation.
Deforestation reduces precipitation.
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