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Lecture 7
23-Aug-2016
PRECIPITATION
In the last class, we were discussing about atmospheric water.
Talked about water vapor
Precipitable water in an atmospheric air column
We also said that two important process involving atmospheric water are:
Evaporation
Precipitation
Today we shall discuss about precipitation.
The methods through which water will fall to the earth’s surface from atmosphere is called
precipitation
Some of them are
Rainfall
Snowfall
Hail
Sleet etc.
In precipitation what happens is
Air mass is lifted in the atmosphere
It gets cooled as it is lifted
Some of the cool air condenses the moisture
There are three mechanisms of air lifting
Frontal lifting(warm air lifted over cooler air)
Orthographic lifting (air mass rises over mountains etc)
Convective lifting (air drawn upward by convective action)
The formation of precipitation in clouds can be briefly stated as below:
Air rises through air mass lifting
Air cools and water condenses from vapor
Usually condensation requires nucleus or seed , which may be particles of dust floating in
air as aerosols
The droplets formed by condensation grows in size as it merges with other droplets
The droplets move downwards
As the droplet move downwards, evaporation occurs and its size decreases
This size reduction causes the droplets to move up to near size of aerosols
That is a cycle of condensation, falling, evaporation, and rising occurs.
The cycle repeats many times till the droplets reach a critical size. That makes it fall
through the bottom of the cloud.
These drops falling are called rainfall.
Raindrops move with terminal velocity through the atmosphere
The terminal velocity is
Where
= terminal velocity of rain drops.
D= diameter of drops.
=coefficient of drag for the drops
=density of water
=density of air
THE THUNDERSTORM CELL MODEL
We said that one method of air mass lifting is the convective air mass lifting
In thunderstorm cell, convective air mass lifting occurs.
3
P3,V3,T3,
2
1
Incipitation
P1, V1,T1,
Consider a vertical column of air, where thunderstorm is intended to occur
This is the control volume
As said earlier, for precipitation to occur air mass has to be lifted, cooled and condensed and
precipitated
Therefore in that vertical column of air we can devise three regions
1. Inflow region
2. Uplift region
3. Outflow region
The warm moist air is drawn into the cell through the inflow region.
It raises through the uplift region and cools and condenses
Through the outflow region, the cooler drier air outflows
The out flown air at the top of the column may descend downwards, outside our control
volume, to form convective cell circulation.
Let us analyze the thunderstorm cell using the continuity equation for water vapor.
Recall,
If the intensity of precipitation is ‘i’ and if the sectional area on the earth’s surface at which
precipitation occurs is A.
Then
i.e.
(Steady)
Let us assume that the thunderstorm cell convection is in steady state:
Then,
Let the cell diameter =D
Let the cell be cylindrical in shape
Inflow region height=
Outflow region height=
Also
If we take the mass of dry air as the extensive property =
Then according to continuity
In steady conditions