Liquid water path - in units of g/m² is a measure of the total amount of liquid water present between two points in the atmosphere. LWP is an important quantity in understanding radiative transfer in the atmosphere. It is defined as the integral of liquid water content between two points in the atmosphere. For nadir observations and whole atmospheric column we have :

LWP=\int_^\infty \rho_ r_L dz'

where is the liquid water mixing ratio and is the density of air (including water loading). The atmosphere is in approximate

hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetar ...

and hydrostatic equation for

atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibar ...

is given by :

\frac= - \rho_ g

which gives :

LWP=\int_0^  r_L dp/g

where is

gravitational acceleration In physics, gravitational acceleration is the acceleration of an object in free fall within a vacuum (and thus without experiencing drag (physics), drag). This is the steady gain in speed caused exclusively by the force of gravitational attract ...

, is the pressure increment between two layers in the atmosphere and integration is between surface and top of the atmosphere. Liquid water path can also be defined between any two selected points. The liquid water path can be approximately retrieved from passive and active remote sensing such as

microwave radiometer A microwave radiometer (MWR) is a radiometer that measures energy emitted at one millimeter-to-metre wavelengths (frequencies of 0.3–300 GHz) known as microwaves. Microwave radiometers are very sensitive receivers designed to measure thermally- ...

instruments, for example SSM/I. Typical values of liquid water path in marine stratocumulus can be of the order of 20-80 g/m². The liquid water path also contributes to important cloud properties. As the value of the liquid water path increases, so does the

albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refl ...

of the cloud. This increase in albedo is seen most quickly at the lower end of the liquid water path spectrum, that is, the smaller the amount of total water, the quicker the albedo increases. The radiative absorption of clouds is also dependent on the liquid water path. An increase of liquid water path leads to an increase in absorption. Again, the largest increase is seen with lower levels of liquid water path. These connections are due to the proportionality between the liquid water path and the

optical depth In physics, optical depth or optical thickness is the natural logarithm of the ratio of incident to ''transmitted'' radiant power through a material. Thus, the larger the optical depth, the smaller the amount of transmitted radiant power throug ...

of the cloud.

References

Atmospheric thermodynamics {{Climate-stub

See also

References