Precipitable rainwater
Under precipitable precipitation water ( English precipitable water , abbreviation PW ) is understood in the meteorology and climatology that amount of liquid water contained in a column with a defined surface area over the entire height of the atmosphere as the (integrated) water vapor (English integrated water vapor , abbreviation IWV ) is available. The maximum achievable amount of precipitation or water column height is specified in millimeters (mm). If you translate the term precipitable waterdirectly from English into German, this is the provisional (not entirely correct) result “ rainwater ”. However, this is the amount of water that rain supplies in a defined period of time, regardless of the available rainwater.
application
The determination of the precipitation water that can be precipitated, i.e. of the possible, achievable precipitation values , is always important. In particular, weather services and climate researchers need this information in order to be able to create models and thus forecasts (examples):
- Mountain rescue and alpine rescue - snow reports and avalanche warnings
- Disaster control - flood warnings etc.
- General - Heavy rain warnings and other threats (e.g. hail )
example
In a cube with a volume of one cubic meter and a base area of one square meter, there is “humid” air close to saturation at 20 ° C, but no clouds or droplets. Since it is only a small cube, one can also assume a constant density. The water vapor present in the air in the cube weighs around 18 g. Now you stack 1,000 such cubes on top of each other (assuming that this is the thickness of a normal cloud and neglecting other variables such as pressure, temperature, etc.). The water vapor present in this tower with a height of 1,000 m is therefore 18 kg. If you could “squeeze out” this tower like a citrus fruit, you would get 18 kg or around 18 liters of water. These 18 liters of water in a basin with a floor area of 1 m² would result in a water level of 18 mm. Consequently, our cloud with a height of 1,000 m would result in precipitation water of 18 mm or 18 l / m² at the saturation point.
In real terms, the height to be included extends over the entire atmosphere and all essential parameters such as pressure, temperature etc. must be taken into account in the density function.
calculation
With the help of a density function of the water vapor, the complete water vapor of the overlying atmosphere (column) is integrated over a defined area and thus IWV is obtained
If you have calculated IWV, you need the density of water in order to get to the precipitation water PW
Measurement
There are different methods here:
- Measurement of the radiation intensity of two different wavelengths of the sun, one of which is absorbed by water. The calculation is based on the Lambert-Beer law .
- Radiosonde measurement ( relative humidity , air pressure and temperature ) and combination of all parameters in a density function, as well as integration over the complete height of the atmosphere (see above).