Raindrop size distribution
According to the time spent in the cloud, the vertical movement in it and the ambient temperature, drops have a very varied history and a distribution of diameters from a few micrometers to a few millimeters.
The most well-known study about raindrop size distribution is from Marshall and Palmer done at McGill University in Montréal in 1948.
Where As the different precipitations (rain, snow, sleet, etc...), and the different types of clouds that produce them vary in time and space, the coefficients of the drop distribution function will vary with each situation.
The Marshall-Palmer relationship is still the most quoted but it must be remembered that it is an average of many stratiform rain events in mid-latitudes.
[4] The upper figure shows mean distributions of stratiform and convective rainfall.
The bottom one is a series of drop diameter distributions at several convective events in Florida with different precipitation rates.
We can see that the experimental curves are more complex than the average ones, but the general appearance is the same.
Many other forms of distribution functions are therefore found in the meteorological literature to more precisely adjust the particle size to particular events.
Over time researchers have realized that the distribution of drops is more of a problem of probability of producing drops of different diameters depending on the type of precipitation than a deterministic relationship.
[4] The Marshall and Palmer distribution uses an exponential function that does not simulate properly drops of very small diameters (the curve in the top figure).
Several experiments have shown that the actual number of these droplets is less than the theoretical curve.
Carlton W. Ulbrich developed a more general formula in 1983 taking into account that a drop is spherical if D <1 mm and an ellipsoid whose horizontal axis gets flattened as D gets larger.
It is mechanically impossible to exceed D = 10 mm as the drop breaks at large diameters.
From the general distribution, the diameter spectrum changes, μ = 0 inside the cloud, where the evaporation of small drops is negligible due to saturation conditions and μ = 2 out of the cloud, where the small drops evaporate because they are in drier air.
Different devices have been developed to get this distribution more accurately: Knowledge of the distribution of raindrops in a cloud can be used to relate what is recorded by a weather radar to what is obtained on the ground as the amount of precipitation.
We can find the relation between the reflectivity of the radar echoes and what we measure with a device like the disdrometer.
): The radar reflectivity Z is: Z and R having similar formulation, one can solve the equations to have a Z-R of the type:[5] Where a and b are related to the type of precipitation (rain, snow, convective (like in thunderstorms) or stratiform (like from nimbostratus clouds) which have different
[6] It is still one of the most used because it is valid for synoptic rain in mid-latitudes, a very common case.
