Spectral microphysics in weather forecast models
with special emphasis on cloud droplet nucleation

Heat bubble over flat terrain

Setup
A well known test case for microphysics schemes is a heat bubble over flat terrain. We have used this bubble for testing both, the warm and the mixed phase microphysics scheme having been implemented in LM-SPECS, using the following setup:

• Initialization of atmosphere by idealized initial sounding after Weisman and Klemp [1].
• Initial temperature disturbance of up to 2 K.
• 80x80 gridpoints, horizontal resolution 1 km.
• 48 vertical layers, model top 18 km, resolution about 90 m near ground up to 600 m near top.
• The initial wind was set to zero.

Results

The applied temperature disturbance leads to the development of a deep convective cloud which heavily precipitates after a while. The picture to the right shows results from sensitivity studies with respect to the initial number concentration of aerosols (numbers to the right, denoted in cm^-3) and with respect to the heterogeneous ice nuclei type (see top of the diagrams). The solid black lines refer to the total liquid water content (the sum over the whole spectrum including rain drops) and the coloured contours refer to the total ice water content.



To the right and further below, mass spectra resulting from the warm case are shown for three different heights and three different times. At 12 minutes, only cloud droplets are present. After 24 minutes, rain drops start to form, especially in higher regions. The onset of rain drop formation for the most polluted case is delayed. After 36 minutes, all three cases precipitate heavily.



The resulting accumulated rain from the warm cases below the center point of the clouds is illustrated in the diagram to the left. The initial aerosol number density has a significant influence on the amount of precipitation - the cleanest case precipitates most, the most polluted precipitates least. This points to the importance of considering aerosol particles when it comes to a correct parameterization of rain formation.

The results of this studies have been submitted to Atmospheric Research (see Publications).
References
[1] Weisman and Klemp (1982), Mon. Wea. Rev., 110, 504-520.