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Treatment of large scale scavenging in LMDZ.

Various schemes may be used depending on the value of the flag 
iflag_lscav:

   iflag_lscav=0 ==> no large scale scavenging
   iflag_lscav=1 ==> Genthon (1992) scheme
   iflag_lscav=2 ==> Genthon (1992) scheme modified by Ph. Heinrich
   iflag_lscav=3 ==> Reddy-Boucher (2004) scheme
   iflag_lscav=4 ==> Reddy-Boucher (2004) scheme modified by R. Pilon

The basic equations of the last scheme are in the appendix of:
Pilon et al (2014) Representation of transport and scavenging of trace 
particles in the Emanuel moist convection scheme.  Q. J. Roy. Meteorol. 
Soc., 141, 1244-1258.

In the last two schemes, aerosols are scavenged in-cloud by 
condensation, scavenged below-cloud by impaction, transported by 
precipitation, and released by evaporation. The flux of aerosols at the 
surface due to large scale precipitation (the wet deposition rate due to LS 
precipitation) is presently calculated as the integral of the tendency of 
aerosol concentration due to LS condensation processes. Of course this 
procedure ensures that there is  conservation of aerosols in the large 
scale condensation scheme. However, the concentration of aerosols in the 
precipitation is actually computed (variable qPrls in phytrac) and could 
also be used to compute the large scale wet deposit. This is not what is 
done in the present version of LMDZ.

Original author: Jean-Yves Grandpeix

Other keywords: lessivage, phytrac