Proceedings - C-SRNWP Project

ALARO-0 Physics developments at LACE in 2006
Neva Pristov, Ján Mašek, Jure Cedilnik, André Simon
In this paper the developments in some physics parameterization, made by scientists from the
LACE consortium are presented. The contributions are commonly termed ALARO-0.
ALARO is seen as a continuous transition from ARPEGE/ALADIN towards AROME
(continuity and improvements), while 0 indicates a base-line version and also that the
development is scale-independent. With this approach to physics we also aim to treating the
processes in the 'grey-zone', i.e. around 3-7 km mesh size. Emphasize is given to economical
computation, numerical efficiency and algorithmic flexibility and this combined with the
developments in physics is supposed to make a good basis for further developments.
Inside LACE group main developments were made inside the radiation (Mašek and Geleyn,
2006, Pristov, 2006) and turbulence scheme (Geleyn et al.,2006, Simon, 2006). Short
description is included in this paper. Moist processes parameterizations (3MT Modular Multiscale
Microphysics and Transport framework) are mostly developed in ALADIN non-LACE
countries. A lot of effort was made for including all the developments into the base-line code,
bearing in mind the computational efficiency. Here it can be pointed out (among others) that
i) new prognostic and diagnostic variables (TKE, hydrometeors, cloud fraction for the
updrafts/downdrafts, convective vertical velocity of the updraft/downdraft, ...) have been
included and that ii) a way of describing the water cycle, where the interaction between the
various parameterisations’ takes place, is neither fully parallel nor fully sequential.
1 Radiation
Current radiation scheme divides electromagnetic spectrum in only 2 bands - solar and
thermal. In each band it applies δ-two stream approximation of radiative transfer equation. A
new algorithm for cloud transmissivity and reflectivity, and a more complex statistical
method for the weighting function between maximum and minimum inter-layer gaseous
exchange terms are the main improvements and are shortly presented.
1.1 Parameterization of cloud optical properties for ALARO-0
Availability of prognostic cloud water and ice calls for more sophisticated treatment of cloud
optical properties. Main objectives were to introduce dependency of absorption coefficient
k abs and scattering coefficient k scat on cloud water content and to make so called saturation
effect dependent on cloud thickness and geometry. In order to keep the new scheme as cheap
as possible, division of spectra in just two bands (solar and thermal) was preserved.
Dependency of absorption and scattering coefficients on cloud water content was fitted using
experimental sample of 7 liquid and 16 ice clouds. In each spectral band, fitting was done for
number of individual wavelengths, using simple linear regression with suitable scaling.
Unsaturated broadband values k abs scat
0 , k 0 were then evaluated as weighted averages over
given spectral band. Used weights were either incoming solar flux at top of atmosphere (solar
band) or blackbody radiation with reference temperature 255.8 K (thermal band). Final
dependency of broadband optical coefficients k abs scat
0 , k 0 on cloud water content was fitted
using Pade approximants of order at most (3, 3), again with suitable scaling.
Saturation effect was evaluated explicitly in idealized framework. It employed δ-two stream
radiative transfer model with N layers. Each layer could contain cloud free and cloudy part,
overlaps between adjacent cloud layers being either maximum or random. Effect of gases and
249