This version fixes a problem that occured with 4.20, that the amount of precipitation was much too low. Reasons for that were within the latent heat nudging, but also within the microphysics. Besides that, the Smagorinsky diffusion for u and v has been implemented.
mu_rain | 0.0 |
rain_n0_factor | 1.0 |
mu_rain | 0.5 |
rain_n0_factor | 0.1 |
In order to correct quickly for the negative impact of the bug fix in 4.20 and nevertheless a better accounting for blacklist information and bright band detection the check of those flags has been moved from Subroutine lhn_obs_prep to Subroutine lhn_pr_ana. In the case that the observation is blacklisted or detected as bright band the analysed precipitation rate is set to the modelled precipitation rate. As the analysed precipitation rate will be used for comparison with the modelled precipitation rate further on, no LHN will take place at those grid points.
This will be a temporally change until the approach is redesigned in a more sophisticated way.
Changes in diagnostics:
numblack = SUM(blacklist(:,:)) instead of counting it during a loop.
Sometimes the COSMO-Model aborts due to horizontal shear instabilities. Some of these crashes can be avoided by the 4th order artificial horizontal diffusion with a prescribed constant diffusion coefficient. But in rare events this diffusion is not strong enough and a more physically based diffusion mechanism is necessary.
The nonlinear Smagorinsky diffusion (Smagorinsky (1963) MWR) determines the diffusion coefficient by the horizontal shear (and tension) strain and therefore acts in particular to reduce too strong horizontal shear.
It is switched on by the new DYNCTL namelist parameter l_diff_Smag=.TRUE. (otherwise it has no impact to the results). (Default is .FALSE.)
An internal parameter, the Smagorinsky constant, is currently set to c_smag=0.03. This value is chosen to prevent shear instabilities in COSMO-DE applications, but otherwise to influence as less as possible the verification scores.
Another option was added for computing the maximal wind gusts (itype_diag_gust=4), where the gust factor weakly depends on the mean wind speed at 10 meters. This method is also used in GME.
This subroutine has been updated to reflect recent changes in the microphysics (SR hydci_pp and hydci_pp_gr, provided by Axel Seifert. The reflectivities of cloud ice and cloud droplets have been added to radar_lm_ray, based on a monodisperse size distribution with D_i = 200 (cloud ice) and D_i = 20 (cloud droplets) microns.
A bug has been fixed: A factor involving N0_snow (n0s(1-p_s)) was doubled and lead to underestimation of dbz_snow of 40-50 dB. This has been corrected.
The changes in the microphysics and the Latent Heat Nudging again do change the results slightly.
Also the Smagorinsky diffusion does change the results, if it is switched on. If the new option for the wind gust computation is activated, the wind gusts will change.
There were the following changes for the Namelist variables:
Group | Name | Meaning | Default |
---|---|---|---|
/DYNCTL/ | l_diff_Smag | to switch on/off the Smagorinsky diffusion | .FALSE. |
/TUNING/ | rain_n0_factor | to reduce the evaporation of raindrops (0.0 ≤ rain_n0_factor ≤ 1.0) |
1.0 |
mu_rain Changed default value! |
shape parameter of the rain drop size distribution (0.0 ≤ mu_rain ≤ 5.0) New default value is 0.0 |
0.0 |
The namelist parameter mu_rain can now also be used for the subroutine hydci_pp, which is used in coarser grid applications. Because these applications are still in the majority, we decided to set the default of mu_rain for these applications, which is 0.0! For higher resolutions, which do use the graupel scheme (subroutine hydci_pp_gr), a value of mu_rain=0.5 is recommended and has to be changed in the namelists!!!