Proceedings - C-SRNWP Project

Physical Aspects
1) The introduction of cycle 31r1 involves a substantial number of changes in the
parametrized physics:
• Changes in the cloud scheme related to ice sedimentation, treatment of ice super-saturation
and the conversion to snow.
• Implicit computation of all convective transports.
• Introduction of the turbulent orographic form drag scheme (TOFD) to replace the effective
roughness length concept, exclusion of the blocked layer in the forcing of gravity waves
(cut-off mountain), and a combined implicit computation of turbulence, TOFD and subgrid
orographic drag.
• The wind gust parametrization (for post processing only) has a revised numerical treatment
which behaves better over orography and solves to a large extent the adverse interaction
with stochastic physics of the previous implementation.
• Air/sea interaction assumes 98% relative humidity at the ocean surface instead of 100% to
account for ocean salinity. Apart from cool skin and warm layer effects (which are included
in the code but not activated), this change brings the ECMWF scheme close to the
TOGA/COARE algorithm (Fairall et al. 1996) and results in a minor reduction of latent
heat flux over the ocean.
• The linear convection model, as used in the assimilation of rain-affected radiances, has
been improved.
The model changes improve physical realism (e.g. ice super saturation, salinity effects on
evaporation) and most of these changes are necessary for further development (e.g. TOFD to
allow work on atmosphere/vegetation exchange, implicit convection for numerical stability).
The impact is very positive on the tropical climate of the model which is clearly reflected in
radiation at the top of the atmosphere and in upper tropospheric winds. The impact on
medium range forecasts is mainly neutral except for some seasonal dependent positive signals
related to the surface drag over land and orographic effects.
2) New radiation scheme
A new version of the short-wave part of the radiation scheme RRTM is being developed. This
version uses the “Monte-Carlo independent column approximation” for the cloud overlap and
new cloud radiative properties. More work is needed to reduce the cost of the scheme which
uses 112 spectral intervals and increases the cost of the forecast model by 30%.
The scheme improves dramatically the radiative forcing in the model leading to an improved
climate.
3) New version of the linearized moist physics
The use of the new moist physics in the high-resolution minimization part of 4D-Var leads to
a larger decrease in the cost function. The influence on the verification scores over the
Northern Hemisphere is slightly positive but it improves substantially the scores over North
America in winter.
Work has started on the tangent-linear and adjoint of the surface parameterization.
4) New soil hydrology H-TESSEL
The new scheme uses a recent digital soil map of the world and shows an improved match to
soil temperature observations with respect to the previous scheme TESSEL.
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