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Effects of numerical methods on high resolution modelling
Holger Göttel
Max Planck Institute for Meteorology, Bundesstr. 53, 20146 Hamburg, Germany, holger.goettel@zmaw.de
1. Introduction
One of the remaining challenges in climate predictions as
well as in weather forecasts is the correct modelling of
heavy precipitation. The reason is the high temporal and
spatial variability of cloud formation and precipitation.
Models with a resolution of several kilometres (50 to 10 km
in regional climate models) are not able to resolve small
scale features like convective clouds explicitly. To account
for these features they use convective parameterisations
which are normally only based on few observations. It is
unclear that these parameterisations are still valid under
climate change conditions and for all current convection
types due to the limits of observations.
One solution is to increase the horizontal resolution to few
kilometres where the model is able to resolve convective
clouds explicitly. The increased computer power in the last
decade allows an increase of resolution from 50 to 10
kilometres. A further increase of resolution, which might be
possible in the near future, has consequences for some
simplifications in state of the art climate models. These
approximations (e.g. the hydrostatic balance and the neglect
of advection of precipitation) are not realistic at these scales.
With the regional climate model REMO – a hydrostatic
model which was extended with an evolutionary approach
(Janjic et al., 2001) to a non-hydrostatic model – the limits
of the Tiedtke scheme for convective systems in cold-airoutbreaks
and the ability of the non-hydrostatic model to
simulate such extreme events are investigated.
2. REMO
The regional climate model REMO (Jacob, 2001; Jacob et
al., 2001; Jacob and Podzun, 1997) is a three-dimensional,
hydrostatic atmospheric circulation model which solves the
discretised primitive equations of atmospheric motion. Like
most other RCMs, REMO has been developed starting from
an existing numerical weather prediction (NWP) model: the
Europa-Modell (EM) of the German Weather Service DWD
(Majewski, 1991). Additionally, the physical
parameterisation package of the general circulation model
ECHAM4 (Roeckner et al., 1996) has been implemented,
optionally replacing the original EM physics. In numerous
studies, the latter combination (i.e., the EM dynamical core
plus the ECHAM4 physical parameterisation scheme)
proved its ability to realistically reproduce regional climatic
features and is therefore used as the standard setup in recent
applications, including the present study.
This hydrostatic model was extended in this study to a nonhydrostatic
model using the evolutionary approach proposed
by Janjic et al. (2001). Additionally, a diagnostic advection
scheme for precipitation is implemented which can be used
online and offline (Göttel, 2009).
3. Results
Figure 1. AVHRR channel 5 retrieval from 17
February 1997
Figure 2. HOAPS estimated precipitation fluxes
[mm/h] valid for 9:00 UTC 17 February 1997
The AVHHR (Figure 1. ) show clouds in the postfrontal
area due to a cold air outbreak over the open ocean. The
cold air outbreak induces convection with intense
precipitation. The precipitation estimates of HOAPS
(Figure 2. ) reaches in the postfrontal area precipitation
similar values like in the cold front of the low “Caroline”.
This precipitation which is validated by ship observations
doesn’t appear in the ECMWF model as well as in the
regional model REMO (Figure 3. ). The missing
precipitation in the models is caused by deficits in the
cloud convection scheme. A re-calculation of this event
with the non-hydrostatic version of REMO shows, that
this model is able to reproduce postfrontal precipitation
(Figure 4. ).