Geoscience Reference
In-Depth Information
mm/day
100
60°N
50
°
W
50
20
50°N
40
°
W
10
40°N
1
0.1
30
°
W
20
°
W
10
°
W
0
°
Fig. 11.7
Map of 3-h precipitation accumulations ending at 0000 UTC 10 February 2009 and used
for computing the precipitation cost function inside the target black box over northwestern Europe.
Grey shading
shows precipitation (in mm day
1
), while
black isolines
of mean-sea-level pressure
are also plotted (in hPa)
vertical diffusion (dry and acting mainly close to surface) and surface drag scheme
(
Buizza 1994
) had to be used to avoid strong wind increments close to the surface.
Precipitation and cloud related observations have not been taken assimilated in
order to use the same type of observations in both experiments. Indeed, without
the linearized moist physics in 4D-Var, cloud and precipitation observations cannot
be assimilated since no observation equivalent can be produced from the model.
Including physical processes in the linear model of 4D-Var not only decreases
the background cost function (measuring the distance between the initial state of
the model and the background), but also brings model closer to observations as
indicated by the general decreased observation cost function (measuring the distance
between the model trajectory and corresponding observations) as seen in Fig.
11.9
.
Thus the distance between the model and the observations is better optimized when
the linearized physics is used in the 4D-Var minimization.
The significance of the impact coming from including the linearized physical
parametrization schemes in the 4D-Var system on the subsequent forecast is
illustrated in Fig.
11.10
for the period of July-September 2011. The forecasts are
scored against operational analyses in terms of anomaly correlation. A systematic
and significant improvement for all plotted parameters, levels and regions is clearly
obvious. Close to analysis time (where obviously the impact of the linearized
physics in 4D-Var should be the largest), the biggest improvement is found in the
middle and upper troposphere (e.g. 200 hPa wind vector scores) and overall in the
Search WWH ::
Custom Search