overview motivation of the new reference atmosphere and description
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15.09.2008 Günther Zängl, DWD 1
A new reference atmosphere for the COSMO model
Günther Zängl
Deutscher Wetterdienst, Offenbach, Germany
15.09.2008 Günther Zängl, DWD 2
Overview
Motivation of the new reference atmosphere and description
Tests considering its impact, including a comparison between Leapfrog and Runge-Kutta cores
Comparison with impact of unapproximated pressure tendency equation (Lucio Torrisi; see subsequent talk)
15.09.2008 Günther Zängl, DWD 3
New reference atmosphere - motivation
The existing reference atmopshere, which is based on the assumption , has the inconvenient property that dT0/dz
gets increasingly negative in the stratosphere This severely limits the allowable vertical extent of the model
domain; for the default values used in COSMO, T0 reaches 0 K at
a height of 28.9 km Another physically questionable property is that the reference
pressure is still nonzero ( 1.05 hPa) where T0 = 0 K
10
lnc
pd
dT
15.09.2008 Günther Zängl, DWD 4
New reference atmosphere - definition
The new reference atmosphere is based on
This expression also allows for an analytical integration of the
hydrostatic equation, yielding
and
Present default values: T00 = 213.15 K, ΔT = 75 K, H = 10 km
H
zTTzT exp)( 000
TT
TTH
z
TR
gHpzp
d 00
00
00000
explnexp)(
TTTp
p
gH
TR
TTTpT
d
)(lnexp
)(
0000
000
000000
15.09.2008 Günther Zängl, DWD 5
New reference atmosphere (cont’d)
As the new reference profile approaches an isothermal stratosphere, there is no longer a limit to the vertical extent of the model domain (and it is much closer to reality)
In the context of implementing the new reference atmosphere, an inconsistency in the calculation of the reference pressure at full levels was discovered: it is taken to be the arithmetic mean of the adjacent half levels, but this is also done for geopotential height in the RK core
Instead, we now use either the analytical formula or integrate the hydrostatic equation to get the full-level reference pressure
15.09.2008 Günther Zängl, DWD 6
Test program
Selected cases: 03/03/2008, 30/05/2008 (each +72h) Leapfrog core (operational configuration) Runge-Kutta core (standard implementation) RK core using discretized hydrostatic equation to compute full-
level reference pressure RK core using analytical formula to compute full-level reference
pressure RK core with new reference atmosphere, combined with both
consistent methods to compute reference pressure Unapproximated pressure tendency equation (L. Torrisi) Generally: Initialization from GME assimilation run (cold start),
lateral boundary conditions from operational GME forecast
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COSMO-EU „cold start“ forecast initialized at 00 UTC 03/03/2008, operational setup, validation against GME assimilation run t + 12h
Sea-level pressure Pressure difference forecast-analysis
15.09.2008 Günther Zängl, DWD 8
Sea-level pressure Pressure difference forecast-analysis
COSMO-EU „cold start“ forecast initialized at 00 UTC 03/03/2008, operational setup, validation against GME assimilation run t + 24h
15.09.2008 Günther Zängl, DWD 9
Sea-level pressure Pressure difference forecast-analysis
COSMO-EU „cold start“ forecast initialized at 00 UTC 03/03/2008, operational setup, validation against GME assimilation run t + 36h
15.09.2008 Günther Zängl, DWD 10
Sea-level pressure Pressure difference forecast-analysis
COSMO-EU „cold start“ forecast initialized at 00 UTC 03/03/2008, operational setup, validation against GME assimilation run t + 48h
15.09.2008 Günther Zängl, DWD 11
Sea-level pressure Pressure difference forecast-analysis
COSMO-EU „cold start“ forecast initialized at 00 UTC 03/03/2008, operational setup, validation against GME assimilation run t + 60h
15.09.2008 Günther Zängl, DWD 12
Sea-level pressure Pressure difference forecast-analysis
COSMO-EU „cold start“ forecast initialized at 00 UTC 03/03/2008, operational setup, validation against GME assimilation run t + 72h
15.09.2008 Günther Zängl, DWD 13
Comparison between reference run (operational setup; right) and experiment with (unmodified) RK core t + 12h
Runge-Kutta Leapfrog
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Runge-Kutta Leapfrog
Comparison between reference run (operational setup; right) and experiment with (unmodified) RK core t + 24h
15.09.2008 Günther Zängl, DWD 15
Runge-Kutta Leapfrog
Comparison between reference run (operational setup; right) and experiment with (unmodified) RK core t + 36h
15.09.2008 Günther Zängl, DWD 16
Runge-Kutta Leapfrog
Comparison between reference run (operational setup; right) and experiment with (unmodified) RK core t + 48h
15.09.2008 Günther Zängl, DWD 17
Runge-Kutta Leapfrog
Comparison between reference run (operational setup; right) and experiment with (unmodified) RK core t + 60h
15.09.2008 Günther Zängl, DWD 18
Runge-Kutta Leapfrog
Comparison between reference run (operational setup; right) and experiment with (unmodified) RK core t + 72h
15.09.2008 Günther Zängl, DWD 19
Runge-Kutta new Leapfrog
Comparison between reference run (operational setup; right) and experiment with RK core and new reference atmosphere (discrete hydrost. eqn.) t + 12h
15.09.2008 Günther Zängl, DWD 20
LeapfrogRunge-Kutta new
Comparison between reference run (operational setup; right) and experiment with RK core and new reference atmosphere (discrete hydrost. eqn.) t + 24h
15.09.2008 Günther Zängl, DWD 21
LeapfrogRunge-Kutta new
Comparison between reference run (operational setup; right) and experiment with RK core and new reference atmosphere (discrete hydrost. eqn.) t + 36h
15.09.2008 Günther Zängl, DWD 22
LeapfrogRunge-Kutta new
Comparison between reference run (operational setup; right) and experiment with RK core and new reference atmosphere (discrete hydrost. eqn.) t + 48h
15.09.2008 Günther Zängl, DWD 23
LeapfrogRunge-Kutta new
Comparison between reference run (operational setup; right) and experiment with RK core and new reference atmosphere (discrete hydrost. eqn.) t + 60h
15.09.2008 Günther Zängl, DWD 24
LeapfrogRunge-Kutta new
Comparison between reference run (operational setup; right) and experiment with RK core and new reference atmosphere (discrete hydrost. eqn.) t + 72h
15.09.2008 Günther Zängl, DWD 25
Temporal evolution of pressure bias, case 1 (3/3/08)
15.09.2008 Günther Zängl, DWD 26
RMS error (after bias removal), case 1 (3/3/08)
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Temporal evolution of pressure bias, case 2 (30/5/08)
15.09.2008 Günther Zängl, DWD 28
RMS error (after bias removal), case 2 (30/5/08)
15.09.2008 Günther Zängl, DWD 29
Summary of test results
The new reference atmosphere tends to improve the pressure forecast; in both cases, the results are similar to those obtained with the old leapfrog core
The way of computing the reference pressure (analytical / numerical integration of the hydrostatic equation) has a marked systematic impact on the pressure bias
The unapproximated pressure tendency equation affects the temporal evolution of the pressure bias; however, more tests are needed for quality assessment
Tests (not shown here) with higher model top (28 instead of 22 km) do not indicate a systematic impact
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