Operational NWP System - ICON - DWD

Forschung und Entwicklung - Abteilung Meteorologische Analyse und Modellierung
                                    Operationelles NWV-System Änderungsmitteilung

                          Operational NWP System - ICON

On Tuesday, 19 May 2020, the following modifications in the global data assimilation
system and the ICON model will become effective with the 09 UTC assimilation run:

1) Use of global Aeolus horizontal line-of-sight (HLOS) winds in the global
assimilation system

On 22 August 2018, the first European Space Agency (ESA) Earth Explorer satellite
mission Aeolus was successfully launched, providing the first globally distributed
profiles of horizontal wind information. On board of Aeolus, an active remote sensing
UV laser sends out pulses into the atmosphere and measures the change in
frequency of the backscattered light (Doppler shift) due to the motion of scattering air
molecules (Rayleigh scattering) and aerosol/cloud particles (Mie scattering). In a
variety of processing steps the Doppler shift is converted into a horizontal line-of-
sight wind speed. Responsible for the processing is the ECMWF where the Aeolus
level 2b HLOS winds are produced and distributed via ftp and at a later stage over
GTS via Eumetsat/DWD. The first Aeolus HLOS winds arrived in the observation
data base of DWD in December 2018. The complexity of the Aeolus wind
measurements required a considerable upgrade of the data assimilation system. A
new processing scheme, including a new observation operator, a bias correction
scheme, and a new observation error scheme had to be set up. In several monitoring
and impact experiments a clear benefit of using Aeolus HLOS wind data in the
analyses and forecasting system of DWD was found. The largest positive impact was
detected, as expected, in the upper troposphere and lower stratosphere of the
tropics. However, a substantial positive impact was also found in the Northern and
Southern Hemisphere (Fig. 1). One of the latest experiments showed some
indications that the Aeolus wind data can, to a small extent, compensate for the loss
of aircraft observation over Europe during the Covid-19 pandemic.

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Figure 1: Comparison of observation departure (obs-FG) errors between an experiment using Aeolus
wind data and control run for radio occultation data as zonal mean (left). Green and blue colors
indicate an improvement with respect to radio occultation data in the experiment using Aeolus winds.
On the right, a summary of the relative forecast improvement against radiosonde data is depicted for
an experiment period of Jan. 2 – Feb. 7, 2020. The variables considered here are wind direction (DD),
wind speed (FF), relative humidity (RH), temperature (T), dew point (TD) and geopotential (Z). Green
bars indicate an improvement by using Aeolus data.

2) Use of radiosonde descent data over Germany in the global assimilation
system

In Germany and some other European countries the Vaisala RS 41 is the standard
radiosonde providing vertically ascending profiles of temperature, pressure, wind and
humidity two or four times a day. In addition, the RS 41 is able to provide the same
meteorological elements from the descent. Observations from the descent phase of
all German radiosonde stations are collected and delivered, together with some
stations from the UK and Switzerland, over the GTS into the observation data base
system of DWD, from where it can be used by the data assimilation system. The
quality of the radiosonde descent observations is comparable to the observations
from radiosonde ascents (Fig. 2). For wind speed, the measurement error for descent
observations is even smaller than for ascent observations, and solely the
temperature bias in the stratosphere is larger for descent data. Consequently, the
temperature observations from radiosonde descents are rejected above the 70 hPa
level. In a NUMEX experiment and during the parallel routine a small positive impact
could be found.
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Figure 2: Comparison of radiosonde observation minus first guess (3-hourly forecast) standard
deviation for ascending profiles (red) and descending profiles (blue) averaged over all German
stations and for a three-week period. Left for wind speed and right for temperature.

3) Introduction of a new formulation of the surface temperature in TERRA: The
skin temperature

A realistic representation of the land surface processes in atmospheric models like
ICON is essential. However, the diurnal temperature range at the surface, i. e. the
difference between the daily maximum and minimum temperature, simulated by the
multi-layer land surface scheme TERRA, was found to be systematically
underestimated. On the other hand, the diurnal temperature range in the soil is
overestimated. Consequently, also other components of the energy and water cycles
at the surface exhibit systematic model errors. This applies to e. g. the turbulent heat
fluxes or the soil water content. This is partly due to the fact that in TERRA the
vegetation is not sufficiently represented in the surface energy balance. The plants
can not have a different (canopy) temperature than the ground. The incoming solar
radiation is directly used to heat the soil, there is no shading exerted by the
vegetation, insulating the ground against the solar radiation.

In order to improve these systematic model deficiencies, the so-called skin
temperature formulation (Viterbo and Beljaars, 1995) was selected and implemented
in TERRA (Schulz and Vogel, 2020). It comprises a canopy description of
intermediate level of complexity, which is consistent with the one of TERRA and does
not introduce several new parameters which need to be determined, like other more
complex schemes would do.

In the current model version of TERRA, the surface temperature Ts is represented by
the temperature of the uppermost soil layer. The surface energy balance equation
has the following form:

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where Cs is the heat capacity per unit area, and t is the time. RSW and RLW are the net
shortwave and longwave radiation flux, respectively, while LE and H denote the latent
and sensible heat flux, and G the ground heat flux.

The new description of the surface temperature in TERRA is based on the skin
temperature formulation by Viterbo and Beljaars (1995). They introduced an
additional temperature of the leaves or the canopy, the skin temperature Tsk, as a
representation of the vegetation in the surface energy balance:

The behaviour of this equation is mainly determined by the new parameter Λsk, the
skin layer conductivity. Large values represent a strong coupling between the skin
temperature Tsk and the surface temperature Ts of the mineralic soil, their diurnal
cycles stay similar. In contrast, small values of Λsk describe a weak coupling, the
diurnal cycle of Tsk can become considerably larger than the one of Ts. The leaves
can become warmer during day, and cooler during night. This can improve the
surface temperature simulated by the soil-vegetation system. In the current
implementation of the skin temperature formulation in TERRA, grid elements covered
with snow are excluded. They can be included as part of a further model
development.

The effects of the new formulation of the skin temperature were investigated with the
ICON global atmospheric model. Two experiments were carried out, the first one with
the unmodified model as reference, and the second one using the new formulation.
Figure 3 shows a comparison of this experiment with the reference simulation for the
European domain. The experimental period is 16 May to 16 June 2019. The figure
shows the domain-averaged diurnal cycles of the bias and the root mean square
error of the 2-m temperature, its minimum and maximum values, and the 2-m dew
point temperature, averaged over the whole period. The reference model version
shows a substantial warm bias at night which is considerably reduced in the
experiment. A pronounced cold bias during daytime in the reference model is slightly
improved in the experiment. Furthermore, a substantial moist bias is reduced,
particularly at night. The root mean square errors of the temperature and humidity
fields are significantly reduced, in particular during the first three days of the
simulations.

References:
Schulz, J.-P. and G. Vogel, 2020: Improving the processes in the land surface scheme TERRA: Bare
soil evaporation and skin temperature. Atmosphere, accepted.

Viterbo, P. and A. C. M. Beljaars, 1995: An improved land surface parameterization scheme in the
ECMWF model and its validation. J. Climate, 8, 2716–2748.

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Figure 3: European domain-averaged diurnal cycles the mean error (ME), or bias, and the root mean
square error (RMSE) of the atmospheric temperature (T2M, left panels), minimum temperature
(TMIN_12H, second panels), maximum temperature (TMAX_12H, third panels) and dew point
temperature (TD2M, right panels), all at 2 m above ground, averaged over the whole experimental
period from 16 May to 16 June 2019. The ICON reference version is depicted in black, the experiment
in red. Filled circles indicate that the differences are statistically significant.

4) Use of analyzed 2m-temperature bias to vary uncertain parameters affecting
evapotranspiration also in EPS

Since July 30, 2019, the analyzed 2m-temperature bias providing input for the soil
moisture analysis is used in addition to vary some uncertain model parameters
affecting bare soil and plant evaporation in the determinstic part of ICON. This was
found to reduce the RMSE of 2m temperature and humidity by 1-2% in regions or
time periods with significant model biases. This coupling is now extended to the
ensemble members of ICON both in the assimilation cycle and the forecasts,
showing improvements in the 2m-variables of the same order as previously found in
the deterministic part.

In case of possible questions please contact:

Alexander Cress (Tel.: ++49 69 8062-2716, E-Mail: Alexander.Cress@dwd.de) for 1)
and 2)
Jan-Peter Schulz (Tel.: -2751, E-Mail: Jan-Peter.Schulz@dwd.de) for 3)
Günther Zängl (Tel.: -2728, E-Mail: Guenther.Zaengl@dwd.de) for 4)

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