P1.1 Climate Model Diagnoses from a Weather Modeler

1
P1.1 Climate Model Diagnoses from a Weather
Modelers Point of View Hua-Lu Pan and Suranjana
Saha Environmental Modeling Center NOAA/NWS/NCEP
Weather in the NCEP CFS model simulations and
forecasts
T126
It is becoming increasingly clear that good
climate models must also be good weather models.
What this means is that climate models must be
realistic in the simulations of the weather
events in the entire globe. Most of the current
climate models have very weak climate
variabilities especially in the tropical regions.
Since the most prominent interannual event that
are of interests to seasonal to interannual
climate forecasts is the ENSO and it takes place
in the tropical Pacifics, realistic variabilities
in the tropical weather simulations may be very
important to the prediction challenge of
ENSO.   In this study, we take a look at the
daily output from several simulation run of the
NCEP Climate Forecast System (CFS) model running
in fully coupled mode and study the realism of
the model simulated weather events. The most
prominent weather event in the tropical Pacific
is the trains of easterly waves which have been
extensively studied since the 1970s (Reed and
Recker, 1971, etc) Even in the early work of
Riehl (1945), these easterly waves have been
noted and studied. Easterly waves also exist in
the tropical Atlantics as well as equatorial
Africa. Many of the hurricanes that form in the
Atlantic can be traced to these easterly waves.
Many of the typhoons in the western Pacific can
also be traced to the easterly waves originating
from eastern or central Pacific. Another area of
interest in the tropics is the Indian monsoon
region.   We have been doing comparisons of the
tropical analyses from the NCAR/NCEP reanalysis,
the NCEP/DOE reanalysis-2, and the more recent
NCEP operational Global Data Assimilation System
products (GDAS). Our confidence in the daily
operational GDAS product is much higher since the
year 2000. All tropical storms are faithfully
captured in the GDAS (Albeit with weaker
intensity when the storm becomes hurricanes or
typhoons) while it is well known that the
reanalysis tends to be too weak on the tropical
variabilities and the reanalysis2 is too intense
with the tropical easterly waves. However, the
wind analyses of reanalysis2 at the 850 hPa and
the 200 hPa levels compares quite favorably with
the GDAS products. We believe that this is the
case because of the presence of the cloud-tracked
wind data at these two levels which forces the
analysis fields to reality. We are therefore
making good use of the 850 hPa wind analysis from
the reanalysis2.   One of the easiest way to
look at the easterly waves is to examine the
Hovmuller diagram of the 850 hPa meridional wind
field. Along a given latitude, the easterly wave
in the 850 hPa meridional wind will go from
northerly to southerly across the trough line
when traced from west to east. In the Hovmuller
diagram, the trough line will be seen to
propagate westward. In the following figures, we
will present the Hovmuller diagrams of the 850
hPa meridional wind field from the reanalysis2,
from the CFS model simulation in a free run at
the T62L64 resolution and from the
higher-resolution CFS model simulation free run
at the T126L64 resolution. It can been seen that
the model simulations have a lot of similarity to
the observed time series suggesting that the CFS
simulation of the tropical synoptic variability
is realistic.
The two figures on the left are two 2-year long
Hovmuller diagram from the reanalysis2 data The
two figures on the right are two 2-year long
Hovmuller diagram from the T62 and the T126
versions of the CFS In general, the model
simulation of the synoptic frequency seems
reasonable. The easterly waves are prominent
during the northern summer season while both
easterly and westerly disturbances can be seen in
both data and simulation during northern winter.
The T126 version of the CFS displays much more
realistic tropical storms as shown in the above
set of maps. In addition, the Indian monsoon
depression is also well simulated.
Together with the 850 hPa meridional wind fields,
we will also present the precipitation Hovmuller
diagrams. These figures show that the easterly
waves depicted by the GFS have a coherent
structure with the accompanying precipitation
fields. In nature, the major contributions to the
tropical precipitations are from these
disturbances and are episodic. It is possible
that the realistic simulation of such
disturbances are needed to generate the low
frequency responses of the tropical
ocean-atmosphere system such as the ENSO.
For a recent period, this figure shows that the
850 hPa meridional wind in the GDAS and in
reanalysis2 are quite similar
The two sets of figures above shows time series
at a given point from both the CFS62 hindcasts
and from the NCEP GDAS analyses. The episodic
nature of the time series from the hindcasts
resemble that of the analyses.
The above two sets of time series show the
2-meter temperature and the precipitation at
points in the maritime continent from the CFS
(T62) displaying synoptic signals. It is clear
that the precipitation patterns are more episodic
during northern summer.

• Summary and discussion
• The tropical weather features are represented in
  both the CFS hindcasts and the CFS simulations.
  Due to the coarse resolution nature of the CFS at
  T62, the tropical disturbances are less realistic
  than the T126 version. This is to be expected.
• The episodic nature of the tropical easterly
  waves and the tropical storms are part of the
  nature and may be the so-called stochastic
  forcing on the climate signal. The CFS model
  exhibits the episodic synoptic-scale disturbances
  similar to nature.
• The actual strength and phase speed of the
  simulated disturbances need to be studied against
  the observed ones. Here we are hampered by the
  question on the validity of the reanalysis. Only
  in the last few years have the NCEP operational
  analysis (GDAS) of the tropics showed realism. We
  need a better reanalysis to help us decide the
  realism of the model simulations of the tropics.
• We are continuing this effort to examine the
  details of the CFS simulation of the tropical
  disturbances. We plan to study the frequency of
  occurrence of these disturbances and the more
  intense tropical cyclones for the Pacific, the
  Atlantic and the Indian oceanic regions. We will
  explore the relationship of the tropical
  variability in the ENSO, the MJO and the synoptic
  time scales through further diagnostic work in
  the future

This sequence of maps show the formation of a
tropical storm in west Pacific in the T62 version
of the CFS. At the same time, it also shows
storms formation in the Indian sub-continent. The
T62 version of the CFS is very active in the
Indian Ocean
The above Hovmuller diagrams showed the v850
field from GDAS and from the CFS hindcasts