Unified Gravity Wave Physics in the NCEP Global Forecast System (GFS)
AGU Fall Meeting
10-14 December, 2017
New Orleans, LA, USA.
J. C. Alpert1 Valery A. Yudin2, T. J. Fuller-Rowell2, and R. A. Akmaev3
NOAA/NWS/NCEP/Environmental Modeling Center, NOAA Center Weather and Climate Prediction, College Park, Maryland 20740
Cooperative Institute for Research in Environmental Sciences, University of Colorado, NWS/SWPC , DSRC, Boulder, CO 80027
NOAA/NWS/NWS Space Weather Prediction Center, Boulder CO 80305
Gravity Waves in the GFS
Gravity waves are generated by a variety of sources including orographic
gravity waves (GWD) and other wave sources, for example, nonorographic GWD (nGWD), excited primarily by jets, fronts and convective
systems in the troposphere and lower stratosphere. The sources are highly
variable in space and time, implying similar variability for the generated
Gravity waves propagate vertically, eventually dissipate, and deposit
energy and momentum to the mean flow once they reach their critical
levels or become unstable. This process occurs on a variety of scales
by NWP by
which is the GWD
. The middle
topic here.of radiation and wave drag arising from the deposition of
momentum from the breaking of small-scale non-orographic gravity waves
and large-scale planetary waves
Underestimation of the poleward circulation between the summer and
winter hemispheres and downwelling over the winter pole inferred in the
GFS below with sub-grid scale GWD present but no nGWD, indicate that
forcing of the mean flow is unrealistically weak. If orographic GWD is
neglected it is associated with
excessively cold winter polar
tropospheric and stratospheric
The middle atmosphere is
dominated by a westerly jet in
the winter hemisphere, an
easterly jet in the summer
hemisphere, and a meridional
GFS T1534 initial conditions
circulation comprised of
averaged over 2 months (JJ2016),
(upper left) 10 mb Height [m],
(above) Zonal mean wind [m/s], and downwelling over the winter
pole, referred to as the Brewer
(lower) T (plots from G.White)
The NWS/NCEP Environmental Modeling Center
Next Generation Global Prediction System (NGGPS)
Atmospheric flow is significantly influenced by orography creating lift and
frictional forces with influence in numerical weather prediction models that are
necessarily divided into the resolvable scales of motion and treated by primitive
equations, and sub-grid scales that must be treated by parameterization.
In addition to orographic GWD the GFS physics parameterizes Mountain
Blocking of wind flow around sub-grid scale orography, a process that retards
motion at various model vertical levels near or in the boundary layer. Flow
around the mountain encounters larger frictional forces by being in contact with
the mountain surfaces for longer time as well as the interaction of the
atmospheric environment with vortex shedding.
Integrating Unified Gravity Wave Physics into the
The weather service GFS, is planning predictions into the upper atmosphere with
128 level Finite Volume (FV3) model compared to the current 64L in Operations. It
is the intention to converge the Whole Atmosphere Model (600km WAM) into this
Unification of models is in development to streamline the interaction of analysis,
forecast, and post-processing systems within NCEP. The NEMS architecture is a
foundation of the NGGPS.
GFS-extension above 50 km is needed to resolve the gravity wave field emerging
from the troposphere to play a critical role in the features of the strato-mesosphere
shown to drive QBO, SAO and tropospheric effects, e.g., stratosphere-troposphere
Integrating Unified Gravity Wave Physics into the NGGPS project has demonstrated
that with increased model vertical levels (L91) and with operational resolution and
low resolution experiments (T670) with the WAM model, improvements in the
stratosphere structure and climatology. Presently model experiments are running at (2)
high resolution (SLG T1534) and testing increased vertical resolution to FV3 128L.
GFSGSM-forecast in 64L & 91L models with Rayleigh
Friction and non orographic Gravity Waves
Issues/Metrics: Implementation of Rayleigh
Friction (wind damping) handles two issues:
(1) The top lid model effects, sponge layer to
suppress resolved wave reflections; (GFS-64L);
(2) The winter-summer zonal wind drag in the
Zonal mean flow:
GFS forecasts for Jun of 2014
20-day GFS forecasts from June 1 of 2014
Issues with RF-schemes:
vs MLS 2014-06-30 (zonal mean temperatures
- Erroneous reflections of Planetary Waves;
-Absence of the U-wind reversals above ~70-80 km;
-Warm mesosphere relative to EOS-Aura MLS and TIMED-SABER multi-year observations.
Advantage of GW-physics: handle above-listed GFS-biases.
2014 GFS-T574 forecasts w/ GW sources: Jan (15 day, Left) and Jun (25 day, Right)
Prediction is what makes EMC unique!
Integrating Unified Gravity Wave Physics into the NGGPS is a project to
develop the vertically extended configurations of GFS model across the
stratopause with realistic representations of sub-grid scale eddies by unified
Gravity Wave schemes. The goal is to improve the troposphere-stratosphere
coupling, predictors of Arctic and North Atlantic Oscillations and propagation
of atmospheric tides and planetary waves.
Vertical levels of FV3/GFS-128L follow IFS-91 or
137L and resemble GEOS-5/FV3 (72L, ~80 km);
Needs to be discussed:
- Sponge Layer Location (# of levels) in FV3
- Rayleigh Friction time scale
- Divergent damping of FV3
- Initialization of VE-models for FV3-dycore
Orchestration FV3-dissiaption with EDDY-mixing of
GWs and vertical Turbulent mixing
(moninq.f/EDMFcurrent choice for FV3-64L )
The above physical process needs to be calculated in
budget form for the upper atmosphere including
divergence damping and other dissipation and surface
processes such as energy balance and
Scale-aware adaptations of WAM unified nGWD
solvers with dissipation to FV3GF: =>Upgrades and
retirement of convective GWD and unification of
GWD, that is, a single SOLVER for orographic
stationary GWD and nGWD non-stationary sources of
What kind of GWs can resolve GEOS-5/FV3?
QBO and SAO in FV3/GFS -> for Reanalysis
Equatorial Dynamics in Vertically
Role of RF (wind damping), it attempts to resolve
-The top lid model effects, sponge layer to suppress
resolved wave reflections; (GFS-64L); extraheating;
-The winter-summer zonal wind drag in the stratomesosphere.
Vertically Extended Configuration under FV3-dycore:
NEMS/WAM-150L+ & FV3/GFS-128L (~80-90 km)
Sensitivity to specification of GW-sources: constant, time-lat & latitude-only dependent
Attempts to make QBO-like oscillations in
WAM-EUL forecasts at T62, T254, and
T382 were not successful.
Similar GW schemes placed to
CESM/CAM were able to reproduce
QBO-likes zonal wind oscillations
Moving to FV3 we can explore
QBO/SAO dynamics in 64L- and 127Lmodels with and w/o GW physics (later in
Desirable configuration: ~500m vertical
resolution between 100-10 hPa; Moderate
vertical diffusion ~ 0.01 m2/s; additional
tune-up of GW drag for IGWs in the
NEMS-WAM GW-physics in CAM83L with FV-dycore produce similar
tropical wind (QBO-like) oscillations
Compare with recent WACCM6
Summary and Future Plans: Moving forward to FV3GFS
The first implementation of GW physics in GFS-91L provides apparent
improvements of the global forecasts beyond 5-days relative to GFS-91RF and
Annual Run of FV3GFS-64L(65L) and its evaluation against GDAS, GEOS-5,
SABER & MLS
Generate ICs for extended Model runs: FV3GFS-TL80km (LTE-radiation)
FV3-TL100km, FV3WAM (non-LTE rad + extra UA phys).
GFS 20-day forecasts w/91L model with different RF and GW sources: June of 2014 Repeat Jan-Feb of 2016 NEMS/GSM-91L/TL80km -> NEMS/FV3GFS-TL80km
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