> **Improving the GFDL/URI Coupled Hurricane-Ocean Model for**

**Transition to Operations**

# Semi-Annual Progress Report

Isaac Ginis

Graduate School of Oceanography

University of Rhode Island

February 6, 2004

## 1 Introduction

This is the first semi-annual report for this program. During this time
period, significant progress has been made toward our overall project
objectives, which are to:

-   To improve the ocean component of the GFDL/URI coupled model in the
    Atlantic basin and implement the ocean coupling into the GFDL model
    used for operational forecasting in the East Pacific

-   To evaluate and transition to operations a new high resolution
    version of the GFDL/URI coupled model

-   To test and implement operationally new air-sea flux
    parameterizations in the coupled model

We have finished development and testing of explicit initialization of
the location and structure of the Gulf Stream and Loop Current in the
GFDL/URI coupled model. It is planned to be implemented into operations
in 2004. Coupling of the GFDL model with 1-D ocean model in the Eastern
Pacific has been implemented and initial testing has begun. The
high-resolution version of the GFDL hurricane has implemented at NCEP
computers and is presently being tested for hurricanes in the 2003
season. Sensitivity studies were conducted to evaluate the sensitivity
of the GFDL model hurricane predictions to various surface flux
parameterizations.

We now briefly elaborate on the present status of the project, including
personnel and recent results.

## 2 Personnel, Logistics and Facilities

There are three scientists committed to this project: Isaac Ginis and
Biju Thomas at URI and Alexandr Falkovich at NCEP. We work in close
collaboration with Morris Bender and Tim Marchok at GFDL and NCEP
scientists. The majority of the model development is done on the URI
Silicon Graphics (SGI) ORIGIN 2000, 10-processor computer. The more
computer-intensive numerical simulations and testing of the operational
version of the GFDL/URI coupled model are performed on the GFDL and NCEP
supercomputers.

## 3 Tasks Completed and Work in Progress

*a.* *Improving the ocean model initialization in the Atlantic*

We have developed a new data ocean assimilation and initialization
procedure to improve simulation of the Gulf Stream (GS) and Loop Current
(LC) in the GFDL/URI operational coupled hurricane prediction system.
This procedure is based on feature modeling and involves cross-frontal
"sharpening" of the background temperature and salinity fields according
to data obtained in specialized field experiments in the GS. We are
presently testing it for past hurricanes in the Atlantic basin using the
data for the 2002 and 2003 hurricane seasons. Fig. 1 shows an example of
the surface ocean currents after ocean model initialization during a
forecast of Hurricane Isidore initiated at 12 UTC, September 19, 2002.
The developed methodology allows initializing the LC and GS with any
given paths in the GFDL/URI hurricane model. In order to take advantage
of this technology satellite-retrieved sea surface height (SSH) data
available at the time of a hurricane forecast can be used in future.

![](media/image1.png){width="3.24375in" height="2.752083333333333in"}

Fig. 1 Surface current after ocean model initialization during Hurricane
Isidore forecast (initial time: 12Z, September 19, 2002). White line
indicates the GS North Wall in September.

*b.* *Implementation of ocean coupling in the East Pacific*

We have developed a 1-D ocean model for its coupling with the GFDL
hurricane model in the Eastern Pacific. The 1-D model was derived from
the 3-D Princeton Ocean Model (POM) used in the GFDL/URI coupled model
in the Atlantic. It was first tested in the Atlantic basin for
simulations of the ocean response to hurricanes and results were
compared with similar simulations using the POM. For the Eastern
Pacific, the ocean model is configured on a 40 x 40 degree relocatable
grid with a horizontal resolution of 1/6 degree and 16 levels in
vertical. The center of the grid coincides with the center of the GFDL
hurricane model's outer mesh which is determined at the beginning of
each forecast. The monthly Levitus climatology is used to specify
initial temperature and salinity fields.

We have recently coupled the 1-D ocean model with the GFDL hurricane
model and started to test the new coupled system. Fig. 2 shows an
example of the coupled model forecast of Hurricane Ignacio initiated at
1200 UTC Aug 24.

![](media/image2.png){width="1.9638888888888888in"
height="2.175in"}![](media/image3.png){width="2.029861111111111in"
height="2.173611111111111in"}![](media/image4.png){width="1.8756944444444446in"
height="2.201388888888889in"}

Fig. 2 The GFDL/URI coupled model forecast of Hurricane Ignacio
initiated at 1200 UTC, Aug 2003: Sea surface temperature in the
operational, uncoupled model (left panel), coupled model (middle panel)
and central pressure forecast (right panel; red -- operational uncoupled
model, green - coupled model, black -- observations).

*c.* *Investigation of the momentum flux under hurricane conditions*

In strong winds, typical under hurricane conditions, momentum exchange
at the sea surface should be described by a sea-state-dependent drag
coefficient. However, presently the GFDL hurricane model, as most
atmospheric models do, utilizes the bulk parameterization, i.e., the
boundary layer parameterization based on the Monin-Obukhov similarity
theory with the behavior of the drag coefficient (C~d~ =
u~\*~^2^/U~10~^2^) based on extrapolations from field measurements in
much weaker winds. These extrapolations describe an increase in C~d~
with wind speed. Recent theoretical, laboratory and observational
studies suggest that the C~d~ levels off or even decrease as the wind
speed increases above hurricane force.

Using a Coupled Wave-Wind CWW) model developed in our group recently
(Moon et al. 2003) we began investigating the effects of waves on the
momentum fluxes in high wind conditions. The CWW model was applied for
constant winds from 10 m/s to 50 m/s. We found for growing seas with
winds less than 30 m/s, Cd increases with wind speed, consistent with
earlier studies. For winds higher than 30 m/s, however, the CWW model
produces a different trend, that is, very young waves yield less drag
and Cd levels-off and even decreases. This is because the wave-induced
stress due to very young waves makes a small contribution to the total
wind stress in extremely high wind conditions. The upper and lower
bounds of C~d~ in our experiments are shown in Fig. 3. Our results are
consistent with the recent observations of Powell et al. (2003).

![](media/image5.png){width="2.4916666666666667in" height="3.4375in"}

Figure 3. Comparison of drag coefficient (C~d~) between various
observation-based values, formulas, and model outputs as a function of
U~10~. Symbols represent observations from GPS sonde wind profiles
(Powell et al. 2003) above hurricane forces. Vertical bars represent 95%
confidence limits. Solid line is an extrapolation of the Large and Pond
(1981) formula. Dash-dot line is the Bulk formula used in GFDL model.
Shaded and hatched areas represent ranges between upper and lower bound
of C~d~ obtained by the URI coupled wave-wind model and internal
estimation of NCEP WAVEWATCH model for hurricane Bonnie in 1988,
respectively. Both observations and URI model results show leveling--off
of the drag coefficient as the wind speed increases.

# 

# 

# *d. Sensitivity of GFDL hurricane intensity prediction to momentum flux parameterization*

We started experiments with the GFDL/URI coupled hurricane-ocean model
to investigate sensitivity of the hurricane intensity to the momentum
flux parameterization at the sea surface. The air-sea exchange in the
operational GFDL/URI model is parameterized using the Monin-Obukhov
similarity theory as described in Kurihara and Tuleya (1974). Shown in
Fig. 4 is a set of idealized experiments in which an idealized hurricane
is embedded into a 5 m/s easterly flow. In the control run model physics
is the same as in the operational GFDL/URI model. In another run, the
roughness length, z~0,~ is set constant (capped) for wind speeds \> 35
m/s (this is consistent with the results from our CWW model). The
capping of z~0~ at high wind speed leads to a substantial increase of
maximum surface wind (Fig. 3a) and a reduction of wind stress and heat
flux (Fig. 4b & 4c). The changes of the wind speed extend through the
boundary layer (Fig. 4d). Therefore, we may conclude that the GFDL/URI
model intensity predictions measured by the surface winds could be
significantly affected by improved parameterization of the air-sea
exchange at high wind speeds.

![](media/image6.png){width="4.510416666666667in"
height="3.5680555555555555in"}

Fig. 4. Evolution with time of (a) surface wind speed, (b) wind stress,
and (c) latent heat flux, and (d) vertical wind profiles at 24 hours,
averaged around the maximum wind point, obtained by two idealized
sensitivity simulations using the GFDL/URI hurricane model. The
"Control" run uses the exchange coefficients for heat and momentum
fluxes as in the operational version of the model which are based on
parameterization described in Kurihara and Tuleya (1974). The "Cap on
z~0~" run uses the same coefficients, but z~0~ is kept constant at wind
speeds of above 35 m/s.

**References**

Moon I.-J., I. Ginis, T. Hara, 2003. The effect of waves on air-sea
momentum exchange: Behavior of drag coefficient under tropical cyclones,
*J. of Atmos. Sci.,* under review.

Kurihara, Y. and R.E. Tuleya, 1974. Structure of a tropical cyclone
developed in a three-dimensional numerical simulation model. *J. of
Atoms. Sci.,* **31***,* 893-905.

Powell, Mark D., P. J. Vickery, and T. A. Reinhold, 2003. Reduced drag
coefficient for high wind speeds in tropical cyclones. *Nature*,
**422**, 279-283.