The introduction to Mr. Songs manuscript
Original
Air-sea interaction plays a very important
role in the coupled ocean-atmosphere system. Because the ocean surface
represents a large potential source of sensible heat and moisture, variations in
marine atmospheric boundary layer (MABL) structure in the vicinity of horizontal
gradients in sea surface temperature (SST) are expected be apparent. In the
Joint Air-Sea Interaction (JASIN) Experiment, some effects on the MABL due to
SST changes were observed, particularly in the wind stress and low-level cloud
structure as observed from aircraft measurements and ships (Guymer et al, 1983).
Guymer observed that fluxes modulated by mesoscale pattern in SST. Other air-sea
interaction observations, such as the Agulhas Current Air-Sea Exchange
Experiment (Lee-Thorp et al, 1999), also showed that the SST front produces
distinct signature in the marine boundary layer thermal, moisture and wind
fields.
Satellite-borne scatterometer and infrared
data collected over Gulf Stream indicate that the scatterometer wind field is
strongly modified near the sharp horizontal gradients of SST (SST front) and
Gulf Stream warm/cold core rings (Park, Cornillon, 2001). The direction of the
wind vector inside the Gulf Stream warm core ring is deflected to the right
associate an accelerated wind speed when compared with the outside wind field
over the relatively cold water.
The overall objective of the Frontal
Air-Sea Interaction Experiment (FASINEX) was to study air-sea interaction on
1100 km horizontal scales in a region characterized by strong horizontal SST
gradients. Friehe et al studied the modification of the lower MABL by a sharp
front with SST changes of up to several degrees Celsius per 10 km located in
Sargasso Sea (1991) with aircraft and ships. When the wind blew from the warm to
the cold side with a direction near perpendicular to the front, surface
stability became stable or near neutral with a decrease in wind speed.
In order to better understand the
modification to the scatterometer-derived wind field as it passes over the edge
of the Gulf Stream we would like to examine the effect of an SST front on a
homogeneous wind field using a full dynamics meteorological numerical model that
contains the marine boundary layer. For this work we have chosen the PSU/NCAR
mesocale model MM5 model. The first step in our analysis was to determine how
well the MM5 model behaves in the vicinity of an SST front; i.e., does the model
contain the dynamics needed for our work. To this end we compare MM5 model
predictions with observations made during FASINEX at the meso-gamma-scale
(220 km) (Friehe, 1991).
During FASINEX, changing synoptic weather
on 3 successive days provided cases of wind direction both approximately
parallel and perpendicular to a SST front. On February 18, with the wind blowing
from the warm to the cold water, ship soundings were made on each side of SST
front at 0000UTC and 1500UTC. The detailed observations of wind speed,
direction, stress, stress direction, heat flux, and water vapor flux from the
aircraft Electra at 30 m height acrossing the SST front were also made during
that day (Friehe, 1991).
The spatial resolution of scatterometer
wind is 25 km, while the resolution of AVHRR SST is near 1 km. In order to
resolve We are particularly interested in the meso-gamma-scale (120
km).
The next section describes the
modifications that we made to the MM5 preprocessing steps which are used to
initialize the model as well as the model setup. The model results are compared
with the observations in Section 3.
Revised
The ocean and the atmosphere interact
strongly. Because the ocean surface represents a large potential source of
moisture and sensible heat, the structure of the marine atmospheric boundary
layer (MABL) should vary near horizontal gradients of sea-surface temperature
(SST). Some effects of this type were seen during the Joint Air-Sea Interaction
(JASIN) Experiment, particularly in wind stress and low clouds observed from
aircraft and ships (Guymer et al, 1983), as well as modulations in heat fluxes
produced by mesoscale variations in SST. SST fronts have also been shown to
produce distinct signatures in the fields of heat, moisture, and wind in the
marine boundary layer (Agulhas Current Air-Sea Exchange Experiment, Lee-Thorp et
al., 1999). Strong modifications in wind fields near SST fronts and
warm/cold-core rings in the Gulf Stream have been revealed by scatterometer and
infrared data from satellites (Park, Cornillon, 2001), with winds inside
warm-core rings being accelerated and deflected to the right relative to winds
over colder water.
Air-sea interaction on horizontal scales
of 1100 km were studied in a region of strong horizontal SST gradients by the
Frontal Air-Sea Interaction Experiment (FASINEX). Modifications of the lower
MABL by a sharp front in the Sargasso Sea whose SST changed by up to several
degrees Celsius per 10 km was observed by aircraft and ships (Friehe et al.,
1991). When surface air moved perpendicularly across the front from warm to
cold, its speed decreased and it became neutral or stable.
In order to better understand the
modification to the scatterometer-derived wind field as it passes over the edge
of the Gulf Stream we have used a fully dynamic meteorological model with an
explicit marine boundary layer to examine how an SST front affects a homogeneous
wind field. We chose the PSU/NCAR mesocale model MM5. We first determined how
well the model behaved near an SST front; i.e., whether it contained the
necessary dynamics. To do this, we compared its predictions at the meso-γ-scale
(220 km) with observations during FASINEX by Friehe, 1991.
Changes in synoptic-scale weather on three
successive days of FASINEX provided wind directions parallel and perpendicular
to an SST front. On February 18, with the wind blowing from warm to cold water,
ship soundings were made on each side of the front at 0000 and 1500 UTC. Wind
speed, direction, stress, stress direction, heat flux, and water vapor flux were
observed in detail across the front on the same day from an Electra aircraft at
30-m height (Friehe, 1991).
The next section describes the
modifications that we made to the MM5 preprocessing steps that are used to
initialize the model [as well as the model setup] ??. The third section compares
the model results to the observations.
[Note sure where to put this.] The scatterometer resolves wind to 25 km, while AVHRR resolves the SST to 1 km. We are particularly interested in the meso-γ-scale (120 km).