3.2.6 Air-Sea interactions and coupled ocean-atmosphere response on TIVs
(Bourras, Menkes, Eymard, Richards, Vialard, Weill).
* Data analysis.
Air-sea interactions will be studied on the basis of in situ flux measurements performed
onboard l’Atalante. Turbulent fluxes will be derived using eddy correlation and inertial
dissipation techniques (Dupuis et al. 1997, Weill et al. 2002). The purpose of the flux
measurements is twofold, namely improving bulk flux parameterizations and validating
satellite flux products.
# Flux Parameterizations
Bulk parameterizations are reasonably accurate at wind speeds ranging from 4 to 8 m/s, and in
near neutral conditions. However, several aspects of the Monin and Obukhov theory (bulk
theory) such as the value of the drag coefficient at low wind speeds, its relation to stability,
sea state, and even biology, or the effect of horizontal heterogeneity on the calculation of the
fluxes are still open questions.
The sea experiment will provide a very good opportunity to work on these subjects for two
reasons. Not only MATI will be ideally located for having low winds (Tropics) as well as sea
state and stability variations (presence of large SST gradients), but it will most of all occur at
a time when the French workgroup on air-sea fluxes has made considerable improvements in
interpretation of data and validation of the flux instruments. Indeed, the effect of flow
distortion around the body of the ship on measured winds and fluxes is better understood after
the works of Nacass et al. (2002), and Butet et al. (2002) on data of the FETCH experiment.
Next, the INSU refractometer (developed at CETP), from the measurements of which specific
humidity fluctuations are derived, was recently upgraded and is now fully operational as
revealed by a validation campaign conducted at the Wharf de la Salie, France, in November
2003 (CAMPINTER, Weill et al. in preparation).
A new technique is being developed at CETP for deriving sea state estimates for ship heave
observations inferred from an inertial platform (Hauser et al. 2001, Schulz et al. in
preparation). This technique should be applied to MATI data. The estimates should be
compared (for validation) to sea state data measured by a SPEAR buoy. The resulting sea
state estimates will be used for creating a bulk parameterization of the fluxes.
Sea state will also be a subject of study through its link with biology. It is hypothesized that
the presence of plankton can modify the mechanical properties of the sea surface such as
surface tension, which in turn would affect the characteristics of waves (at low wind speeds).
During MATI, the amount of plankton will be measured, and the relationship between
plankton and sea state will be investigated.
The impact of horizontal heterogeneity in wind, temperature, and humidity on the calculation
of bulk fluxes will be another field of investigations. Indeed, horizontal homogeneity of the
mean flow is one of the fundamental hypotheses of the Monin-Obukhov theory that is often
violated when fluxes are calculated. That is, bulk fluxes should not be calculated when
horizontal gradients of wind, temperature, and humidity are different from zero. Geernaert
(2003) started to address this issue, but his study was restricted to the impact of varying fetch
on the drag coefficient, in coastal areas. Since strong SST, wind, and air temperature gradients
systematically occur in the vicinity of TIVs, their impact on fluxes will be investigated when
the ship crosses SST fronts (section xx).
# Satellite derived fluxes
Specific satellite flux products will be developed for the tropics. Next, these flux products
will be validated against flux measurements performed onboard L’Atalante.
In order to derive net heat flux fields from satellite sensor data, several retrieval algorithms
can be used (Jones et al, 1999, Schulz et al. 1997, Bourras et al. 2002, and LeHugeur 2003).
The main weakness of the flux algorithms in the tropical areas is that they use as inputs a
combination of measurements from several instruments, including space-borne microwave
and infrared radiometers. The latter sensors are extremely sensitive to the cloud amount, while
the former are sensitive to the water vapor and/or liquid water content in the atmosphere.
Consequently, the existing algorithms will be adapted to the specific conditions of the tropics.
Part of the improvements planned are already described in Bourras (2003, PNTS proposal). In
the context of MATI, they consist in modifying retrieval algorithms of qA, latent heat flux,
and sensible heat flux.
For qA, the synergistic use of microwave and infrared sensors will be investigated, as
already successfully initiated by Wick (2003) and Porcu et al. (1999). For the latent heat flux,
the Bourras et al. (2002) algorithm, which is based on a neural network, will have its
coefficients optimized for the tropics (TAO data will have a large weight in the learning
dataset). Finally, the sensible heat flux algorithm of LeHugeur (2003) will use the improved
qA algorithm (above), and its coefficients (cooling rate, vertical flux gradient) will be fitted to
TAO data.
* Modeling aspects
There are numerous evidences that TIWs affect the organization of the atmospheric
boundary layer (e.g. Hashizume et al., 2002). However, the process through which the
atmosphere responds to the SST gradients is still a topic of active research, as one can not
firmly state whether it is based on the effects of pressure gradient forces or horizontal
gradients of turbulent mixing (e.g. Lindzen and Nigam, 1987, Hashizume et al. 2002, and
Cronin et al., 2003). The validity of both assumptions will be investigated. In this goal,
radiosonde data will be launched from l’Atalante, for studying the dynamical vertical
structure of the boundary layer. Additionally, the three dimensional structure of the MATI
IOP will be reconstructed by using an atmospheric model (MM5, Dudhia 1993) in which
satellite data as well as analyzed SST fields will be assimilated.
Marked TIW variability in the atmosphere (Chelton et al., 2000, Liu et., 2000, Hashizume
et al., 2002, 2001) suggest that there may be coupled feedbacks on TIV dynamics. However,
initial experiments suggest that such coupling is weak (Pezzi et al., 2003). Further
investigation is required to fully demonstrate this result. Coupled OPA ocean-atmosphere
simulations have been conducted using statistical SST/stress relationships on TIW scales
calculated from TMI/QSCAT data (Pezzi et al., 2003). Thus far, there is a weak retroaction of
the atmosphere to the oceanic properties of the TIVs. However, there is ample room for
improvements in the crude ocean/atmosphere parameterization that we can explore.
Specifically, the imprint of the surface currents on the stress calculations were not considered
in the first configuration. Subsequent work will include them to understand how the surface
current affect the instability characteristics. This will be pursued in collaboration with K
Richards (University of Hawaii).
This work addresses mainly questions 4-5.
---------------------------------------------------------------------------------------------------------------Experimental strategy
Effects of horizontal heterogeneity on the calculation of the fluxes
It is expected that the ships will cross two large SST fronts (10°C per 100km) along their
clockwise Lagrangian survey around the TIV. A hundred kilometers ahead of each SST front,
the following strategy will be applied. Both the US and French ships will start measuring
coincidently the mean variables (wind speed and direction, temperature, pressure, humidity,
and SST) at the same location for one hour. Next, the French ship will depart from its
position, measuring continuously the fluxes plus the meteorological variables. For half an
hour, the French ship will stay on the other side of the front, which will allow us to derive first
estimates of wind, temperature, an humidity gradients. Finally, the US ship will cross the
front, and its data will be used to validate the spatial gradients found using data from the
French ship. Subsequent lab work will consist in relating the spatial gradients to the difference
between bulk flux estimates and eddy correlation and inertial dissipation fluxes.
Response of the boundary layer : vertical sounding
Eighty one radiosondes (RS) will be launched during the 22 days of the experiment. From the
point of view of atmospheric sounding, the IOP will be split into five periods. Two periods
(IOP2-4) that correspond to the crossing of the two SST fronts, and three remaining periods
(IOP1-3-5). During IOP1-3-5, 3 RS will be launched every day (21). On the other hand,
IOP1-2 will consist in two periods of 5 days each, during which 6 RS will be launched each
day (in order to have diurnal variability).
IOP1
Duration:
RS per day:
3
SST Front 1
IOP2
5 days
6
IOP3
3
SST Front 2
IOP4
5days
6
IOP5
3
---------------------------------------------------------------------------------------------------------------DETAILED METHODS AND MEASUREMENTS
Physics
0-400 T, S during CTD casts. ADCP (current and backscatter for biological purposes),
Salinometer, SVP-WOCE floats. An instrument mast will be installed on the ship to measure
the air-sea flux measurements by inertial dissipation and eddy correlation methods (Weill et
al., 2002, Dupuis et al., 1997, Delahaye et al., 2001). The flux measurement instruments
consist in the following set : sonic anemometer (temperature and wind speed), microwave
refractometer (humidity fluctuations, see 3.2.6), meteorological station, inertial platform (6
degrees of freedom), Lymann alpha or KH20 (humidity). Acquisition rate : 50 Hz.
References
Bourras, D., 2003: Développement de nouveaux algorithmes pour restituer le flux net à la
surface des océans, submitted to PNTS.
Butet, A., 2002 : Distorsion du vent autour du N.O. Thalassa. Atelier de Mosélisation et
d’Expérimentation de l’INSU, Toulouse.
Cronin, M. F., S.-P., Xie, and H. Hashizume, 2003: Barometric Pressure Variations
Associated with Eastern Pacific Tropical Instability Waves, J. Climate, 3050-3057.
Dudhia, J., 1993: A Nonhydrostatic version of the Penn State/NCAR mesoscale model:
Validation tests and simulations of an Atlantic cyclone and cold front. Mon. Wea. Rev., 121,
1493-1513.
Geernaert , G. L., 2002: On extending the Flux Profile Similarity Theory to include Quasi
Homogeneous Conditions in the Marine Atmospheric Surface Layer, Boundary Layer
Meteorology, 433-450.
Hauser D.: Presentation of the FETCH experiment , in preparation for Jour. Geophys. Res. ,
2001
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