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James W. Stevenson

Abstract

A Kraus-Turner (1967) seasonal mixed-layer model with constant background dissipation is used to study the mixed-layer response to the vertical motions of linear Rossby waves that are not driven by direct local atmospheric forcing. The model equations are solved using analytical and simple numerical techniques. A strong seasonal variation in the response to Rossby waves is found. The mixed layer buoyancy (or sea surface temperature) response is a maximum toward the end of the heating season or beginning of the cooling season (early fall). The mixed-layer depth response is largest at the end of the cooling season. The sea surface temperature (SST) pattern in response to the two baroclinic Rossby waves that McWilliams and Flierl (1976) fit to the MODE data is very different from the temperature pattern at a fixed depth below the mixed layer. While the shape of the “eddies” as modeled by the two Rossby waves stays fixed, the shape of the SST response changes with time. In addition the SST response propagates at about half the speed of the Rossby waves. While the SST response initially leads the forcing it finally lags behind the forcing. Using the POLYMODE XBT data it is concluded that the mixed-layer response to the vertical motions of “eddies” could be important in the fall in the POLYMODE region of the North Atlantic Ocean.

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James W. Stevenson

Abstract

This paper considers several parameterizations of dissipation in a Kraus-Turner (1967) seasonal mixed-layer model with no net annual surface buoyancy flux. By studying the behavior of the turbulent kinetic energy equation near the end of the cooling season it is determined for a given parameterization of dissipation whether or not the mixed layer deepens to infinite depth at the end of the cooling season. It is concluded that the following two conditions together form a sufficient condition for the maximum mixed-layer depth to be finite. First, the dissipation must be able to balance the wind generation of turbulent kinetic energy in the absence of a surface buoyancy flux. Second, the dissipation must exactly balance the turbulent kinetic energy generated by the wind and released by convection near the end of the cooling season. For two parameterizations of dissipation, cyclic solutions of the Kraus-Turner model are obtained. With the constant background dissipation used by Niiler (1977) the mixed-layer depth stays finite; whereas, with the parameterization of dissipation used by Kamenkovich and Khar'kov (1975) the mixed layer deepens to infinite depth at the end of the cooling season. It is also mentioned that with the parameterizations of dissipation used by Gill and Turner (1976) and Elsberry et al. (1976) the mixed layer deepens to infinite depth at the end of the cooling season. Finally, the potential usefulness of these findings for numerical ocean modeling is discussed.

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James W. Stevenson
and
Pearn P. Niiler

Abstract

Heat flux, CTD and current profile data from the Hawaii-to- Tahiti Shuttle Experiment are used to study the upper ocean heat budget in order to better understand the seasonal evolution of sea surface temperature (SST) in the central tropical Pacific Ocean between February 1979 and June 1980. The surface heat flux is estimated using bulk formulas and the standard meteorological data taken aboard ship. Upper ocean heat storage is computed from CTD data in such a way (using temperature vertically averaged between the sea surface and fixed isotherm depths) as to filter internal waves. It is found that the surface heat flux plays a large role in the seasonal evolution of SST. A time-latitude correlation coefficient of 0.70 is found between the surface heat flux and heat storage. The seasonal evolution of the vertically averaged temperature whose time rate of change determines storage is very closely correlated with the seasonal evolution of SST.

At 155°W, there is no evidence for a relation between changes of main thermocline depths and changes in SST. Also, we see no feedback from the ocean to the atmosphere through SST governed heat flux. Horizontal heat advection is estimated from Firing et al. profiling current meter data. The advection of cold water from the east is important in the 15-cruise (16-month) mean but the data are too noisy to estimate the seasonal evolution of heat advection.

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Galina Chirokova
,
John A. Knaff
,
Michael J. Brennan
,
Robert T. DeMaria
,
Monica Bozeman
,
Stephanie N. Stevenson
,
John L. Beven
,
Eric S. Blake
,
Alan Brammer
,
James W. Darlow
,
Mark DeMaria
,
Steven D. Miller
,
Christopher J. Slocum
,
Debra Molenar
, and
Donald W. Hillger

Abstract

Visible satellite imagery is widely used by operational weather forecast centers for tropical and extratropical cyclone analysis and marine forecasting. The absence of visible imagery at night can significantly degrade forecast capabilities, such as determining tropical cyclone center locations or tracking warm-topped convective clusters. This paper documents ProxyVis imagery, an infrared-based proxy for daytime visible imagery developed to address the lack of visible satellite imagery at night and the limitations of existing nighttime visible options. ProxyVis was trained on the VIIRS day/night band imagery at times close to the full moon using VIIRS IR channels with closely matching GOES-16/17/18, Himawari-8/9, and Meteosat-9/10/11 channels. The final operational product applies the ProxyVis algorithms to geostationary satellite data and combines daytime visible and nighttime ProxyVis data to create full-disk animated GeoProxyVis imagery. The simple versions of the ProxyVis algorithm enable its generation from earlier GOES and Meteosat satellite imagery. ProxyVis offers significant improvement over existing operational products for tracking nighttime oceanic low-level clouds. Further, it is qualitatively similar to visible imagery for a wide range of backgrounds and synoptic conditions and phenomena, enabling forecasters to use it without special training. ProxyVis was first introduced to National Hurricane Center (NHC) operations in 2018 and was found to be extremely useful by forecasters becoming part of their standard operational satellite product suite in 2019. Currently, ProxyVis implemented for GOES-16/18, Himawari-9, and Meteosat-9/10/11 is being used in operational settings and evaluated for transition to operations at multiple NWS offices and the Joint Typhoon Warning Center.

Significance Statement

This paper describes ProxyVis imagery, a new method for combining infrared channels to qualitatively mimic daytime visible imagery at nighttime. ProxyVis demonstrates that a simple linear regression can combine just a few commonly available infrared channels to develop a nighttime proxy for visible imagery that significantly improves a forecaster’s ability to track low-level oceanic clouds and circulation features at night, works for all current geostationary satellites, and is useful across a wide range of backgrounds and meteorological scenarios. Animated ProxyVis geostationary imagery has been operational at the National Hurricane Center since 2019 and is also currently being transitioned to operations at other NWS offices and the Joint Typhoon Warning Center.

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