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A. F. G. Jacobs and K. G. McNaughton

Abstract

In outdoor experiments a temperature sensor is always subjected to a radiation load that results in a temperature excess error. Not only the mean temperature is effected by this radiation load but also, for a fast-responding sensor, the measured temperature fluctuations. In the present paper the mean excess temperature error is discussed and the fluctuation error is analyzed. A technique is presented for estimating the importance of this error in eddy correlation measurements as well as temperature variance measurements. It appears that the mean excess error as well as the fluctuation error can be considerably reduced by coating the sensor with while paint.

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S. Daniel Jacob and Lynn K. Shay

Abstract

Oceanic mixed layer (ML) response to Hurricane Gilbert in the western Gulf of Mexico is investigated in this paper using the Miami Isopycnic Coordinate Ocean Model (MICOM). Three snapshots of oceanic observations indicated that a Loop Current Warm Core Eddy (LCWCE) contributed significantly to the ML heat and mass budgets. To examine the time evolution of different physical processes in the ML, MICOM is initialized with realistic, climatological, and quiescent conditions for the same realistic forcing. The ML evolves differently for the realistic background condition with the LCWCE in the domain; differences between climatological and quiescent conditions remain small. Mixed layer temperature (MLT) and ML depth (MLD) differences of up to 1°C and 30 m are directly attributed to horizontal advective processes in the LCWCE regime due to preexisting velocities. Comparison of simulated temperatures using realistic conditions in the model shows improved agreement with profiler observations. Using four entrainment mixing parameterizations, the spatial and temporal ML evolution is investigated in MICOM simulations. Although the rates of simulated cooling and deepening differ for the four schemes, the overall pattern remains qualitatively similar. For the three schemes that use surface-induced turbulence to predict entrainment rate, the cooling pattern extends farther away from the track. Based on linear regression analysis, MLTs simulated using the bulk Richardson number closure fit the observed temperatures better than did the other schemes. Averaged surface fluxes ranged from 10% to 30% in the directly forced region, with larger values in the LCWCE regime. Overall, entrainment mixing remains the dominant mechanism in controlling the heat and mass budgets.

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Tido Semmler, Daniela Jacob, K. Heinke Schlünzen, and Ralf Podzun

Abstract

The Arctic plays a major role in the global circulation, and its water and energy budget is not as well explored as that in other regions of the world. The aim of this study is to calculate the climatological mean water and energy fluxes depending on the season and on the North Atlantic Oscillation (NAO) through the lower, lateral, and upper boundaries of the Arctic atmosphere north of 70°N. The relevant fluxes are derived from results of the regional climate model (REMO 5.1), which is applied to the Arctic region for the time period 1979–2000. Model forcing data are a combination of 15-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-15) data and analysis data. The annual and seasonal total water and energy fluxes derived from REMO 5.1 results are very similar to the fluxes calculated from observational and reanalysis data, although there are some differences in the components. The agreement between simulated and observed total fluxes shows that these fluxes are reliable. Even if differences between high and low NAO situations occur in our simulation consistent with previous studies, these differences are mostly smaller than the large uncertainties due to a small sample size of the NAO high and low composites.

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Jossy P. Jacob, Eric P. Chassignet, and William K. Dewar

Abstract

An analytical and numerical study of isolated coherent vortices and topography is presented. The motivation for this work comes from many observations of vortices influenced in trajectory, propagation, and decay by encounters with midocean ridges, seamounts, and bottom slopes. In particular, analytical predictions relevant to vortex propagation and evolution are compared with numerical results for lenses on bottom slopes and mixed barotropic–baroclinic eddies over a variety of topographies. The latter case includes examination of short-term and long-term behavior. Analytical theories are found to work well for the bottom lenses, and short-term behavior is captured well by a simple theory that emphasizes barotropic dynamics for mixed vortices. The exception for the latter case occurs for counterrotating eddies (i.e., eddies with opposing upper- and lower-layer swirl), for which the evolution is dominated by vortex instability. Long-term evolution has no comparable theory, and the various possibilities for vortex behavior are delineated by means of exploratory numerical work. A specific application to the case of North Brazil current rings, which are observed to move at anomalous rates, is presented.

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Tido Semmler, Daniela Jacob, K. Heinke Schlünzen, and Ralf Podzun

Abstract

The influence of two simple descriptions for the sea ice distribution on boundary layer values is investigated by comparing model results from the regional climate model REMO with measured data in the Fram Strait in April 1999. One method for determining the sea ice distribution in REMO is to diagnose the sea ice cover from the prescribed surface temperature and allow each grid cell to be either completely free of ice or completely covered by ice (REMO-original). The other one is to employ a partial sea ice concentration in each REMO grid cell with the input data derived from satellite data (REMO-partial). Surface fluxes are average values of the ice and water partial fluxes. There is a clearly better agreement between measured and simulated surface and boundary layer temperatures and humidities when using REMO-partial compared to REMO-original. The closed ice cover in REMO-original leads to downward sensible heat fluxes over ice, whereas the ice cover with leads and polynyas in REMO-partial leads to smaller downward or even upward sensible heat fluxes. The introduction of the partial sea ice concentration smoothes unrealistically sharp gradients between ice-covered and ice-free regions. which can influence cloud cover and precipitation. An additional result of the study is that the simulation of the albedo could be improved in allowing a larger range of sea ice albedos and introducing a water albedo dependent on sun zenith angle.

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Neil A. Jacobs, Daniel J. Mulally, and Alan K. Anderson

Abstract

A method for correcting the magnetic deviation error from planes using a flux valve heading sensor is presented. This error can significantly degrade the quality of the wind data reported from certain commercial airlines. A database is constructed on a per-plane basis and compared to multiple model analyses and observations. A unique filtering method is applied using coefficients derived from this comparison. Three regional airline fleets hosting the Tropospheric Airborne Meteorological Data Reporting (TAMDAR) sensor were analyzed and binned by error statistics. The correction method is applied to the outliers with the largest deviation, and the wind observational error was reduced by 22% (2.4 kt; 1 kt = 0.51 m s−1), 50% (8.2 kt), and 68% (20.5 kt) for each group.

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S. Daniel Jacob, Lynn K. Shay, Arthur J. Mariano, and Peter G. Black

Abstract

Upper-ocean heat and mass budgets are examined from three snapshots of data acquired during and after the passage of Hurricane Gilbert in the western Gulf of Mexico. Measurements prior to storm passage indicated a warm core eddy in the region with velocities of O(1) m s−1. Based upon conservation of heat and mass, the three-dimensional mixed layer processes are quantified from the data. During and subsequent to hurricane passage, horizontal advection due to geostrophic velocities is significant in the eddy regime, suggesting that prestorm oceanic variability is important when background flows have the same magnitude as the mixed layer current response. Storm-induced near-inertial currents lead to large vertical advection magnitudes as they diverge from and converge toward the storm track. Surface fluxes, estimated by reducing flight-level winds to 10 m, indicate a maximum wind stress of 4.2 N m−2 and a heat flux of 1200 W m−2 in the directly forced region. The upward heat flux after the passage of the storm has a maximum of 200 W m−2 corresponding to a less than 7 m s−1 wind speed.

Entrainment mixing across the mixed layer base is estimated using three bulk entrainment closure schemes that differ in their physical basis of parameterization. Entrainment remains the dominant mechanism in controlling the heat and mass budgets irrespective of the scheme. Depending on the magnitudes of friction velocity, surface fluxes and/or shear across the mixed layer base, the pattern and location of maximum entrainment rates differ in the directly forced region. While the general area of maximum entrainment is in the right-rear quadrant of the storm, shear-induced entrainment scheme predicts a narrow region of cooling compared to the the stress-induced mixing scheme and observed SST decreases. After the storm passage, the maximum contribution to the mixed layer dynamics is associated with shear-induced entrainment mixing forced by near-inertial motions up to the third day as indicated by bulk Richardson numbers that remained below criticality. Thus, entrainment based on a combination of surface fluxes, friction velocity and shear across the entrainment zone may be more relevant for three-dimensional ocean response studies.

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Lynn K. Shay, Arthur J. Mariano, S. Daniel Jacob, and Edward H. Ryan

Abstract

The three-dimensional hurricane-induced ocean response is determined from velocity and temperature profiles acquired in the western Gulf of Mexico between 14 and 19 September 1988 during the passage of Hurricane Gilbert. The asymmetric wind structure of Gilbert indicated a wind stress of 4.2 N m−2 at a radius of maximum winds (R max) of 60 km. Using observed temperature profiles and climatological temperature–salinity relationships, the background and storm-induced geostrophic currents (re: 750 m) were 0.1 m s−1 and 0.2 m s−1, respectively. A Loop Current warm core ring (LCWCR) was also located to the right of the storm track at 4–5 R max, where anticyclonically rotating near-surface and 100-m currents decreased from 0.9 m s−1 to 0.6 m s−1 at depth. The relative vorticity in the LCWCR was shifted below the local Coriolis parameter by about 6%.

In a storm-based coordinate system, alongtrack residual velocity profiles from 0 to 4 R max were fit to a dynamical model by least squares to isolate the near-inertial content over an e-folding timescale of four inertial periods (IP ≈ 30 h). Observed frequency shifts in the mixed layer currents ranged from 1.03 to 1.05f in agreement with both the backrotated velocity profiles at 1.04f relative to the storm profile (where maximum correlation coefficients were 0.8) and the predicted frequency shift from the mixed-layer Burger number. This frequency was increasingly blue shifted in the upper 100 m to 1.1f, decreasing toward f within the thermocline. Near-inertial currents rotated anticyclonically by 90°–180° in the upper ocean, providing the velocity shear for layer cooling and deepening observed on the right-hand side of the track. A summation of the first four baroclinic modes described up to 77% of this near-inertial current variability during the first 1.75 IP. However, the variance explained by this modal summation decreased to a minimum of 36% after 2.9 IP following passage due to phase separation between the first baroclinic mode and higher-order modes in the mixed layer. Although the response was complicated by the LCWCR, the evolving three-dimensional current structure can be described by linear, near-inertial wave dynamics.

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G. R. Halliwell Jr., L. K. Shay, S. D. Jacob, O. M. Smedstad, and E. W. Uhlhorn

Abstract

To simulate tropical cyclone (TC) intensification, coupled ocean–atmosphere prediction models must realistically reproduce the magnitude and pattern of storm-forced sea surface temperature (SST) cooling. The potential for the ocean to support intensification depends on the thermal energy available to the storm, which in turn depends on both the temperature and thickness of the upper-ocean warm layer. The ocean heat content (OHC) is used as an index of this potential. Large differences in available thermal energy associated with energetic boundary currents and ocean eddies require their accurate initialization in ocean models. Two generations of the experimental U.S. Navy ocean nowcast–forecast system based on the Hybrid Coordinate Ocean Model (HYCOM) are evaluated for this purpose in the NW Caribbean Sea and Gulf of Mexico prior to Hurricanes Isidore and Lili (2002), Ivan (2004), and Katrina (2005). Evaluations are conducted by comparison to in situ measurements, the navy’s three-dimensional Modular Ocean Data Assimilation System (MODAS) temperature and salinity analyses, microwave satellite SST, and fields of OHC and 26°C isotherm depth derived from satellite altimetry. Both nowcast–forecast systems represent the position of important oceanographic features with reasonable accuracy. Initial fields provided by the first-generation product had a large upper-ocean cold bias because the nowcast was initialized from a biased older-model run. SST response in a free-running Isidore simulation is improved by using initial and boundary fields with reduced cold bias generated from a HYCOM nowcast that relaxed model fields to MODAS analyses. A new climatological initialization procedure used for the second-generation nowcast system tended to reduce the cold bias, but the nowcast still could not adequately reproduce anomalously warm conditions present before all storms within the first few months following nowcast initialization. The initial cold biases in both nowcast products tended to decrease with time. A realistic free-running HYCOM simulation of the ocean response to Ivan illustrates the critical importance of correctly initializing both warm-core rings and cold-core eddies to correctly simulate the magnitude and pattern of SST cooling.

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W. J. Teague, G. A. Jacobs, H. T. Perkins, J. W. Book, K-I. Chang, and M-S. Suk

Abstract

High resolution, continuous current measurements made in the Korea/Tsushima Strait between May 1999 and March 2000 are used to examine current variations having time periods longer than 2 days. Twelve bottom-mounted acoustic Doppler current profilers provide velocity profiles along two sections: one section at the strait entrance southwest of Tsushima Island and the second section at the strait exit northeast of Tsushima Island. Additional measurements are provided by single moorings located between Korea and Tsushima Island and just north of Cheju Island in Cheju Strait. The two sections contain markedly different mean flow regimes. A high velocity current core exists at the southwestern section along the western slope of the strait for the entire recording period. The flow directly downstream of Tsushima Island contains large variability, and the flow is disrupted to such an extent by the island that a countercurrent commonly exists in the lee of the island. The northeastern section is marked by strong spatial variability and a large seasonal signal but in the mean consists of two localized intense flows concentrated near the Korea and Japan coasts. Peak nontidal currents exceed 70 cm s−1 while total currents exceed 120 cm s−1. The estimated mean transport calculated from the southwest line is 2.7 Sv (Sv ≡ 106 m3 s−1). EOF analyses indicate total transport variations in summer are due mainly to transport variations near the Korea coast. In winter, contributions to total transport variations are more uniformly distributed across the strait.

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