Search Results

You are looking at 1 - 10 of 15 items for

  • Author or Editor: A. Birol Kara x
  • All content x
Clear All Modify Search
A. Birol Kara, Peter A. Rochford, and Harley E. Hurlburt

Abstract

Efficient and computationally inexpensive simple bulk formulas that include the effects of dynamic stability are developed to provide wind stress, and latent and sensible heat fluxes at the air–sea interface in general circulation models (GCMs). In these formulas the exchange coefficients for momentum and heat (i.e., wind stress drag coefficient, and latent and sensible heat flux coefficients, respectively) have a simple polynomial dependence on wind speed and a linear dependence on the air–sea temperature difference that are derived from a statistical analysis of global monthly climatologies according to wind speed and air–sea temperature difference intervals. Using surface meteorological observations from a central Arabian Sea mooring, these formulas are shown to yield air–sea fluxes on daily timescales that are highly accurate relative to those obtained with the standard algorithm used by the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE), where the latter includes the effect of dynamic stability in calculating wind stress and air–sea heat fluxes. Direct comparisons in calculating the wind stress, and latent and sensible heat fluxes with these formulas and the TOGA COARE algorithm demonstrate that the methodology presented here is computationally inexpensive because iterative calculations are not required in the present methodology. Wind stress and air–sea fluxes can be calculated ≈30 times faster with these bulk formulas than by using the TOGA COARE algorithm. This methodology is of direct practical value for GCMs of high spatial resolution, where the severe computational demands of performing GCM simulations encourage computing air–sea fluxes in the most computationally efficient manner possible. The combination of accuracy and ease of computation of this method makes it the preferred one for computing air–sea fluxes in such GCMs.

Full access
A. Birol Kara, Alan J. Wallcraft, and Harley E. Hurlburt

Abstract

A 1/25° × 1/25° cos(lat) (longitude × latitude) (≈3.2-km resolution) eddy-resolving Hybrid Coordinate Ocean Model (HYCOM) is introduced for the Black Sea and used to examine the effects of ocean turbidity on upper-ocean circulation features including sea surface height and mixed layer depth (MLD) on annual mean climatological time scales. The model is a primitive equation model with a K-profile parameterization (KPP) mixed layer submodel. It uses a hybrid vertical coordinate that combines the advantages of isopycnal, σ, and z-level coordinates in optimally simulating coastal and open-ocean circulation features. This model approach is applied to the Black Sea for the first time. HYCOM uses a newly developed time-varying solar penetration scheme that treats attenuation as a continuous quantity. This scheme includes two bands of solar radiation penetration, one that is needed in the top 10 m of the water column and another that penetrates to greater depths depending on the turbidity. Thus, it is suitable for any ocean general circulation model that has fine vertical resolution near the surface. With this scheme, the optical depth–dependent attenuation of subsurface heating in HYCOM is given by monthly mean fields for the attenuation of photosynthetically active radiation (k PAR) during 1997–2001. These satellite-based climatological k PAR fields are derived from Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) data for the spectral diffuse attenuation coefficient at 490 nm (k 490) and have been processed to have the smoothly varying and continuous coverage necessary for use in the Black Sea model applications. HYCOM simulations are driven by two sets of high-frequency climatological forcing, but no assimilation of ocean data is then used to demonstrate the importance of including spatial and temporal varying attenuation depths for the annual mean prediction of upper-ocean quantities in the Black Sea, which is very turbid (k PAR > 0.15 m−1, in general). Results are reported from three model simulations driven by each atmospheric forcing set using different values for the k PAR. A constant solar-attenuation optical depth of ≈17 m (clear water assumption), as opposed to using spatially and temporally varying attenuation depths, changes the surface circulation, especially in the eastern Black Sea. Unrealistic sub–mixed layer heating in the former results in weaker stratification at the base of the mixed layer and a deeper MLD than observed. As a result, the deep MLD off Sinop (at around 42.5°N, 35.5°E) weakens the surface currents regardless of the atmospheric forcing used in the model simulations. Using the SeaWiFS-based monthly turbidity climatology gives a shallower MLD with much stronger stratification at the base and much better agreement with observations. Because of the high Black Sea turbidity, the simulation with all solar radiation absorbed at the surface case gives results similar to the simulations using turbidity from SeaWiFS in the annual means, the aspect of the results investigated in this paper.

Full access
A. Birol Kara, James B. Elsner, and Paul H. Ruscher

Abstract

Nighttime minimum temperatures at the Tallahassee Regional Airport (TLH) are colder in comparison with surrounding locations and other parts of the city, especially during the cool season (TLH minimum temperature anomaly). These cold events are examined using the one-dimensional Oregon State University atmospheric boundary layer (ABL) model including a two-layer model of soil hydrology. The model is used for 12-h forecasts of the ABL parameters, such as surface fluxes, surface inversion height, and minimum temperature when clear, calm synoptic conditions existed over the region at night. The minimum temperature forecasts are performed at TLH and a nearby location. Cooling in the surface inversion layer is examined in terms of turbulence and clear-air radiative effects, and it is confirmed that the lower temperatures at TLH are related to the clear-air radiative cooling even in the lower part of the inversion layer but not to cold-air drainage. Stability, ABL height, and surface inversion height are examined with respect to a potential temperature curvature. Turbulent exchanges in the surface boundary layer are also taken into account. The model is able to simulate the nocturnal evolution of air temperatures well. Besides the soil moisture, the value of the roughness length momentum has a substantial effect on temperature forecasts in the model. The best overall agreement for the minimum temperature prediction over TLH is obtained using equal values for the roughness lengths of heat and momentum. Finally, use of the ABL model with its surface energy balance and crude radiative parameterization package under negligible synoptic-scale forcing can be valuable to a forecaster in predicting the daily maximum temperature drop.

Full access
A. Birol Kara, James B. Elsner, and Paul H. Ruscher

Abstract

The return-flow of low-level air from the Gulf of Mexico over the southeast United States during the cool season is studied using numerical models. The key models are a newly developed airmass transformation (AMT) model and a one-dimensional planetary boundary layer (PBL) model. Both are employed to examine the thermodynamic structure over and to the north of the Gulf. Model errors for predicting minimum, maximum, and dewpoint temperatures at the surface during both offshore and onshore phases of the return-flow cycle are analyzed. PBL model forecasts indicate soil moisture values obtained from the Eta Model improve accuracy. It is shown that forecasts of maximum temperature for coastal locations are sensitive to the soil moisture used in the PBL model. The AMT model performs well in determining boundary layer parameters since it includes horizontal advective processes. The AMT model is also able to predict the regional differences caused by different surface forcing while passing over land or sea. Results lead to a strategy for making predictions during cool-season return-flow events over and around the Gulf of Mexico.

Full access
A. Birol Kara, Alan J. Wallcraft, and Harley E. Hurlburt

Abstract

Ocean models need over-ocean atmospheric forcing. However, such forcing is not necessarily provided near the land–sea boundary because 1) the atmospheric model grid used for forcing is frequently much coarser than the ocean model grid, and 2) some of the atmospheric model grid over the ocean includes land values near coastal regions. This paper presents a creeping sea-fill methodology to reduce the improper representation of scalar atmospheric forcing variables near coastal regions, a problem that compromises the usefulness of the fields for ocean model simulations and other offshore applications. For demonstration, atmospheric forcing variables from archived coarse-resolution gridded products—the 1.125° × 1.125° 15-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-15) and 1.0° × 1.0° Navy Operational Global Atmospheric Prediction System (NOGAPS)—are used here. A fine-resolution [1/25° × 1/25° cos(lat)], (longitude × latitude) (∼3.2 km) eddy-resolving Black Sea Hybrid Coordinate Ocean Model (HYCOM) is then forced with/without sea-filled atmospheric variables from these gridded products to simulate monthly mean climatological sea surface temperature (SST). Using only over-ocean values from atmospheric forcing fields in the ocean model simulations significantly reduces the climatological mean SST bias (by ∼1°–3°C) and rms SST difference over the seasonal cycle (by ∼2°–3°C) in coastal regions. Performance of the creeping sea-fill methodology is also directly evaluated using measurements of wind speed at 10 m above the surface from the SeaWinds scatterometer on the NASA Quick Scatterometer (QuikSCAT) satellite. Comparisons of original monthly mean wind speeds from operational ECMWF and NOGAPS products with those from QuikSCAT give basin-averaged rms differences of 1.6 and 1.4 m s−1, respectively, during 2000–03. Similar comparisons performed with sea-filled monthly mean wind speeds result in a much lower rms difference (0.7 m s−1 for both products) during the same time period, clearly confirming the accuracy of the methodology even on interannual time scales. Most of the unrealistically low wind speeds from ECMWF and NOGAPS near coastal boundaries are appropriately corrected with the use of the creeping sea fill. Wind speed errors for ECWMF and NOGAPS (mean bias of ≥ 2.5 m s−1 with respect to QuikSCAT during 2000–03) are substantially eliminated (e.g., almost no bias) near most of the land–sea boundaries. Finally, ocean, atmosphere, and coupled atmospheric–oceanic modelers need to be aware that the creeping sea fill is a promising methodology in significantly reducing the land contamination resulting from an improper land–sea mask existing in gridded coarse-resolution atmospheric products (e.g., ECMWF).

Full access
A. Birol Kara, Alan J. Wallcraft, and Harley E. Hurlburt

Abstract

This paper examines the sensitivity of sea surface temperature (SST) to water turbidity in the Black Sea using the eddy-resolving (∼3.2-km resolution) Hybrid Coordinate Ocean Model (HYCOM), which includes a nonslab K-profile parameterization (KPP) mixed layer model. The KPP model uses a diffusive attenuation coefficient of photosynthetically active radiation (k PAR) processed from a remotely sensed dataset to take water turbidity into account. Six model experiments (expt) are performed with no assimilation of any ocean data and wind/thermal forcing from two sources: 1) European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA) and 2) Fleet Numerical Meteorology and Oceanography Center (FNMOC) Navy Operational Global Atmospheric Prediction System (NOGAPS). Forced with ECMWF, experiment 1 uses spatially and monthly varying k PAR values over the Black Sea, experiment 2 assumes all of the solar radiation is absorbed at the sea surface, and experiment 3 uses a constant k PAR value of 0.06 m−1, representing clear-water constant solar attenuation depth of 16.7 m. Experiments 4, 5, and 6 are twins of 1, 2, and 3 but forced with NOGAPS. The monthly averaged model SSTs resulting from all experiments are then compared with a fine-resolution (∼9 km) satellite-based monthly SST climatology (the Pathfinder climatology). Because of the high turbidity in the Black Sea, it is found that a clear-water constant attenuation depth (i.e., expts 3 and 6) results in SST bias as large as 3°C in comparison with standard simulations (expts 1 and 4) over most of the Black Sea in summer. In particular, when using the clear-water constant attenuation depth as opposed to using spatial and temporal k PAR, basin-averaged rms SST difference with respect to the Pathfinder SST climatology increases ∼46% (from 1.41°C in expt 1 to 2.06°C in expt 3) in the ECMWF forcing case. Similarly, basin-averaged rms SST difference increases ∼36% (from 1.39°C in expt 4 to 1.89°C in expt 6) in the NOGAPS forcing case. The standard HYCOM simulations (expts 1 and 4) have a very high basin-averaged skill score of 0.95, showing overall model success in predicting climatological SST, even with no assimilation of any SST data. In general, the use of spatially and temporally varying turbidity fields is necessary for the Black Sea OGCM studies because there is strong seasonal cycle and large spatial variation in the solar attenuation coefficient, and an additional simulation using a constant k PAR value of 0.19 m−1, the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) space–time mean for the Black Sea, did not yield as accurate SST results as experiments 1 and 4. Model–data comparisons also revealed that relatively large HYCOM SST errors close to the coastal boundaries can be attributed to the misrepresentation of land– sea mask in the ECMWF and NOGAPS products. With the relatively accurate mask used in NOGAPS, HYCOM demonstrated the ability to simulate accurate SSTs in shallow water over the broad northwest shelf in the Black Sea, a region of large errors using the inaccurate mask in ECMWF. A linear relationship is found between changes in SST and changes in heat flux below the mixed layer. Specifically, a change of ∼50 W m−2 in sub-mixed-layer heat flux results in a SST change of ∼3.0°C, a value that occurs when using clear-water constant attenuation depth rather than monthly varying k PAR in the model simulations, clearly demonstrating potential impact of penetrating solar radiation on SST simulations.

Full access
A. Birol Kara, Harley E. Hurlburt, and Alan J. Wallcraft

Abstract

This study introduces exchange coefficients for wind stress (CD), latent heat flux (CL), and sensible heat flux (CS) over the global ocean. They are obtained from the state-of-the-art Coupled Ocean–Atmosphere Response Experiment (COARE) bulk algorithm (version 3.0). Using the exchange coefficients from this bulk scheme, CD, CL, and CS are then expressed as simple polynomial functions of air–sea temperature difference (TaTs)—where air temperature (Ta) is at 10 m, wind speed (Va) is at 10 m, and relative humidity (RH) is at the air–sea interface—to parameterize stability. The advantage of using polynomial-based exchange coefficients is that they do not require any iterations for stability. In addition, they agree with results from the COARE algorithm but at ≈5 times lower computation cost, an advantage that is particularly needed for ocean general circulation models (OGCMs) and climate models running at high horizontal resolution and short time steps. The effects of any water vapor flux in calculating the exchange coefficients are taken into account in the polynomial functions, a feature that is especially important at low wind speeds (e.g., Va < 5 m s−1) because air–sea mixing ratio difference can have a major effect on the stability, particularly in tropical regions. Analyses of exchange coefficients demonstrate the fact that water vapor can have substantial impact on air–sea exchange coefficients at low wind speeds. An example application of the exchange coefficients from the polynomial approach is the recalculation of climatological mean wind stress magnitude from 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data in the North Pacific Ocean over 1979–2002. Using ECMWF 10-m winds and the authors’ methodology provides accurate surface stresses while largely eliminating the orographically induced Gibb’s waves found in the original ERA-40 surface wind stresses. These can have a large amplitude near mountainous regions and can extend far into the ocean interior. This study introduces exchange coefficients of air–sea fluxes, which are applicable to the wide range of conditions occurring over the global ocean, including the air–sea stability differences across the Gulf Stream and Kuroshio, regions which have been the subject of many climate model studies. This versatility results because CD, CL, and CS are determined for Va values of 1 to 40 m s−1, (TaTs), intervals of −8° to 7°C, and RH values of 0% to 100%. Exchange coefficients presented here are called the Naval Research Laboratory (NRL) Air–Sea Exchange Coefficients (NASEC) and they are suitable for a wide range of air–sea interaction studies and model applications.

Full access
A. Birol Kara, Alan J. Wallcraft, and Harley E. Hurlburt

Abstract

A fine-resolution (≈3.2 km) Hybrid Coordinate Ocean Model (HYCOM) is used to investigate the impact of solar radiation attenuation with depth on the predictions of monthly mean sea surface height (SSH), mixed layer depth (MLD), buoyancy and heat fluxes, and near-sea surface circulation as well. The model uses spatially and temporally varying attenuation of photosynthetically available radiation (k PAR) climatologies as processed from the remotely sensed Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) to take water turbidity into account in the Black Sea. An examination of the k PAR climatology reveals a strong seasonal cycle in the water turbidity, with a basin-averaged annual climatological mean value of 0.19 m−1 over the Black Sea. Climatologically forced HYCOM simulations demonstrate that shortwave radiation below the mixed layer can be quite different based on the water turbidity, thereby affecting prediction of upper-ocean quantities in the Black Sea. The clear water constant solar attenuation depth assumption results in relatively deeper MLD (e.g., ≈+15 m in winter) in comparison to standard simulations due to the unrealistically large amount of shortwave radiation below the mixed layer, up to 100 W m−2 during April to October. Such unrealistic sub–mixed layer heating causes weaker stratification at the base of the mixed layer. The buoyancy gain associated with high solar heating in summer effectively stabilizes the upper ocean producing shallow mixed layers and elevated SSH over the most of the Black Sea. In particular, the increased stability resulting from the water turbidity reduces vertical mixing in the upper ocean and causes changes in surface-layer currents, especially in the easternmost part of the Black Sea. Monthly mean SSH anomalies from the climatologically forced HYCOM simulations were evaluated against a monthly mean SSH anomaly climatology, which was constructed using satellite altimeter data from TOPEX/ Poseidon (T/P), Geosat Follow-On (GFO), and the Earth Remote Sensing Satellite-2 (ERS-2) over 1993–2002. Model–data comparisons show that the basin-averaged root-mean-square (rms) difference is ≈4 cm between the satellite-based SSH climatology and that obtained from HYCOM simulations using spatial and temporal k PAR fields. In contrast, when all solar radiation is absorbed at the sea surface or clear water constant solar attenuation depth values of 16.7 m are used in the model simulations, the basin-averaged SSH rms difference with respect to the climatology is ≈6 cm (≈50% more). This demonstrates positive impact from using monthly varying solar attenuation depths in simulating upper-ocean quantities in the Black Sea. The monthly mean k PAR and SSH anomaly climatologies presented in this paper can also be used for other Black Sea studies.

Full access
A. Birol Kara, Alan J. Wallcraft, and Harley E. Hurlburt

Abstract

The Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) with an embedded mixed layer submodel is used to predict the climatological monthly mean sea surface temperature (SST) and surface ocean mixed layer depth (MLD) over the global ocean. The thermodynamic model simulations presented in this paper are performed using six dynamical layers plus the embedded mixed layer at 1/2° resolution in latitude and 0.703125° in longitude, globally spanning from 72°S to 65°N. These model simulations use climatological wind and thermal forcing and include no assimilation of SST or MLD data. To measure the effectiveness of the NLOM mixed layer, the annual mean and seasonal cycle of SST and MLD obtained from the model simulations are compared to those from different climatological datasets at each grid point over the global ocean. Analysis of the global error maps shows that the embedded mixed layer in NLOM gives accurate SST with atmospheric forcing even with no SST relaxation/assimilation. In this case the model gives a global root-mean-square (rms) difference of 0.37°C for the annual mean and 0.59°C over the seasonal cycle over the global ocean. The mean global correlation coefficient (R) is 0.91 for the seasonal cycle of the SST. NLOM predicts SST with an annual mean error of <0.5°C in most of the North Atlantic and North Pacific Oceans. For the MLD the model gave a global rms difference of 34 m for the annual mean and 63 m over the seasonal cycle over the global ocean in comparison to the NRL MLD climatology (NMLD). The mean global R value is 0.62 for the seasonal cycle of the MLD. Additional model–data comparisons use climatological monthly mean SST time series from 18 National Oceanic Data Center (NODC) buoys and 11 ocean weather station (OWS) hydrographic locations in the North Pacific Ocean. The median rms difference between the NLOM SSTs and SSTs at these 29 locations is 0.49°C for the seasonal cycle. Deepening and shallowing of the MLD at the all OWS locations in the northeast Pacific are captured by the model with an rms difference of <20 m and an R value of >0.85 for the seasonal cycle.

Using several statistical measures and climatologies of SST and MLD we have demonstrated that NLOM with an embedded mixed layer is able to simulate with substantial skill the climatological SST and MLD when using accurate and computationally efficient surface heat flux and solar radiation attenuation parameterizations over the global ocean. Further, this was accomplished using a model with only seven layers in the vertical, including the embedded mixed layer. Success of climatological predictions from the NLOM with an embedded mixed layer is a prerequisite for simulations using interannual atmospheric forcing with high temporal resolution. NLOM gives accurate upper-ocean quantities with atmospheric forcing even with no SST relaxation or assimilation, a strong indication that the model is a good candidate for assimilation of SST data. Finally, the techniques and datasets used here can be applied to evaluation of other ocean models in predicting the SST and MLD.

Full access
A. Birol Kara, Harley E. Hurlburt, Peter A. Rochford, and James J. O'Brien

Abstract

The Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) with an embedded bulk-type mixed layer model is used to examine the effects of ocean turbidity on sea surface temperature (SST) and ocean mixed layer depth (MLD) simulations over the global ocean. The model accounts for ocean turbidity through depth-dependent attenuation of solar radiation in the mixed layer formulation as determined from the diffusive attenuation coefficient at 490 nm (k 490) obtained by the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS). Interannual model simulations are used to assess the first-order effects of ocean turbidity on SST and MLD simulation. Results are reported from three model experiments performed using different values for the attenuation of photosynthetically available radiation (k PAR). It is shown that, although allowing incoming solar radiation to vary in time and space is desirable for predicting SST, in an OGCM use of a constant k PAR with a value of 0.06 m−1 is generally sufficient in the deep ocean. The daily averaged SST time series from the three NLOM simulations are verified against daily in situ SSTs reported from 12 moored buoys in 1996 and 1997. Model results show that allowing the possibility of solar heating below the mixed layer reduces the root-mean-square error (rmse) difference between the daily yearlong model and buoy SST time series by up to 0.4°C and reduces the rmse at 11 of the 12 buoy locations. Although using spatially and temporally varying k PAR versus a constant k PAR = 0.06 m−1 (which is representative over most of the global ocean) had low impact overall, using it generally reduced the rmse at low latitudes, and using it can have a substantial impact locally in space and time. The model MLD results show low sensitivity to the k PAR value used.

Full access