Search Results

You are looking at 1 - 5 of 5 items for

  • Author or Editor: Hyun-Sook Kim x
  • All content x
Clear All Modify Search
Hyun-Sook Kim and D. Randolph Watts

Abstract

Contours of the main thermocline (12°C isotherm depth: Z12) topography objectively generated from Inverted Echo Sounder observations in the Gulf Stream may be treated as a baroclinic, geostrophic streamfunction ψ. As preliminary steps, the authors developed techniques to estimate geostrophic velocity V ψ = k×(g */f)∇Z12 and geostrophic vorticity ζψ =(g */f)∇2Z12. In doing this the authors also determined a reduced gravity g * = 1.53 cm s−2 by least-squares fitting the estimated V ψ to observed V bc, velocities (i.e., fine-tuned for the ψ at 400 m relative to 3000 m).

Accuracy of the objective maps of ψ is investigated by comparing V ψ, against V bc directly measured from tall current meter moorings and ζψ against vorticity ζp separately estimated using a “rigid-stream” velocity section and path curvature. The V ψ, values, with speeds up to 100 cm s−1, are correlated with V bc at the 99% confidence level, and the two measurements differ by only 10 cm s−1 rms error. Vorticity estimates, ranging between −0.44f and +0.64f, also show excellent agreement between ζψ and ζp within 0.77 × 10−5 s−1 (or 0.09f) rms difference. This study carefully documents the objective error-estimation techniques for these fields that, bemuse they are derivatives of the measured ψ field, are sensitive to noise. The objective estimates of error in V ψ and ζψ agree in each case with the rms differences from observations.

The authors also illustrate the consequent utility of the objective ψ maps by applying quasigeostrophic calculations to diagnose ageostrophic motions Va, in strong events in the Gulf Stream. The authors found Va/V ψ to have peak values of 0.2–0.6. Roughly equal contributions to Va came in events of large curvature or with high temporal rate of exchange. Vertical stretching ∂w/∂z at the 400-m level was estimated using the quasigeostrophic vorticity equation, finding downwelling as large as −4 mm s−1 downstream of meander crests, and upwelling as large as 3 mm s−1 downstream of meander troughs.

Full access
Hyun-Sook Kim, Carlos Lozano, Vijay Tallapragada, Dan Iredell, Dmitry Sheinin, Hendrik L. Tolman, Vera M. Gerald, and Jamese Sims

Abstract

This paper introduces a next-generation operational Hurricane Weather Research and Forecasting (HWRF) system that was developed at the U.S. National Centers for Environmental Prediction. The new system, HWRF–Hybrid Coordinate Ocean Model (HYCOM), retains the same atmospheric component of operational HWRF, but it replaces the feature-model-based Princeton Ocean Model (POM) with the eddy-resolving HYCOM. The primary motivation is to improve enthalpy fluxes in the air–sea interface, by providing the best possible estimates of the balanced oceanic states using data assimilated Real-Time Ocean Forecast System products as oceanic initial conditions (IC) and boundary conditions.

A proof-of-concept exercise of HWRF–HYCOM is conducted by validating ocean simulations, followed by the verification of hurricane forecasts. The ocean validation employs airborne expendable bathythermograph sampled during Hurricane Gustav (2008). Storm-driven sea surface temperature changes agree within 0.1° and 0.5°C of the mean and root-mean-square difference, respectively. In-storm deepening mixed layer and shoaling 26°C isotherm depth are similar to observations, but they are overpredicted at similar magnitudes of their ICs. The forecast verification for 10 Atlantic hurricanes in 2008 and 2009 shows that HWRF–HYCOM improves intensity by 13.8% and reduces positive bias by 43.9% over HWRF–POM. The HWRF–HYCOM track forecast is indifferent, except for days 4 and 5, when it shows better skill (8%) than HWRF–POM. While this study proves the concept and results in a better skillful hurricane forecast, one well-defined conclusion is to improve the estimates of IC, particularly the oceanic upper layer.

Full access
Myung-Sook Park, Myong-In Lee, Dongmin Kim, Michael M. Bell, Dong-Hyun Cha, and Russell L. Elsberry

Abstract

The effects of land-based convection on the formation of Tropical Storm Mekkhala (2008) off the west coast of the Philippines are investigated using the Weather Research and Forecasting Model with 4-km horizontal grid spacing. Five simulations with Thompson microphysics are utilized to select the control-land experiment that reasonably replicates the observed sea level pressure evolution. To demonstrate the contribution of the land-based convection, sensitivity experiments are performed by changing the land of the northern Philippines to water, and all five of these no-land experiments fail to develop Mekkhala.

The Mekkhala tropical depression develops when an intense, well-organized land-based mesoscale convective system moves offshore from Luzon and interacts with an oceanic mesoscale system embedded in a strong monsoon westerly flow. Because of this interaction, a midtropospheric mesoscale convective vortex (MCV) organizes offshore from Luzon, where monsoon convection continues to contribute to low-level vorticity enhancement below the midlevel vortex center. In the no-land experiments, widespread oceanic convection induces a weaker midlevel vortex farther south in a strong vertical wind shear zone and subsequently farther east in a weaker monsoon vortex region. Thus, the monsoon convection–induced low-level vorticity remained separate from the midtropospheric MCV, which finally resulted in a failure of the low-level spinup. This study suggests that land-based convection can play an advantageous role in TC formation by influencing the intensity and the placement of the incipient midtropospheric MCV to be more favorable for TC low-level circulation development.

Full access
Jili Dong, Ricardo Domingues, Gustavo Goni, George Halliwell, Hyun-Sook Kim, Sang-Ki Lee, Michael Mehari, Francis Bringas, Julio Morell, and Luis Pomales

Abstract

The initialization of ocean conditions is essential to coupled tropical cyclone (TC) forecasts. This study investigates the impact of ocean observation assimilation, particularly underwater glider data, on high-resolution coupled TC forecasts. Using the coupled Hurricane Weather Research and Forecasting (HWRF) Model–Hybrid Coordinate Ocean Model (HYCOM) system, numerical experiments are performed by assimilating underwater glider observations alone and with other standard ocean observations for the forecast of Hurricane Gonzalo (2014). The glider observations are able to provide valuable information on subsurface ocean thermal and saline structure, even with their limited spatial coverage along the storm track and the relatively small amount of data assimilated. Through the assimilation of underwater glider observations, the prestorm thermal and saline structures of initial upper-ocean conditions are significantly improved near the location of glider observations, though the impact is localized because of the limited coverage of glider data. The ocean initial conditions are best represented when both the standard ocean observations and the underwater glider data are assimilated together. The barrier layer and the associated sharp density gradient in the upper ocean are successfully represented in the ocean initial conditions only with the use of underwater glider observations. The upper-ocean temperature and salinity forecasts in the first 48 h are improved by assimilating both underwater glider and standard ocean observations. The assimilation of glider observations alone does not make a large impact on the intensity forecast due to their limited coverage along the storm track. The 126-h intensity forecast of Hurricane Gonzalo is improved moderately through assimilating both underwater glider data and standard ocean observations.

Full access
Ricardo Domingues, Matthieu Le Hénaff, George Halliwell, Jun A. Zhang, Francis Bringas, Patricia Chardon, Hyun-Sook Kim, Julio Morell, and Gustavo Goni

Abstract

Major Atlantic hurricanes Irma, Jose, and Maria of 2017 reached their peak intensity in September while traveling over the tropical North Atlantic Ocean and Caribbean Sea, where both atmospheric and ocean conditions were favorable for intensification. In situ and satellite ocean observations revealed that conditions in these areas exhibited (i) sea surface temperatures above 28°C, (ii) upper-ocean heat content above 60 kJ cm−2, and (iii) the presence of low-salinity barrier layers associated with a larger-than-usual extension of the Amazon and Orinoco riverine plumes. Proof-of-concept coupled ocean–hurricane numerical model experiments demonstrated that the accurate representation of such ocean conditions led to an improvement in the simulated intensity of Hurricane Maria for the 3 days preceding landfall in Puerto Rico, when compared to an experiment without the assimilation of ocean observations. Without the assimilation of ocean observations, upper-ocean thermal conditions were generally colder than observations, resulting in reduced air–sea enthalpy fluxes—enthalpy fluxes are more realistically simulated when the upper-ocean temperature and salinity structure is better represented in the model. Our results further showed that different components of the ocean observing system provide valuable information in support of improved TC simulations, and that assimilation of underwater glider observations alone enabled the largest improvement over the 24 h time frame before landfall. Our results, therefore, indicated that ocean conditions were relevant for more realistically simulating Hurricane Maria’s intensity. However, further research based on a comprehensive set of hurricane cases is required to confirm robust improvements to forecast systems.

Restricted access