Quality Assessment Techniques Applied to Surface Radial Velocity Maps Obtained from High-Frequency Radars

Kim³

2018

Self Cite

Submesoscale coastal surface currents at hourly and O(1) km resolutions, obtained from an array of high‐frequency radars in a coastal region off the east coast of Korea over a period of 1 year (2013), are described in the frequency and wavenumber domains. The low‐frequency surface currents exhibit more consistent variability with the regional geostrophic currents in summer than those in winter because of the relatively weak wind conditions in summer. The clockwise near‐inertial surface currents show onshore phase propagation and decreasing amplitudes. The kinetic energy spectra of the surface currents in the wavenumber domain (k) become steeper, increasing from a slope of k−5/3 at a length scale of approximately 10 km to slopes between k−2 and k−3 at a length scale of 2 km. These kinetic energy spectra exhibit anisotropy and weak seasonality with an injection scale of O(10) km, consistent with dominant length scales of the regional submesoscale eddies and the zero‐crossing wavenumber in the estimated kinetic energy fluxes. We suggest possible mechanisms to explain the above findings. Submesoscale processes in this region are primarily initiated by surface frontogenesis caused by regional mesoscale currents, including low‐frequency circulations and topographically linked shear currents, then maintained by baroclinic instability in the mixed layer with moderate seasonality.

Section: Datamentioning

confidence: 99%

Section: Datamentioning

confidence: 99%

Spectral Descriptions of Submesoscale Surface Circulation in a Coastal Region

Yoo

Kim³

2018

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“…This network consists of two 25‐MHz radars located at Namhae (NAM4;

sans-serifR1

) and Hyangil (HYIL;

sans-serifR2

) and two 44‐MHz radars located at Namhae Sports Park (NHSP;

sans-serifR3

) and Odong‐do (ODNG;

sans-serifR4

; Figure c; Table ). The spatial and temporal availability of the radial velocity data, which are processed with respect to the measured beam patterns, at the individual radar sites during the 2‐year period are shown in Figure (see Kim, , for more details). The effective spatial data availability reflects the footprint of the coastline boundary, which is related to the land interference and reduction in the radar signal‐to‐noise ratio (SNR; Figures a‐d).…”

Section: Datamentioning

confidence: 99%

“…We evaluate the correlations of the paired radial velocities and the standard deviations of their sums (e.g., Kim, 2015), but this approach may have limitations because there are places in which the paired radial velocities are persistently absent in our study domain; these absences can yield the anomalous correlations and standard deviations.…”

Section: 1029/2018jc014892mentioning

confidence: 99%

Submesoscale Surface Tidal, Vortical, and Residual Circulations in a Semienclosed Bay

Won

2019

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We scrutinize three different components of submesoscale surface tidal, vortical, and residual circulations in a semienclosed bay using (1) observations of high‐frequency radar‐derived surface current maps and (2) numerical model simulations run under realistic vertical stratification and boundary conditions at O(1)‐km spatial and 1‐hourly temporal resolutions over a 2‐year period (2013‐2014). The tidal circulation is characterized by (1) the tidal ellipses and spectral contents having more baroclinic motions at the M2 and S2 frequencies and more barotropic motions at the K1 frequency and (2) the temporal and spatial variability in the tidal fronts appearing as tidal ellipses with clustered shapes and opposite rotational directions. The regional vortical circulation is presented with the submesoscale eddies at diameters ranging from 3 to 12 km and normalized vorticity magnitudes of 0.2 to 2 for both clockwise and counterclockwise rotations. Based on the spatial statistics of the identified submesoscale eddies and eddy kinetic energy budget analysis, the submesoscale eddies are primarily generated by the detachment of shoreline‐following tidal currents at the coastal boundaries, persist for less than 1.5 days, and are dissipated dominantly via vertical buoyancy fluxes associated with bottom bathymetric interactions of the tidal currents. The residual circulation is clearly shown with the nontidal geostrophic currents associated with the pressure gradients generated by wind‐driven Ekman transports against the coast and the ageostrophic low‐frequency currents.

“…These radial velocity maps were optimally interpolated into a surface vector current map with a spatial resolution of 1 km using an exponential correlation function with an isotropic decorrelation length scale of 2 km to minimize spatial smoothing (Kim et al, ; Figure a). The uncertainty of the HFR‐derived surface currents in the study area is less than

6 cm {normals}^{- 1}

, which was estimated from the standard deviations of the sum of the oppositely facing radial velocity pairs sampled at nearby radial grid points by two independent radars (e.g., Kim, ; Kim et al, ; Paduan et al, ). An hourly averaged radial velocity was obtained from a temporal mean of at most six samples of the radial velocities within 1 h because of the irregular occurrence of missing data in both time and space (e.g., Kim et al, ; Schmidt, ).…”

Section: Datamentioning

confidence: 99%

Diagnostic Characteristics of Submesoscale Coastal Surface Currents

Soh

2018

Self Cite

Submesoscale kinetic energy (KE) spectra and fluxes at the length scales ranging from 2 to 25 km are estimated from hourly and O(1) km‐scale coastal surface current maps observed from shore‐based high‐frequency radars off southern San Diego. The one‐dimensional wave number‐domain KE spectra of the surface currents have decay slopes between k−2 and k−3 at a wave number (k) of 0.5 km−1. The KE spectra exhibit anisotropy associated with anisotropic circulation, which is constrained by the shoreline and bottom bathymetry. Moreover, the KE spectra exhibit weak seasonality related to the regional submesoscale eddies and low‐frequency circulation with weak seasonal variability. The estimated KE fluxes are categorized into four cases of purely forward cascades and inverse cascades at all wave numbers, inverse‐then‐forward cascades with a single zero‐crossing within the range of wave numbers (0.04 to 0.5 km−1), and residuals, which account for approximately 33%, 39%, 19%, and 9% of the total number of realizations, respectively. An injection scale where forward enstrophy cascade and inverse energy cascade occur is estimated to be 5 to 10 km from the cases of the inverse‐then‐forward cascade, which is consistent with the length scales of the regional submesoscale eddies. Thus, the regional submesoscale processes are initiated by surface frontogenesis due to the weak seasonal low‐frequency surface circulation and topography‐related currents, then maintained by baroclinic instabilities associated with the seasonal mixed layer and O(10) km‐scale submesoscale eddies with weak seasonal variability.