Nirnimesh Kumar,Ph.D


Image Courtesy; Dr. Ata Suanda.

Modeling multi-scale interactions on the Pt. Sal inner-shelf.


As a part of ONR funded multi-institutional collaborative project with Dr. Ata Suanda as the lead scientist  we are modeling circulation pattern adjacent to Pt. Sal (north of Pt. Concepcion) due to winds, surface gravity waves, tides, buoyancy and local bathymetry. 


The modeling involves using the ocean-circulation Regional Ocean Modeling System (ROMS) and a wave propagation model Simulating WAves Nearshore (SWAN) in a series of one-way nesting by resolving regional processes (Mesoscale, Sub-mesoscale and Wind-driven) with a 1 km grid resolution, followed by resolution of local processes (barotropic and baroclinic tides, bathymetry) with grid resolution of 600m and 200m. The next nesting levels (66m and 22m) resolve inner-shelf and surf zone processes. 


Preliminary results suggest the local variability is strongly controlled by local upwelling favorable winds and reversal events (downwelling) during which warm water from the Southern California Bight inundates the Pt. Sal region. Semi-diurnal internal waves are generated locally during spring tides. Also, during periods of strong wave heights (> 3m), local bathymetrically controlled rip currents make it to the inner-shelf.  

​​​Interaction between Transient Rip Currents and Stratification.


Transient rip currents (TRCs) on alongshore uniform beaches are formed by short-crested wave breaking, and exchange tracers from the surf zone to the potentially stratified inner-shelf. Influence of TRCs on the inner-shelf stratification and vice versa has not yet been investigated. Wave-resolving (WR) Boussinesq models include finite crest length breaking needed to accurately simulate TRCs. However, WR models are depth-integrated and do not account for stratification and vertically-sheared flow structure. Wave-averaged (WA) circulation models includes stratification, but lack surfzone eddy generation needed to simulate TRCs. Accurate simulation of surfzone to inner-shelf exchange requires combined application of WA and WR models.  


Here, WR depth-integrated Boussinesq model funwaveC is coupled to stratification and depth-resolving WA Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system and applied on 1 km cross-shore region from the surf zone to the inner-shelf with a constant buoyancy frequency and Coriolis effects. The TRC forcing is extracted from a funwaveC simulation of random normally incident, directionally spread waves on a planar beach, and specified as a depth-uniform force to COAWST. Inner-shelf interaction between TRCs and stratification are examined with simulations including TRC with or without stratification.


​Preliminary results suggest that TRCs rapidly mix the surf zone and eject the relatively colder water to the inner-shelf (e.g., (a)). The eddies associated with the TRCs are three-dimensional and pull colder water up in the water column (e.g., (b)) leading to transformation from kinetic to potential energy. 

​​Parameterization of Finite-Crested Wave Breaking in Wave-averaged Models 

In this collaborative project with Dr. Ata Suanda and Falk Feddersen, we are investigating methodology to parameterize the effects of short-crested waves in a wave-averaged model (i.e., a model which does not resolve individual waves).  


Surface gravity waves become depth-limited in shallow waters and break injecting turbulence, momentum and mass flux in the water column (e.g., (a)). In addition, finite length wave crests inject vorticity in the water column (e.g., (b)). These vorticity ejection events are responsible of generation of surf zone eddies, primary mechanism for mixing in the surf zone. These surf zone eddies coalesce to create transient rip current.


The effect of finite crested wave breaking is studied through wave-resolving Boussinesq models, which are similar to nonlinear shallow water equations but allow for higher order dispersive terms. Nonetheless, wave-resolving models are computationally expensive and at present cannot be used for nowcasting/forecasting surf zone ocean circulation. 

 

We are extracting the rotational component of wave breaking force (e.g., (c)) and its stream-function (e.g., (d)), which can be represented in wave-averaged models.