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M. Ikeda

Abstract

Wind effects on buoyancy-driven circulation in a two-level rectangular basin are studied. The ocean is driven by positive and negative buoyancy fluxes in the northern and southern portions as well as wind stress of constant curl. In a model with a flat and frictionless bottom, a barotropic component is determined only by wind forcing. A baroclinic component of the wind-driven circulation, associated with horizontal density gradient, is reduced by horizontal diffusion; i.e., the wind-driven circulation is more barotropic with stronger diffusion.

Meridional overturn induced by buoyancy fluxes is modified by the wind-driven circulation, for example, the southward upper-level flow, produced by positive and negative buoyancy fluxes in the northern and southern portions, greatly shifts to the western (eastern) boundary by cyclonic (anticyclonic) wind-driven circulation with realistic intensity. Relative importance of the wind-driven circulation to the buoyancy-driven circulation for meridional density transport is dependent on total Sverdrup transport of the wind-driven circulation, but independent of the buoyancy flux intensifies: the wind-driven circulation is less important with weaker wind stress and in a smaller zonal-size basin. A variable wind stress is also given, to examine effects of seasonal variabilities in wind.

The results are applied to the Baffin Bay/Labrador Sea system, and suggest that the circulation pattern is changed by wind stress cud of ±10−7 N m−3. With a steady wind stress, the meridional density transport is essentially determined by the buoyancy-driven overturn. However, with the variable wind stress, the density transport varies by more than ±50%, as the system tends to adjust to the wind stress.

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M. Ikeda

Abstract

A new modeling approach is proposed for representing a subpolar ocean, whose upper layer is partly convected with the lower layer. A simple box model, built as a reference, has one active box to be the upper layer of a colder ocean, which interacts with the warmer box and the lower box. The active box receives atmospheric forcing (cooling and precipitation) and a parameterized freshwater (or sea ice) flux as well, while the other boxes have their properties fixed. The active box, interacting with the warmer box, possesses a thermal-driven state, at which the warmer water enters the active box, is cooled by the atmosphere, and becomes denser. The lower box adds another solution: a convected state appears in the vicinity of the nonconvected state. The nonconvected state either separates from or is absorbed into the convected state; that is, the entire upper box is convected with the lower box, once the lower-box density is close to the upper-box density.

The new component to the simple model is a probability distribution function (PDF) on the temperature–salinity (TS) plane for the active box. The PDF in this probability box model represents heterogeneity in the upper layer, whereas one box has to be homogeneous in an ordinary box model. A TS distribution retains only the probabilities of different water types, while their locations are discarded. The mechanisms to increase and reduce heterogeneity are modeled by the divergence and convergence of the PDF on the TS plane, respectively. The heterogeneity is generated by the intrusion of exterior water as well as variability in the atmospheric forcing and freshwater flux, while the heterogeneity is reduced by horizontal diffusion within the box. Convection with the lower box tends to concentrate the PDF to the T, S of the lower box. Under the exterior condition that could produce both nonconvected and convected states in the simple box model, there is only one state of the upper box, which is partly convected, in the probability model. This intermediate state is possible when the divergent mechanism is intense, and the convergent mechanism is weak. Thus, the on–off convection in the simple box model is replaced with an intermediate state between the convected and the nonconvected states. It is suggested that, once mesoscale variability maintains heterogeneity, convection in the subpolar ocean is more robust against freshwater input.

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M. Ikeda

Abstract

NOAA satellite visible images exhibit two alternative ice-edge patterns off Labrador; one is a straight pattern, and the other is a large-meander pattern with alongshore wavelengths of 200–300 km and offshore extensions corresponding to banks over the continental shelf. As strong northwesterly wind events occurred, the straight patterns changed into the large-meander patterns. Assimilating results from both a continental shelf model driven by alongshore wind and a reduced gravity model constrained by a vertical wall with alongshore variation, this progression is interpreted as follows: the strong wind induces downwelling and a resultant alongshore flow over the shelf break, and then, the intensified Labrador Current interacts with alongshore variation in the shelf break more effectively than the weak one does. The topography produces propagating meanders of the Labrador Current and is enhanced by offshore and offshore extensions corresponding to banks and saddles, respectively, several days after the wind event. The ice edge, following the Labrador Current, comes to have the large-meander pattern. Heat fluxes due to warm Labrador Sea water carried by the shoreward meanders may be important to a heat balance over the shelf in winter.

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M. Ikeda and W. J. Emery

Abstract

Infrared satellite images from the west coasts of Oregon and northern California are used to identify meander patterns in sea surface temperature which appear as large cold tongues extending offshore. Two relatively long series of images from 1982 and a few examples from 1980 and 1983 demonstrate the evolution of the cold tongues from an initial variety of scales (60–200 km), to the fastest growing waves (110–130 km) and then finally to tongues with longer wavelengths (400 km). This is observed to occur over periods of about three months in summer and fall when the coastal circulation is composed of a southward surface current over a northward undercurrent. The initial shorter scale features are believed to be excited by the interaction between the mean current and the coastal topography. Baroclinic instability associated with the vertical shear between the surface current and the undercurrent is found to be responsible for the growth of the features observed in the satellite imagery. A nonlinear numerical model is used to simulate the evolution of these features in summer/fall including the initial excitation, the growth of the dominant waves and the red cascade to longer wavelengths. In winter or spring when the current no longer reverses with depth but flows north or south respectively, the meanders have scales of about 100–120 km consistent with the horizontal scale of features in the bottom topography.

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T. G. Prasad and M. Ikeda

Abstract

Using a level 2 one-dimensional turbulent closure model, the evolution of the Arabian Sea High Salinity Water mass (ASHSW) has been studied. Model-derived heat fluxes, sea surface temperature (SST), and mixed layer depths (MLD) are found to be in reasonable agreement with the observations. Winter cooling is characterized by a net heat loss of 40 W m−2 from the ocean, while there is a mild heat loss in summer. Wind-driven turbulent mixing is considerably stronger than buoyancy forcing during the summer monsoon and explains the mechanism for governing deep MLD and cool SST. During winter, a large latent heat release by the ocean due to prevailing dry air from the north, together with reduced solar radiation and increased longwave radiation due to lower cloudiness, decreases the net heat flux considerably. Negative buoyancy flux plays a major role in the formation of ASHSW during winter, while kinetic energy-driven vertical mixing is relatively weaker than during the summer monsoon. A sensitivity test has confirmed that the humidity plays an important role in the heat budget of the northern Arabian Sea during winter.

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M. Ikeda, J. A. Johannessen, K. Lygre, and S. Sandven

Abstract

A series of NOAA satellite images shows that the Norwegian Coastal Current (NCC) over the Norwegian Trench was disturbed by mesoscale meanders with 60–100 km wavelengths. In the first several days, the meanders grew and propagated northward. Some meanders were pinched off seaward, forming anticyclonic eddies. The flow pattern became very chaotic around 23 February, and then, on 25–27 February it changed into a systematic pattern again with three cyclonic vortices developing on the offshore side of the NCC axis, accompanied by seaward meanders or separated anticyclonic eddies on their northern sides. Acoustic Doppler current profiler measurement showed these vortices to have significant barotropic components and to move northward at 5 km day−1.

A quasi-geostrophic two-layer model is employed first to show the basic behavior of the system. Model sensitivity is examined with various vertical profiles of the initial jet and various bottom topography. Baroclinic instability is an essential mechanism to generate the observed mesoscale features. Anticyclonic eddies separate seaward from some meanders, and a cyclonic vortex develops south of each eddy or meander. A submarine ridge in the upstream flow plays an important role in redevelopment of the systematic flow pattern during the second half of the observations.

A simulation model is constructed to hindcast the observed flow pattern for a two-week period, starting with initial perturbations estimated from the 13 February satellite image. The general progression (initial meander growth-chaotic pattern-systematic pattern) is well duplicated in the simulation, with three stationary cyclonic vortices at the locations comparable to those of the observed vortices, and it is robust to variations in subsurface structure of the initial perturbations.

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Kyoko Ikeda, Roy M. Rasmussen, William D. Hall, and Gregory Thompson

Abstract

Observations of supercooled drizzle aloft within two storms impacting the Oregon Cascades during the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) field project are presented. The storms were characterized by a structure and evolution similar to the split-front model of synoptic storms. Both storms were also characterized by strong cross-barrier flow. An analysis of aircraft and radar data indicated the presence of supercooled drizzle during two distinct storm periods: 1) the intrafrontal period immediately following the passage of an upper cold front and 2) the postfrontal period. The conditions associated with these regions of supercooled drizzle included 1) temperatures between −3° and −19°C, 2) ice crystal concentrations between 1 and 2 L−1, and 3) bimodal cloud droplet distributions of low concentration [cloud condensation nuclei (CCN) concentration between 20 and 30 cm−3 and cloud drop concentration <35 cm−3].

Unique to this study was the relatively cold cloud top (<−15°C) and relatively high ice crystal concentrations in the drizzle region. These conditions typically hinder drizzle formation and survival; however, the strong flow over the mountain barrier amplified vertical motions (up to 2 m s−1) above local ridges, the mountain crest, and updrafts in embedded convection. These vertical motions produced high condensate supply rates that were able to overcome the depletion by the higher ice crystal concentrations. Additionally, the relatively high vertical motions resulted in a near balance of ice crystal fall speed (0.5–1.0 m s−1), leading to nearly terrain-parallel trajectories of the ice particles and a reduction of the flux of ice crystals from the higher levels into the low-level moisture-rich cloud, allowing the low-level cloud water and drizzle to be relatively undepleted.

One of the key observations in the current storms was the persistence of drizzle drops in the presence of significant amounts of ice crystals over the steepest portion of the mountain crest. Despite the high radar reflectivity produced by the ice crystals (>15 dBZ) in this region, the relatively high condensate supply rate led to hazardous icing conditions. The current study reveals that vertical motions generated by local topographic features are critical in precipitation processes such as drizzle formation and thus it is essential that microphysical models predict these motions.

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Julie M. Thériault, Roy Rasmussen, Kyoko Ikeda, and Scott Landolt

Abstract

Accurate snowfall measurements are critical for a wide variety of research fields, including snowpack monitoring, climate variability, and hydrological applications. It has been recognized that systematic errors in snowfall measurements are often observed as a result of the gauge geometry and the weather conditions. The goal of this study is to understand better the scatter in the snowfall precipitation rate measured by a gauge. To address this issue, field observations and numerical simulations were carried out. First, a theoretical study using finite-element modeling was used to simulate the flow around the gauge. The snowflake trajectories were investigated using a Lagrangian model, and the derived flow field was used to compute a theoretical collection efficiency for different types of snowflakes. Second, field observations were undertaken to determine how different types, shapes, and sizes of snowflakes are collected inside a Geonor, Inc., precipitation gauge. The results show that the collection efficiency is influenced by the type of snowflakes as well as by their size distribution. Different types of snowflakes, which fall at different terminal velocities, interact differently with the airflow around the gauge. Fast-falling snowflakes are more efficiently collected by the gauge than slow-falling ones. The correction factor used to correct the data for the wind speed is improved by adding a parameter for each type of snowflake. The results show that accurate measure of snow depends on the wind speed as well as the type of snowflake observed during a snowstorm.

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Kyoko Ikeda, Edward A. Brandes, and Roy M. Rasmussen

Abstract

An unusual multiple freezing-level event observed with polarimetric radar during the second phase of the Improvement of Microphysical Parameterization through Observational Verification Experiments (IMPROVE-2) field program is described. The event occurred on 28 November 2001 when a warm front moved over the Oregon Cascade Mountains. As the front approached, an elevated melting layer formed above a preexisting melting layer near ground. Continued warming of the lower atmosphere eventually dissipated the lower melting layer.

The polarimetric measurements are used to estimate the height of the freezing levels, document their evolution, and deduce hydrometeor habits. The measurements indicate that when the two freezing levels were first observed melting was incomplete in the upper melting layer and characteristics of particles that passed through the two melting layers were similar. As warming progressed, the character of particles entering the lower melting layer changed, possibly becoming ice pellets or frozen drops. Eventually, the refreezing of particles ended and only rain occurred below the elevated melting layer.

The Doppler radial winds showed a well-defined wind maximum apparently associated with a “warm conveyor belt.” The jet intensified and descended through the elevated melting layer with time. However, the increase in wind speed did not appear connected with melting or result in precipitation enhancement.

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M. Ikeda, L. A. Mysak, and W. J. Emery

Abstract

A long time series of satellite infrared images spanning the period 21 July–8 October 1980 reveals the evolution of long (160 km) and short (80 km) wavelength meanders of the California Current System off Vancouver Island. Midway in the series, strong interactions occurred between the two meander scales. Although the shorter meanders were initially more energetic, they were eventually dominated by the longer meanders. Near the end of the time series, detached mesoscale eddies were formed from the longer meanders. Current meter data from the period before meander growth clearly indicates the presence of a strong vertical shear near 150 m, which consists of a southeastward surface current and a northwestward undercurrent. It is argued in this paper that the observed shear flow is the energy source for the meanders and eddies seen in the satellite images.

A four-layer, quasi-geostrophic model is used to represent the California Current System off Vancouver Island. The two upper layers represent the surface current and undercurrent respectively, and the two lower layers describe the deep ocean. Linear stability theory suggests that the vertical shear between the two currents maintains meander growth through the mechanism of baroclinic instability. Nonlinear numerical calculations simulate the engulfment of the shorter meander by the longer meanders and also the formation of the detached eddies. An energy analysis indicates two mechanisms at work during engulfment of the shorter meanders. One involves a stronger stabilization of the shorter meanders resulting from an inverse Reynolds stress, and the other consists of a direct nonlinear interaction which transfers energy from the shorter meanders to the longer meanders.

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