A Modeling Study of Rainbands Upstream from Western Japan during the Approach of Typhoon Tokage (2004)

During 19–20 October 2004, a series of spectacular arc-shaped rainbands developed south or southeast of southwestern Japan when Typhoon Tokage (TY0423) approached the region from the southwest. As the typhoon moved closer and the upstream Froude number (Fr) continued to increase, these rainbands first remained quasi-stationary but eventually retreated backward. Using the Nagoya University Cloud-Resolving Storm Simulator (CReSS) at 1-km grid size, these rainbands were successfully simulated, and their behavior during the transition period from a relatively low-Fr to a high-Fr regime was investigated and compared with idealized two-dimensional (2D) model results from theoretical studies. In the present case, the rainbands were found to develop along a low-level frontal convergence zone between the southerly flow associated with the typhoon and the northerly flow from the Sea of Japan. The northeasterly winds accelerated through gaps between topography and fed the offshore flow at the backside of the rainbands, producing a strong resistance that allowed the rainbands to remain stationary under significantly higher Fr values (at least 1.2) than predicted by 2D simulations (of about 0.3–0.5) for the retreat to occur in conditionally unstable flow with a convective available potential energy of about 1300 J kg−1. Typically ≤ 500 m in depth with a potential temperature (θ) deficit of 2–4 K across the rainband, the cooler offshore flow was also found to be enhanced by evaporative cooling as in some other events. The cooling effect helped the rainbands to hold their position until Fr of the upstream flow became too large, and the rainband with stronger cooling behind was able to withstand a higher Fr before retreat. Once the retreat started, the offshore layer became thinner and the θ deficit also reduced, and the rainbands were washed back by the strengthening upcoming flow.

Regional flow conditions associated with stratocumulus cloud-eroding boundaries over the southeast Atlantic

Large, abrupt clearing events have been documented in the marine stratocumulus cloud deck over the subtropical Southeast Atlantic Ocean. In these events, clouds are rapidly eroded along a line hundreds–to–thousands of kilometers in length that generally moves westward away from the coast. Because marine stratocumulus clouds exert a strong cooling effect on the planet, any phenomenon that acts to erode large areas of low clouds may be climatically important. Previous satellite-based research suggests that the cloud-eroding boundaries may be caused by westward-propagating atmospheric gravity waves rather than simple advection of the cloud. The behavior of the coastal offshore flow, which is proposed as a fundamental physical mechanism associated with the clearing events, is explored using the Weather Research and Forecasting model. Results are presented from several week-long simulations in the month of May when cloud-eroding boundaries exhibit maximum frequency. Two simulations cover periods containing multiple cloud-eroding boundaries (active periods), and two other simulations cover periods without any cloud-eroding boundaries (null periods). Passive tracers and an analysis of mass flux are used to assess the character of the diurnal west-African coastal circulation. Results indicate that the active periods containing cloud-eroding boundaries regularly experience stronger and deeper nocturnal offshore flow from the continent above the marine boundary layer, compared to the null periods. Additionally, we find that the boundary layer height is higher in the null periods than in the active periods, suggesting that the active periods are associated with areas of thinner clouds that may be more susceptible to cloud erosion.

Changes in Atlantic Water circulation patterns and volume transports North of Svalbard over the last 12 years (2008-2020)

<p>Atlantic Water (AW) enters the Arctic through Fram Strait as the West Spitsbergen Current (WSC). When reaching the south of Yermak Plateau, the WSC splits into the Svalbard, Yermak Pass and Yermak Branches. Downstream of Yermak Plateau, AW pathways remain unclear and uncertainties persist on how AW branches eventually merge and contribute to the boundary current along the continental slope. We took advantage of the good performance of the 1/12° Mercator Ocean model in the Western Nansen Basin (WNB) to examine the AW circulation and volume transports in the area. The model showed that the circulation changed in 2008-2020. The Yermak Branch strengthened over the northern Yermak Plateau, feeding the Return Yermak Branch along the eastern flank of the Plateau. West of Yermak Plateau, the Transpolar Drift likely shifted westward while AW recirculations progressed further north. Downstream of the Yermak Plateau, an offshore current developed above the 3800 m isobath, fed by waters from the Yermak Plateau tip. East of 18°E, enhanced mesoscale activity from the boundary current injected additional AW basin-ward, further contributing to the offshore circulation. A recurrent anticyclonic circulation in Sofia Deep developed, which also occasionally fed the western part of the offshore flow. The intensification of the circulation coincided with an overall warming in the upper WNB (0-1000 m), consistent with the progression of AW. This regional description of the changing circulation provides a background for the interpretation of upcoming observations.</p>

A quantitative evaluation of rip current appearance in Argus timex imagery: when and where does offshore flow correspond to visible features?

<p>Modelling changes in nearshore bathymetry (<10m depth) is complicated by the nonlinear interactions between sediment, waves, and currents that can cause complex flow and transport patterns such as rip currents. Rip currents are of particular interest because of their implications for both sediment transport and beach-goer safety. An active area of research is using remote sensing (e.g., radar, video imagery) to estimate the existence and location of rip currents. Radar actively measures surface flow directions at high resolutions, however, the equipment can be expensive and difficult to set up. In contrast, video cameras are less expensive and more accessible, but can only provide passive observations that estimate derived surface quantities such as current speed and direction, and wave runup. Time exposure (timex) images from video cameras also provide information about the location of bright pixels (indications of breaking waves). Previous research has relied on the appearance of elongated, shore-normal regions of dark pixels (intersecting bright white regions) as a clear indicator of rip current presence, making timex images a prime candidate for automated detection of rip currents on beaches with video cameras installed. However, it is also known that rip currents vary widely in appearance, and that a better understanding of these parameters is necessary for automated rip current detection.</p><p>In this study, radar data and Argus camera imagery from the United States Army Corps of Engineers Field Research Facility at Duck, NC, USA were evaluated to determine how often radar measured offshore flow indicative of a rip current spatially correlates with dark, shore-normal features in the camera imagery. Radar data for two different times were processed to obtain surface current directions. Timex imagery from the video cameras on the same dates were evaluated with a machine learning algorithm   (Maryan et al. 2019) to objectively define the dark shore-normal features previously assumed to indicate rip currents’ existence within the imagery. A confusion matrix between these two datasets (surface flow direction and machine-identified rip current regions) confirms that dark, shore-normal features in the timex images are not always rip currents, and that offshore directed surface currents are not always visible as dark features in timex images. These results provide the first quantitative evaluation of how often rip current detections are missed and show that additional information is required for accurate automated rip current detection from camera imagery.</p><p>Further analysis will include using wind and wave data from field instruments at the site to reveal which conditions produce (1) offshore flow that is correlated with dark, shore-normal features in the timex imagery, (2) offshore flow that is not correlated with dark, shore-normal features in the timex imagery, and (3) dark, shore-normal features without focused offshore flow. This ongoing study could lead to the clarification of specific conditions under which the existence of rip currents can be correlated with a particular feature that machine learning techniques can be trained to recognize in camera imagery, thereby improving the accuracy of automated rip current detection. </p>

Numerical Simulations of Seasonal Variations of Rainfall over the Island of Hawaii

The seasonal variations of rainfall over the island of Hawaii are studied using the archives of the daily model run from the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) from June 2004 to February 2010. Local effects mainly drive the rainfall on the Kona coast in the early morning and the lower slopes in the afternoon. During the summer, the incoming trade winds are more persistent and moister than in winter. The moisture content in the wake zone is higher than open-ocean values because of the convergent airflow associated with dual counterrotating vortices. As the westerly reversed flow moves toward the Kona coast, it decelerates with increasing moisture and a moisture maximum over the coastal area, especially in the afternoon hours in summer months. The higher afternoon rainfall on the Kona lower slopes in summer than in winter is caused by a moister (>6 mm) westerly reversed flow bringing moisture inland and merging with a stronger upslope flow resulting from solar heating. Higher nocturnal rainfall off the Kona coast in summer than in winter is caused by the low-level convergence between a moister westerly reversed flow and offshore flow. On the windward slopes, the simulated rainfall accumulation in winter is higher because of frequently occurring synoptic disturbances during the winter storm season. Nevertheless, early morning rainfall along the windward coast and afternoon rainfall over the windward slopes of the Kohala Mountains is lower in winter because the incoming trades are drier.

The Deflection of the China Coastal Current over the Taiwan Bank in Winter

In winter, an offshore flow of the coastal current can be inferred from satellite and in situ data over the western Taiwan Bank. The dynamics related to this offshore flow are examined here using observations as well as analytical and numerical models. The currents can be classified into three regimes. The downwind (i.e., southward) cold coastal current remains attached to the coast when the northeasterly wind stress is stronger than a critical value depending on the upwind (i.e., northward) large-scale pressure gradient force. By contrast, an upwind warm current appears over the Taiwan Bank when the wind stress is less than the critical pressure gradient force. The downwind coastal current and upwind current converge and the coastal current deflects offshore onto the bank during a moderate wind. Analysis of the vorticity balance shows that the offshore transport is a result of negative bottom stress curl that is triggered by the positive vorticity of the two opposite flows. The negative bottom stress curl is reinforced by the gentle slope over the bank, which enhances the offshore current. Composite analyses using satellite observations show cool waters with high chlorophyll in the offshore current under moderate wind. The results of composite analyses support the model findings and may explain the high productivity over the western bank in winter.

Wind-Driven Circulation in a Shelf Valley. Part I: Mechanism of the Asymmetrical Response to Along-Shelf Winds in Opposite Directions

Motivated by observations in Hudson shelf valley showing stronger onshore than offshore flows, this study investigates wind-driven flows in idealized shallow shelf valleys. This first part of a two-part sequence focuses on the mechanism of the asymmetrical flow response in a valley to along-shelf winds of opposite directions. Model simulations show that (i) when the wind is in the opposite direction to coastal-trapped wave (CTW) phase propagation, the shelf flow turns onshore in the valley and generates strong up-valley transport and a standing meander on the upstream side (in the sense of CTW phase propagation) of the valley, and (ii) when the wind is in the same direction as CTW phase propagation, the flow forms a symmetric onshore detour pattern over the valley with negligible down-valley transport. Comparison of the modeled upstream meanders in the first scenario with CTW characteristics confirms that the up-valley flow results from CTWs being arrested by the wind-driven shelf flow establishing lee waves. The valley bathymetry generates an initial excessive onshore pressure gradient force that drives the up-valley flow and induces CTW lee waves that sustain the up-valley flow. When the wind-driven shelf flow aligns with CTW phase propagation, the initial disturbance generated in the valley propagates away, allowing the valley flow to adjust to roughly follow isobaths. Because of the similarity in the physical setup, this mechanism of arrested CTWs generating stronger onshore than offshore flow is expected to be applicable to the flow response in slope canyons to along-isobath background flows of opposite directions.

Development and impact of hooks of high droplet concentration on remote southeast Pacific stratocumulus

Over the southeastern Pacific (SEP), droplet concentration (Nd) in the typically unpolluted marine stratocumulus west of 80° W (> 1000 km offshore) is periodically strongly enhanced in zonally elongated "hook"-shaped features that increase albedo. Here, we examine three hook events using the chemistry version of the Weather Research and Forecasting model (WRF-Chem) with 14 km horizontal resolution, satellite data, and aircraft data from the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx). A particularly strong hook yields insights into the development, decay, and radiative impact of these features. Hook development occurs with Nd increasing to polluted levels over the remote ocean primarily due to entrainment of cloud condensation nuclei (CCN) from the lower free troposphere (FT). The feature advects northwestward until the FT CCN source is depleted, after which Nd decreases over a few days due to precipitation and dilution. The model suggests that the FT CCN source supplying the hook consists of high concentrations of small accumulation-mode aerosols that contribute a relatively small amount of aerosol mass to the MBL, in agreement with near-coast VOCALS measurements of polluted layers in the FT. The aerosol particles in this hook originate mainly from a pulse of offshore flow that transports Santiago-region (33–35° S) emissions to the remote marine FT. To provide pollution CCN that can sustain hooks, the FT transport of pollution plumes to the remote ocean requires strong, deep offshore flow. Such flow is favored by a trough approaching the South American coast and a southeastward shift of the climatological subtropical high-pressure system. The model simulations show precipitation suppression in the hook and a corresponding increase in liquid water path (LWP) compared with a simulation without anthropogenic sources. LWP also increases as the hook evolves over time due to increasing stability and decreasing subsidence. WRF-Chem suggests that dimethyl sulfide (DMS) significantly influences the aerosol number and size distributions in a hook, but that hooks do not form without FT CCN. The Twomey effect contributes ~ 50–70% of the albedo increase due to the presence of the hook, while secondary aerosol indirect effects and meteorological influences also contribute significantly. The source of hook aerosols is difficult to determine with the available observations alone. The model provides further explanation of the factors influencing hook formation. Two other weaker hooks during VOCALS-REx are not as well simulated but are also associated with FT offshore flow near Santiago. Hooks demonstrate the importance of free-tropospheric transport of aerosols in modulating the droplet concentration in the southeastern Pacific stratocumulus deck, and present a formidable challenge to simulate accurately in large-scale models.