Winter Cold Fronts Research Articles

Role of Topography in Triggering Elevated Thunderstorms Associated With Winter Cold Fronts Over the Eastern Yunnan‐Guizhou Plateau

AbstractUsing observations and reanalysis data, the circulation characteristics of elevated thunderstorms behind winter cold fronts (ETCFs) over the eastern Yunnan‐Guizhou Plateau are summarized, and a new triggering mechanism in the cold season related to topography is proposed. ETCFs always occur on the ridge and eastern side of the Dalou Mountain and are triggered at night. Pattern classifications using obliquely rotated T‐mode principal component analysis show that low‐level large‐scale circulation patterns have the following two characteristics: a southwestern airflow at 850 hPa, changing from southwest wind to east wind; and a northeast airflow at 900 hPa, changing from northeast wind to east wind. ETCFs occur in the area of this low easterly wind. A case study of 21 January 2020, shows that it is the intersection of the topography and cold front that provides unique geographical and circulation conditions for triggering elevated thunderstorms. Lateral friction along the mountains causes horizontal wind shear to flow parallel to them, forming positive and negative vortices above and below the front, respectively. This pair of vortices is strengthened by inertial oscillation at night, producing a stronger easterly wind. In the combined effects of frontal lift and topographical lift of the easterly flow, the elevated convection is triggered in the convergence zone above the front. These results provide a new insight into how topography helps to trigger convection in the cold season.

Climate and the latitudinal limits of subtropical reef development

Climate plays a central role in coral-reef development, especially in marginal environments. The high-latitude reefs of southeast Florida are currently non-accreting, relict systems with low coral cover. This region also did not support the extensive Late Pleistocene reef development observed in many other locations around the world; however, there is evidence of significant reef building in southeast Florida during the Holocene. Using 146 radiometric ages from reefs extending ~ 120 km along Florida’s southeast coast, we test the hypothesis that the latitudinal extent of Holocene reef development in this region was modulated by climatic variability. We demonstrate that although sea-level changes impacted rates of reef accretion and allowed reefs to backstep inshore as new habitats were flooded, sea level was not the ultimate cause of reef demise. Instead, we conclude that climate was the primary driver of the expansion and contraction of Florida’s reefs during the Holocene. Reefs grew to 26.7° N in southeast Florida during the relatively warm, stable climate at the beginning of the Holocene Thermal Maximum (HTM) ~ 10,000 years ago, but subsequent cooling and increased frequency of winter cold fronts were associated with the equatorward contraction of reef building. By ~ 7800 years ago, actively accreting reefs only extended to 26.1° N. Reefs further contracted to 25.8° N after 5800 years ago, and by 3000 years ago reef development had terminated throughout southern Florida (24.5–26.7° N). Modern warming is unlikely to simply reverse this trend, however, because the climate of the Anthropocene will be fundamentally different from the HTM. By increasing the frequency and intensity of both warm and cold extreme-weather events, contemporary climate change will instead amplify conditions inimical to reef development in marginal reef environments such as southern Florida, making them more likely to continue to deteriorate than to resume accretion in the future.

Non-native Asian swamp eel, Monopterus albus/javanensis (Zuiew, 1973/Lacepede, 1800), responses to low temperatures.

Asian swamp eel, Monopterus albus/javanensis [Zuiew, 1973/Lacepede 1800], has been established in the southeastern USA since at least 1994, yet little is known about its ability to survive low winter temperatures. We use standard thermal methodologies to quantify low temperature responses and provide a detailed description of swamp eel reactions to cold temperatures. When exposed to chronic temperature decreases of 1.0 °C day-1, swamp eel ceased foraging at 15.0 °C, markedly diminished movements below 11.0 °C, and became incapacitated near 9.6 °C. During critical thermal minima trials, swamp eel exposed to acute temperature drops (0.25 °C min-1) tolerated temperatures as low as 6.2 °C. Swamp eel exhibited a moderate cold acclimation response, gaining 0.23 °C in cold tolerance for every 1 °C drop in acclimation temperature. Progressive time-series critical thermal minimum temperatures (CTmin) estimates for eel acclimated to 20.5 °C followed by an acute temperature decrease to 16.0 °C, revealed that cold acclimation may occur in only 8 days. Fringe populations of swamp eel in their native range periodically experience colder winter temperatures, which may explain the ability of introduced populations to survive winter cold fronts in Florida. Understanding Asian swamp eel acute and chronic thermal limits may be useful in assessing dispersal risk and range expansion in the southeastern USA.

Hydrodynamics and sediment dynamics in Barataria Bay, Louisiana, USA

Barataria Bay is a receiving basin for a large Mid-Barataria Sediment Diversion scheme in Louisiana, USA. In this region, data on sediment transport and hydrodynamics are scarce but essential for the design and planning of future sediment diversion and marsh creation. Four months of bottom boundary layer observations were conducted to study winter and spring hydrodynamics and sediment dynamics in this coastal bay. Hourly waves, tides, currents, and bottom suspended sediment concentrations were measured using multiple optical and acoustic sensors attached to two tripod platforms. High temporal resolution data indicated that the salinity in the northern bay was mainly controlled by northerly winds, and tidal currents kept the salinity high in the southern bay during the winter cold front season. In spring, frequent, pervasive southerly winds and the westward shelf transport of less saline water emerging from the Mississippi River Delta lowered the salinity in the southern bay. Spectral analysis showed that wave-current combined shear stress played the most critical role in triggering sediment resuspension. The Style-Glenn 1-D bottom boundary layer model was applied in sediment flux calculations, and showed that net sediment transport mainly occurred during cold front passages. During two 3-day cold-front events, southward sediment fluxes accounted for 56% of the total sediment flux during the 35-day winter period, revealing their nonlinear, event-driven, and episodic nature. The direction of the sediment transport generally rotated, and its magnitude changed considerably, when southeasterly winds shifted to intensified northwesterly winds. Long-duration southerly pre-frontal winds facilitated wetland sedimentation by transporting sediment to the northern bay during high water level conditions impacted by flooding spring tides or southerly winds. Conversely, northerly winds during cold fronts dominated bidirectional tidal currents and led to southward net sediment transport and possible escape of sediment from the bay. The timing of diversion openings, the orientation of receiving basins, and dominant wind directions in relation to fetch, as well as the dynamic water levels should be considered in the planning and management of future diversion operations in coastal areas.

Climatology and Variability of Warm and Cold Fronts over North America from 1979 to 2018

AbstractThis paper describes the development and analysis of an objective climatology of warm and cold fronts over North America from 1979 to 2018. Fronts are detected by a convolutional neural network (CNN), trained to emulate fronts drawn by human meteorologists. Predictors for the CNN are surface and 850-hPa fields of temperature, specific humidity, and vector wind from the ERA5 reanalysis. Gridded probabilities from the CNN are converted to 2D frontal regions, which are used to create the climatology. Overall, warm and cold fronts are most common in the Pacific and Atlantic cyclone tracks and the lee of the Rockies. In contrast with prior research, we find that the activity of warm and cold fronts is significantly modulated by the phase and intensity of El Niño–Southern Oscillation. The influence of El Niño is significant for winter warm fronts, winter cold fronts, and spring cold fronts, with activity decreasing over the continental United States and shifting northward with the Pacific and Atlantic cyclone tracks. Long-term trends are generally not significant, although we find a poleward shift in frontal activity during the winter and spring, consistent with prior research. We also identify a number of regional patterns, such as a significant long-term increase in warm fronts in the eastern tropical Pacific Ocean, which are characterized almost entirely by moisture gradients rather than temperature gradients.

Wind and tide effects on the Choctawhatchee Bay plume and implications for surface transport at Destin Inlet

Multiple river-dominated estuaries line the northern Gulf coast and introduce substantial density variations. Their plumes have been shown to be highly sensitive to wind and tide effects, but in studies with limited observations and idealized wind forcing. This study explores these effects with a dynamic model that can represent the full behavior from river through estuary to shelf, and for a period with extensive observations. The inner shelf adjacent to Choctawhatchee Bay, a micro tidal estuary situated along the Florida Panhandle, is subject to buoyant, brackish outflows during the ebb-phase of the tidal cycle. In December 2013, experiments were performed in this region to study mechanisms that influence near-shore surface transport. Satellite imagery showed a visible brackish surface plume at Destin during low tide. The goal of the present study is to quantify variability in the plume signature due to changes in tidal and wind forcing. Density-driven flows near Destin Inlet are modeled with the recently-enhanced, three-dimensional, baroclinic capabilities of the ADvanced CIRCulation (ADCIRC) model. Modeled tides, salinities and plume signature are validated against in-situ observations and satellite imagery. Model results reveal substantial changes in the length, width and orientation of the plume as the wind direction varied on consecutive days due to winter cold fronts. During a period of near-constant winds and variability in tidal amplitude, the model predicted a larger plume during spring tides than during neap conditions. Coriolis effects on the plume are minimized due to its small scale nature. Therefore, when the wind forcing is weak, the plume signature spreads radially from the inlet with slight preference to the down-shelf. The Choctawhatchee Bay plume is representative of other small-scale plumes formed in river-dominated and micro-tidal environments, and this work demonstrates the sensitivity of these plumes to changing environmental conditions.

Wind effects on dense shelf water cascades in south-west Australia

Evaporation and cooling during autumn and winter in south-western Australia results in denser water near the coast and a cross-shelf density gradient. These conditions drive a gravitational circulation, with offshore transport of higher density water along the seabed, defined as dense shelf water cascades (DSWCs). A field experiment with data collected from multiple platforms is described to identify the temporal variability and controlling mechanisms of DSWCs during the austral autumn and winter (May–June 2015) when DSWCs were most prevalent. Data analysis confirmed that both the wind intensity and direction were critical parameters to: (1) inhibit DSWC formation through vertical mixing; and, (2) enhance DSWCs, depending on the wind direction. An intense and persistent DSWC was initiated through the passage of a winter cold front with strong onshore winds. Here, onshore winds caused downwelling, resulting in onshore movement of surface water and offshore movement in the bottom layer, enhancing the cross-shelf density gradient and the cascade. Vertical mixing of the water column and shutdown of the DSWC occurred when the winds were strong and had an offshore component or were upwelling-favourable, which resulted in the advection of higher density water offshore at the surface and vertical mixing.

Genesis and growth of a carbonate Holocene wedge on the northern Little Bahama Bank

Abstract The study focuses on the sedimentary dynamics at the transition between the modern Bahamas carbonate platform and related adjacent slopes and deeper environments. A Holocene carbonate sedimentary wedge is developed at the edge of the carbonate platform extending along the northern Little Bahamas Bank (LBB; Bahamas). The transition area between the platform and the wedge shows superposed terraces. The Holocene wedge thickness varies from 0 to 35 m. Its surface is dominated by homogeneous, fine-grained, soft sediments. It lies on a Pleistocene substratum and fills depressions corresponding to lowstand erosional surfaces. On the uppermost slope, this wedge represents the main depocenter of fine-grained bank-derived sediments since the last bank flooding. Twelve gravity cores were sampled from this wedge and allow the identification of both the processes that can supply this wedge and the wedge evolution based on content analyses and 19 radiocarbon ages. At present, particle export from the LBB margins mostly occurs during episodes of winter cold fronts but also occurs during hurricanes. Evidence of hurricane impact in the sedimentary record is suggested by the recovery of two laminated sequences showing both decreasing sedimentation rates and varying velocities during platform export. The local morphology of the platform edge allows the better sorting and export of sediments according to the presence of tidal outlets and sediment sorting on the wedge. However, because no ooids from tidal deltas located on the platform edge were found in cores, tidal currents are probably not energetic enough to both mobilize and export sediment, even if tide–induced currents may help the export of fine-grained carbonate particles remobilized by either cold fronts or hurricanes on the platform and upper slope. The formation of the wedge started at 13.6 ± 3.5 kyr cal BP following the meltwater pulse 1 A. It was supplied by shallow environment-derived particles produced on four narrow terraces between 60 and 20 mbsl after meltwater pulses. Since the LGM (Last Glacial Maximum), terrace formation seems to correspond to periods of sea-level stillstands and wedge formation, whilst escarpments correspond to periods of high rates of rising sea-level due to meltwater pulses. The main period of wedge growth started around 6.5 ± 0.9 kyr cal BP, when sea level finally reached the platform margins. The maximum flooding period around 4 kyr cal BP is characterized by the highest contents of aragonite needles exported from the platform.

Nearshore Influences of Upwelling, Waves, and Currents On A Tropical Carbonate Ramp: Holocene, Northwestern Yucatán Shelf, Mexico

Abstract: Many ancient carbonate strata represent deposition on gently dipping ramp systems, yet processes and patterns of carbonate accumulations of modern carbonate ramps are not as extensively studied as those on modern isolated platforms and rimmed shelves. To better understand controls on sediment accumulations of the shallowest parts of tropical carbonate ramp systems, this study investigates geomorphology, sediment, and organic-matter abundance on the nearshore areas of the Holocene ramp system on the northwestern Yucatan Shelf, Mexico. Currents generated by the day-to-day easterly trade winds, and larger waves caused by winter cold fronts, have the net effect of transporting sediment and upwelled nutrient-rich (occasionally in excess of 5 mg/m 3 , eutrophic to hypertrophic) and cooler (∼ 16–20°C) water southward. The upper shoreface includes shore-parallel belts of mollusk- and Halimeda -rich muddy sand to gravel. Longshore sediment transport along the coast forms barrier-island spits of molluscan coquina beach ridges which have prograded southward and westward. The islands in turn act as barriers to form protected lagoons that accumulate ubiquitous fine sediment. A broad supratidal to freshwater marsh landward of the lagoons includes dominantly fine-grained carbonate sediment. Across the area, mollusks are the primary sediment contributors, although Halimeda is locally ubiquitous offshore. Barnacles, benthic foraminifera, sponges, bryozoans, and serpulid worm tubes dominate the remainder of carbonate sediment producers, although echinoderms and planktonic forams are present as well. Corals are notably absent, as are peloids and ooids. Biosiliceous organisms (diatoms, sponge spicules, dinoflagellates) contribute up to 20% of sediment throughout the shoreface and lagoon, and the greater part of carbonate mud is calcitic. This assemblage, which includes many biota of the heterozoan association, is interpreted to be related to upwelled cool nutrient-rich water, although local freshwater springs may provide nutrients in the lagoons. The results of this study illustrate the roles of energy level, sea-surface temperature, upwelled nutrient-rich waters, and possibly freshwater mixing on the geomorphology, biota, and sedimentology of the nearshore parts of this Holocene ramp system. The insights provide an actualistic model for sedimentological character and depositional heterogeneity of shoreface regions of ancient carbonate ramps influenced by upwelling.

On facies belts and facies mosaics: Holocene isolated platforms, South China Sea

AbstractAlthough the general criteria for recognition and environmental interpretation of different carbonate facies are well‐established, a predictive understanding of the areal extent and spatial patterning of facies bodies and why they might organize into facies belts or facies mosaics is poorly constrained. To explore patterns and process dynamics of facies on isolated carbonate platforms, quantitative analysis of thematic maps derived from remote sensing images of 27 Holocene atolls of the Paracel and Spratly chains in the South China Sea explores variability within and among platforms. On these systems, most annular shelf‐margin reefs are less than 500 m wide on both chains; inboard of the reefs, reef sand aprons range up to 500 m (Spratlys) and 1000 m (Paracels) wide. Around individual platforms, Spratly Chain sand apron widths are wider to the north‐west, whereas apron widths in the Paracel Chain are more symmetrical; collectively, data indicate log‐normal width‐exceedance probability distributions. Platform‐interior patch reefs include area‐exceedance probability distributions and gap size distributions (lacunarity) consistent within chains, but distinct between the chains. To understand the processes underlying distinct distributions, simulations explored distinct growth scenarios. Results suggest that differences may represent distinct process classes: proportional growth processes with multiplicative random effects (reef sand aprons – belts), versus non‐linear, size‐proportional growth of randomly aged and distributed elements (patch reefs – mosaics). The probabilistically distinct sizes and spatial patterns of geomorphic elements within these general process classes are interpreted to represent ‘variations on themes’ related to the different impacts of tropical storms, winter cold fronts and circulation in each chain. The results highlight fundamentally different growth patterns impacting the sizes and distribution of facies belts and mosaics on isolated carbonate platforms. Because these types of bodies ultimately construct stratigraphy, the themes could be applied to understand and predict variability in the architecture of subsurface reservoir analogues.

Occurrence of energetic extreme oceanic events in the Colombian Caribbean coasts and some approaches to assess their impact on ecosystems

Above-normal meteorological and oceanographic conditions that generate damage on coastal ecosystems and associated human communities are called extreme oceanic events. Accurate data are needed to predict their occurrence and to understand their effects. We analyzed available data from four localities in the Colombian Caribbean to study the effect of wave-related extreme events (hurricanes, surges) in three coastal ecosystems, i.e., mangroves, beaches, and reefs. Three localities were continental (Portete Bay mangroves at the Guajira Peninsula, Bocagrande Public Beach at Cartagena City, Tayrona Natural Park reefs near Santa Marta City), and one was oceanic (Old Providence Island reefs in the San Andres and Old Providence Archipelago, SW Caribbean). We gathered data on ocean surface winds (1978–2011) for the four locations, then modeled significant wave heights, then identified extreme events, and finally tried to identify effects on the ecosystems, directly or from published literature. Wave-related extreme surges were also compiled from Colombian press news (1970–2008). Modeled wave maximums (>5m significant wave height) and press-reported events coincided with hurricanes, extreme dry season, mid-summer drought and northern hemisphere winter cold fronts, with neither a relationship to ENSO events, nor a temporal trend of increase, excepting Portete Bay, with a marked increase after 1995. Changes in Portete Bay mangroves were analyzed from aerial photographs before and after Tropical Storm Cesar (1996). In the 38years before Cesar there was mangrove inland colonization, with some loss associated to beach erosion, while during the 8years following the storm there were localized retreats and important changes in vegetation composition related to the falling of large trees and subsequent recolonization by species that are faster colonizers, and changes in soil composition brought about by inundation. Cartagena's Bocagrande Beach was followed between 2009 and 2011 by video, and two events of strong retreat were observed in 2010, one associated to the arriving of cold fronts in March, and the other to the passing of Hurricane Tomas in November–December. Together, they produced >90m beach retreat. We identified modeled wave maximums during Hurricane Lenny (1999) at Santa Marta city, and hurricane Beta (2005) at Old Providence Island, both of which, according to the literature, had transient minor effects on local coral reefs, which had been more affected by diseases and bleaching.

Sea surface temperature influence on a winter cold front position and propagation: air–sea interactions of the ‘Nortes’ winds in the Gulf of Mexico

AbstractA high‐resolution, regional atmospheric model with different sea surface temperature (SST) boundary conditions (BC) is used to examine the air–sea interactions of the winter cold fronts (CF) advancing over the Gulf of Mexico (GoM). Comparison with oceanic‐buoy 10‐m wind, 2 m air temperature (AIR.2 m), sea level pressure (SLP) and SST reveals good agreement with observations. The CF propagation speed was significantly affected by the SST: higher SST produced faster CF traveling speeds. Using a 1‐D ocean mixed layer model as BC reduced the air–sea fluxes and the CF propagated slower but in accordance with the reanalysis data; representing an improvement in the numerical modeling of the CF propagation and airmass modification over the GoM.

Evaluation of a WRF ensemble using GCM boundary conditions to quantify mean and extreme climate for the southwest of Western Australia (1970–1999)

A high resolution (5 km), single initialization, 30 year (1970–1999) Weather Research and Forecast regional climate model (RCM) ensemble for southwest Western Australia (SWWA) is evaluated. The article focuses on the ability of the RCM to simulate winter cold fronts, which are the main source of rainfall for the region, and assesses the spatial and temporal characteristics of climate extremes within the region's cereal crop growing season. To explore uncertainty, a four-member ensemble was run, using lateral boundary conditions from general circulation models (GCMs) of the Coupled Model Intercomparison Project Phase 3; ECHAM5, Model for Interdisciplinary Research on Climate 3.2 (MIROC 3.2), Community Climate System Model version 3 (CCSM3) and Commonwealth Scientific and Industrial Research Organisation (CSIRO) mk3.5. Simulations are evaluated against gridded observations of temperature and precipitation and atmospheric conditions are compared to a simulation using ERA-Interim reanalysis boundary conditions, which is used as a surrogate truth. Results show that generally, the RCM simulations were able to represent the climatology of SWWA well, however differences in the positioning of the subtropical high pressure belt were apparent which influenced the number of fronts traversing the region and hence winter precipitation biases. Systematic temperature biases were present in some ensemble members and the RCM was found to be colder than the driving GCM in all simulations. Biases impacted model skill in representing temperature extremes and this was particularly apparent in the MIROC-forced simulation, which was the worst performing RCM for both temperature and precipitation. The dynamical causes of the biases are explored and findings show that nonetheless, the RCM provides added value, particularly in the spatio-temporal representation of wet season rainfall.

Wave Climate and Trends for the Gulf of Mexico: A 30-Yr Wave Hindcast

Abstract This paper describes wave climate and variability in the Gulf of Mexico based on a 30-yr wave hindcast. The North American Regional Reanalysis wind fields are employed to drive a third-generation spectral wave model with high spatial (0.005°–0.06°) and temporal (3 hourly) resolution from 1979 through 2008. The wave hindcast information is validated using National Data Buoy Center (NDBC) data and altimeter wave information (GlobWave). The model performance is satisfactory (r2 ~ 0.90) in the Gulf of Mexico and to a lesser extent in the Caribbean Sea (r2 ~ 0.87) where only locally generated waves are considered. However, the waves generated by the Caribbean low-level jet (CLLJ) are discussed in this work. Subsequently, the yearly/monthly mean and extreme wave climates are characterized based on the (30 yr) wave hindcast information. The model results show that the mean wave climate is mainly modulated by winter cold fronts (nortes) in the Gulf of Mexico, whereas extreme wave climate is modulated by both hurricane and norte. Extreme wave heights in the Gulf of Mexico have increased at a rate of 0.07–0.08 m yr−1 in September/October because of increased cyclone intensity in the last decade. However, there is no significant trend when considering the annual statistics for extreme events. Furthermore, modeling results also suggest that the CLLJ modulates the mean wave climate in the Caribbean Sea and controls the rate of mean wave height increase (0.03 m yr−1) in the Caribbean. However, these later results need to be corroborated by extending the computational domain in order to include the swell coming from the Atlantic Ocean.

Assimilation of GPS RO Refractivity Data and Its Impact on Simulations of Trade Wind Inversion and a Winter Cold Front

Assimilation of Global Positioning System (GPS) Radio Occultation (RO) refractivity based on WRF-3DVAR is applied to numerical weather predictions (NWP) in Hawaii, where limited conventional observations and poor representation of local circulations in global analysis constrain the quality of numerical weather predictions. For a summer trade wind case, with GPS RO refractivity assimilated, the trade wind inversion is better predicted. For a winter cold front case, the propagation of the cold front is also better simulated when GPS RO refractivity is assimilated. Furthermore, the moist tongue associated with the cold front is better defined and the vertical profiles of temperature and moisture are largely improved when compared to the model run without GPS RO assimilation.

Spatial and seasonal variation in a target fishery for spotted eagle ray Aetobatus narinari in the southern Gulf of Mexico

The target fishery for the spotted eagle ray Aetobatus narinari in the southern Gulf of Mexico is little known. The landings of four small-scale vessels at two fishing localities were sampled and fishermen were interviewed in 2009. Rays landed at Campeche [mean ± standard deviation (SD) 1204 ± 225.3 mm disc width (DW)], fished at 30–50 km from the shore, were larger than rays landed at Seybaplaya (924 ± 206.5 mm DW), fished at 8–15 km from the shore. Ray catches were male biased off Campeche and female biased off Seybaplaya. Catch rate off Campeche was 6.6 (±4.9) rays per vessel trip and off Seybaplaya was 3.0 (±2.9) rays per vessel trip. Fishermen stated that catches of A. narinari are positively influenced by winter cold fronts, turbidity, low sea temperature, and new moon phase, and negatively influenced by the presence of cownose rays Rhinoptera bonasus. Spatial variation in size composition, and sex and maturity ratios of A. narinari were evident between sites. Catch rates of A. narinari varied with individual fisherman and seasonally between months with winter cold fronts versus warmer months. Fishermen reported a general decline in catches of A. narinari over recent decades in this region.

A Confirmatory Snowfall Enhancement Project in the Snowy Mountains of Australia. Part I: Project Design and Response Variables

AbstractThe Snowy Precipitation Enhancement Research Project (SPERP) was undertaken from May 2005 to June 2009 in the Snowy Mountains of southeastern Australia with the aim of enhancing snowfall in westerly flows associated with winter cold fronts. Building on earlier field studies in the region, SPERP was developed as a confirmatory experiment of glaciogenic static seeding using a silver-chloroiodide material dispersed from ground-based generators. Seeding of 5-h experimental units (EUs) was randomized with a seeding ratio of 2:1. A total of 107 EUs were undertaken at suitable times, based on surface and upper-air observations. Indium (III) oxide was released during all EUs for comparison of indium and silver concentrations in snow in seeded and unseeded EUs to test the targeting of seeding material. A network of gauges was deployed at 44 sites across the region to detect whether precipitation was enhanced in a fixed target area of 832 km2, using observations from a fixed control area to estimate the natural precipitation in the target. Additional measurements included integrated supercooled liquid water at a site in the target area and upper-air data from a site upwind of the target.

Measurement-derived Heat-budget Approaches for Simulating Coastal Wetland Temperature with a Hydrodynamic Model

Numerical modeling is needed to predict environmental temperatures, which affect a number of biota in southern Florida, U.S.A., such as the West Indian manatee (Trichechus manatus), which uses thermal basins for refuge from lethal winter cold fronts. To numerically simulate heat-transport through a dynamic coastal wetland region, an algorithm was developed for the FTLOADDS coupled hydrodynamic surface-water/ground-water model that uses formulations and coefficients suited to the coastal wetland thermal environment. In this study, two field sites provided atmospheric data to develop coefficients for the heat flux terms representing this particular study area. Several methods were examined to represent the heat-flux components used to compute temperature. A Dalton equation was compared with a Penman formulation for latent heat computations, producing similar daily-average temperatures. Simulation of heat-transport in the southern Everglades indicates that the model represents the daily fluctuation in coastal temperatures better than at inland locations; possibly due to the lack of information on the spatial variations in heat-transport parameters such as soil heat capacity and surface albedo. These simulation results indicate that the new formulation is suitable for defining the existing thermohydrologic system and evaluating the ecological effect of proposed restoration efforts in the southern Everglades of Florida.

Wintertime Supercell Thunderstorms in a Subtropical Environment: A Diagnostic Study

AbstractThe present study documents the environment, initiation, and evolution of three isolated supercell storms on 19 December 2002, as the first case near Taiwan reported in the literature, mainly using radar data and manual and gridded analyses. In a subtropical environment, the supercells occurred behind a winter cold front that provided a large west-southwesterly vertical wind shear of 6.4 × 10−3 s−1 at 0–3 km. This combined with weak-to-moderate instability (CAPE = 887 J kg−1) above the shallow surface cold air to yield a favorable environment for supercells. An approaching upper-level jet (ULJ) at 200 hPa also provided strong shear through deep layers farther aloft. Prior to storm initiation, significant daytime solar heating occurred over the mountain slopes along the coast of southeastern China, leading to development of local circulation and onshore/upslope winds, resulting in convergence and uplifting. Three storms were initiated about 80 km inland around 1400 LST near the peaks of local terrain with a northeast–southwest alignment. After formation, the three storms evolved into isolated supercells and each experienced multiple splits. The right-moving storms were usually stronger than left-moving ones and traveled eastward rapidly at about 18 m s−1 across the Taiwan Strait. The storms reached their maximum strength over the strait where low-level shear intensified during the day due to cold air surge. The northern storm also registered a peak reflectivity of 72 dBZ, the strongest ever recorded by any radar in Taiwan. Eventually, the three supercell storms made landfall over Taiwan, producing swaths of rain, hail, and property damages. Before they diminished after midnight, each of the three storms had lasted for about 10 h and propagated for over 550 km.

Modeling the Circulation of the Atchafalaya Bay System during Winter Cold Front Events. Part 1: Model Description and Validation

The Atchafalaya Bay system consists of a series of five shallow bays in southern Louisiana (U.S.A.) that are dominated by the circulation of the Atchafalaya River plume. Winter cold fronts have a significant impact on the resuspension and transport of sediments in this region, and a better understanding of the circulation during these events is absolutely necessary for determining the sediment transport patterns of the Atchafalaya Bay system and the adjacent shelf area. Understanding the circulation of this region is also crucial for environmental studies as well. This work describes the implementation of the Navy Coastal Ocean Model (NCOM), a three-dimensional numerical circulation model for tide, river, and wind-forced circulation in the Atchafalaya Bay system. The model has a cell size (Δx) of ∼800 m and is nested to a northern Gulf of Mexico model (Δx ∼5000 m), which is itself nested to the global NCOM (Δx =1/8°). Atmospheric forcing is supplied by the Navy Operational Global Atmospheric Prediction System (NOGAPS) (Δx =1°). These models are used to simulate the hydrodynamics of the Atchafalaya Bay system and Atchafalaya river plume between December 1997 and January 1998 during the passage of three winter cold fronts. The water levels, salinity, and currents predicted by NCOM are in reasonable agreement with available measurements and tide-gauge elevation data. Errors in ebb tides and wind-driven circulation are attributable to uncertainties in the bathymetry and the low spatial and temporal resolution of the NOGAPS wind fields.