Side Of Fronts Research Articles

Statistical Relationships between Two Types of Heavy Rainfall and Low-Level Jets in South China

AbstractTwo types of heavy rainfall, namely warm-sector and frontal heavy rainfall, coexist in South China during the presummer rainy season and manifest as varying mechanisms and features. They both exhibit close relationships with two types of low-level jets (LLJs): the boundary layer jet (BLJ) and synoptic-system-related low-level jet (SLLJ), but in different ways. The motivation of the present study is to elucidate the statistical relations between the two types of heavy rainfall and LLJs over South China using TRMM rainfall data and ERA5 reanalysis. Generally, warm-sector heavy rainfall mainly occurs over coastal areas and during the early morning, which is primarily caused by the interaction between the nocturnal BLJ and land breeze. In contrast, frontal heavy rainfall is mostly concentrated in inland regions and modulated by distinct diurnal forcings at different locations. Statistical analysis indicates that 76% (62%) of the warm-sector (frontal) heavy rainfall events are associated with LLJs. In the presence of heavy rainfall, low-level winds are often strengthened over Beibu Gulf, northern South China Sea, and the south side of fronts, corresponding to two branches of southerly BLJs at ~950 hPa over the ocean and the southwesterly SLLJs at ~850–700 hPa on land, respectively. Furthermore, BLJs are shown to be linked to both types of heavy rainfall and with the most frequent occurrences of rainfall in their exit region, whereas SLLJs are more closely associated with frontal heavy rainfall. The left side (entrance) of the SLLJ axis is favorable for frontal (warm-sector) heavy rainfall production. The regional rainfall distributions are affected by the structures and locations of LLJs.

Wind-Forced Submesoscale Symmetric Instability around Deep Convection in the Northwestern Mediterranean Sea

During the winter from 2009 to 2013, the mixed layer reached the seafloor at about 2500 m in the northwestern Mediterranean Sea. Intense fronts around the deep convection area were repeatedly sampled by autonomous gliders. Subduction down to 200–300 m, sometimes deeper, below the mixed layer was regularly observed testifying of important frontal vertical movements. Potential Vorticity dynamics was diagnosed using glider observations and a high resolution realistic model at 1-km resolution. During down-front wind events in winter, remarkable layers of negative PV were observed in the upper 100 m on the dense side of fronts surrounding the deep convection area and successfully reproduced by the numerical model. Under such conditions, symmetric instability can grow and overturn water along isopycnals within typically 1–5 km cross-frontal slanted cells. Two important hotpspots for the destruction of PV along the topographically-steered Northern Current undergoing frequent down-front winds have been identified in the western part of Gulf of Lion and Ligurian Sea. Fronts were there symmetrically unstable for up to 30 days per winter in the model, whereas localized instability events were found in the open sea, mostly influenced by mesoscale variability. The associated vertical circulations also had an important signature on oxygen and fluorescence, highlighting their under important role for the ventilation of intermediate layers, phytoplankton growth and carbon export.

Submesoscale Flows Associated with Convergent Strain in an Anticyclonic Eddy of the Kuroshio Extension: A High-resolution Numerical Study

Based on fine-resolution simulations, we present a case study of the frontogenetic generation of submesoscale flows associated with a convergence field inside an anticyclonic mesoscale eddy in the Kuroshio Extension. The results reveal that the generation of submesoscale flows on the edge of the eddy is closely related to the convergent zone in the ageostrophic secondary circulation (ASC) that develops from frontal sharpening. Diagnostic analysis of the frontal tendency based on flow decomposition shows that frontal sharpening is initiated by mesoscale strain and accelerated by submesoscale convergence. With convergence on the cold side and divergence on the warm side of fronts, submesoscale divergent fields contribute to the asymmetry of the frontal tendency because the fronts tend to be strengthened by convergent motions and weakened by divergent motions. During frontal sharpening, the cross-front ASCs generate additional positive vorticity in their downwardbranches characterized by convergence via vortex stretching, which involves neighboring submesoscale vortices. Moreover, the enhanced lateral buoyancy gradients and surface buoyancy loss are conducive to triggering symmetric instability (SI) and ageostrophic anticyclonic instability (AAI) on the edge of the eddy. In response to these submesoscale instabilities and the subsequent slumping of fronts, the removal of kinetic energy (KE) associated with shear flows is comparable to the source of submesoscale KE from the eddy, which indicates that mesoscale energy can be drained by submesoscale instabilities during frontal sharpening. This study focuses on and describes the frontogenetic generation of submesoscale flows in mesoscale eddies in the Kuroshio Extension.

Reply to “Comments on ‘A CloudSat–CALIPSO View of Cloud and Precipitation Properties across Cold Fronts over the Global Oceans’”

In Naud et al., a compositing method was utilized with CloudSat– CALIPSO observations to obtain mean transects of cloud vertical distribution and surface precipitation across cold fronts, and to examine their sensitivity to the large-scale properties of the parent extratropical cyclone. This reply demonstrates the value of compositing for evaluating numerical models, and presents additional results that address the issue of the sensitivity of the initial results to the frontal detection methodology and the potential misclassification of occlusions as cold fronts. Here a sensitivity study of the cold front composite transects of cloud cover to the input datasets or the method utilized to locate the cold fronts demonstrates that these composite transects are robust and only marginally sensitive to cold front location methods. The same conclusion is reached for the robustness of the contrast between Northern and Southern Hemisphere cloud transects. While occlusions cannot directly be flagged within the database at this point, comparisons of transects obtained for subsets of cyclones of different age indicate that the misclassification of occluded fronts as cold fronts does not explain the predominance of cloud and precipitation on the warm side of the cold fronts. The strong signal on the warm side might be better explained by a predominance of forward sloping cold fronts, or the presence of the warm conveyor belt.

Short-term variation in copepod community and physical environment in the waters adjacent to the Kuroshio Current

A continuous survey examined short-term variations in the zooplankton community and physical ocean environment from the northeastern Izu Islands to Boso Peninsula in Japan. High copepod abundance and small upwellings in the surface layer and salinity minimum layer in the subsurface were observed on the north side of coastal fronts in the westernmost transect, moving southward as the Kuroshio Current left the Boso Peninsula. Thus, the salinity minimum layer might be a key factor forming upwelling and the fronts, leading to large abundance of coastal copepods off the northeastern Izu Islands. A community structure analysis of calanoid copepods revealed an intermediate belt assemblage between coastal and offshore (Kuroshio) assemblages. Copepod abundance was remarkably low and Ctenocalanus vanus dominated (nearly 37%) in the intermediate belt zone, indicating that C. vanus has a relatively high tolerance to adverse environments for calanoid copepods. As the Kuroshio Current left the Boso Peninsula, the coastal assemblage expanded in the same direction, and the intermediate belt assemblage off the northeastern Izu Islands disappeared. The largest population of Calanus sinicus was found along the two western transects off the northeastern Izu Islands (>1000 m depth), which was assumed to be transported from Sagami Bay and advanced southwestward while growing from copepodite stages CIII to CV. Larvae of C. sinicus would be an important food for fish larvae in addition to Paracalanus parvus s.l., the numerically dominant species in the coastal assemblage, and C. vanus under the adverse conditions for coastal copepods.

Low-frequency and high-frequency oscillatory winds synergistically enhance nutrient entrainment and phytoplankton at fronts

When phytoplankton growth is limited by low nutrient concentrations, full-depth-integrated phytoplankton biomass increases in response to intermittent mixing events that bring nutrient-rich waters into the sunlit surface layer. Here, it is shown how oscillatory winds can induce intermittent nutrient entrainment events and thereby sustain more phytoplankton at fronts in nutrient-limited oceans. Low frequency (i.e. synoptic to planetary scale) along-front wind drives oscillatory cross-front Ekman transport, which induces intermittent deeper mixing layers on the less dense side of fronts. High-frequency wind with variance near the Coriolis frequency resonantly excites inertial oscillations, which also induce deeper mixing layers on the less dense side of fronts. Moreover, we show that low and high frequency winds have a synergistic effect and larger impact on the deepest mixing layers, nutrient entrainment, and phytoplankton growth on the less dense side of fronts than either high-frequency winds or low frequency winds acting alone. These theoretical results are supported by two-dimensional numerical simulations of fronts in an idealized nutrient-limited open-ocean region forced by low and high frequency along-front winds. In these model experiments, higher-amplitude low-frequency wind strongly modulates and enhances the impact of the lower-amplitude high-frequency wind on phytoplankton at a front. Moreover, sensitivity studies emphasize that the synergistic phytoplankton response to low and high frequency wind relies on the high-frequency wind just below the Coriolis frequency. This article is protected by copyright. All rights reserved.

Biological and physical impacts of ageostrophic frontal circulations driven by confluent flow and vertical mixing

Subduction, upwelling, and phytoplankton blooms are commonly observed features at oceanic fronts. This study isolates the role of vertical mixing for enhanced production and water mass subduction near fronts, considering the time-developing problem with a Semi-Geostrophic circulation model coupled to a planktonic ecosystem model. Our model results show that vertical mixing in the surface boundary layer strongly modifies the time evolution of the front and of its associated biology. Ageostrophic flows caused by the combined effects of confluence and vertical mixing enhance primary production on the less dense side and increase water mass subduction on the dense side of the front. Confluence alone results in the intensification of the front by the same advective response, while the phytoplankton bloom on the less dense side does not arise without vertical mixing. Vertical mixing alone slumps the front near the surface and provides weak subduction on the dense side and uplift of the isopycnals at the center of the front. We find that it is possible to sustain an isolated phytoplankton patch above the domed isopycnals at the center of the front with the nutrients supplied by the secondary circulations arising due to vertical mixing. These results suggest that the phytoplankton bloom and patches found on the less dense side of fronts in many field observations are likely caused by fine-scale along-isopycnal upwelling of nutrients forced by adiabatic confluence in the meander trough of fronts and further pumping and entrainment of nutrients by the secondary circulation due to vertical mixing. Isolated patches observed at the center of the front in many frontal surveys could be caused by secondary flows due to vertical mixing.

A Five-Year Climatology of Elevated Severe Convective Storms in the United States East of the Rocky Mountains

Abstract A 5-yr climatology of elevated severe convective storms was constructed for 1983–87 east of the Rocky Mountains. Potential cases were selected by finding severe storm reports on the cold side of surface fronts. Of the 1826 days during the 5-yr period, 1689 (91%) had surface fronts east of the Rockies. Of the 1689 days with surface fronts, 129 (8%) were associated with elevated severe storm cases. Of the 1066 severe storm reports associated with the 129 elevated severe storm cases, 624 (59%) were hail reports, 396 (37%) were wind reports, and 46 (4%) were tornado reports. A maximum of elevated severe storm cases occurred in May with a secondary maximum in September. Elevated severe storm cases vary geographically throughout the year, with a maximum over the south-central United States in winter to a central and eastern U.S. maximum in spring and summer. A diurnal maximum of elevated severe storm cases occurred at 2100 UTC, which coincided with the diurnal maximum of hail reports. The wind reports had a broad maximum during the daytime. Because the forecasting of hail from elevated storms typically does not pose as significant a forecast challenge as severe wind for forecasters and tornadoes from elevated storms are relatively uncommon, this study focuses on the occurrence of severe wind from elevated storms. Elevated severe storm cases that produce only severe wind reports occurred roughly 5 times a year. To examine the environments associated with cases that produced severe winds only, five cases were examined in more detail. Common elements among the five cases included elevated convective available potential energy, weak surface easterlies, and shallow near-surface stable layers (less than 100 hPa thick).

Influence of SST on catches of swordfish and tuna in the Portuguese domestic longline fishery

Catch‐and‐effort data from a longline vessel operating off the west coast of Portugal, and thermal imagery from NOAA satellites were employed to investigate the aggregation and availability of swordfish, bigeye and albacore tuna in relation to transient coastal upwelling features. We found a decreasing trend in swordfish catch per unit effort (CPUE) and an increasing trend in tuna species CPUE during the study period in apparent association with the progressively increasing intensity of the coastal upwelling regime from year to year. Swordfish CPUE were significantly higher on the warm side of surface thermal fronts associated with events of intensification/relaxation of coastal upwelling. The fishing efficiency for tuna species was significantly higher during peak upwelling conditions, in the close vicinity of mushroom‐like structures at the edge of upwelling filaments. The present results do not support the existence of a preferred sea surface temperature (SST) range for these fish species. It is suggested that several mechanisms leading to the local concentration of prey provide adequate explanations for the aggregation of these species in the vicinity of frontal structures associated with coastal upwelling processes.

Local and external components of the summertime plankton community in Lough Hyne, Ireland a stratified marine inlet

Water samples were collected intensively during summer to study the processes affecting the plankton community in Lough Hyne, Ireland. Multivariate analysis of species counts demonstrated a clear separation between lough samples and water taken from incoming tides. Differences between the lough and adjacent water reflected the strong stratification within the lough. The dinoflagellates Gonyaulax verior, Ceratium furca, Ceratium fusus, Prorocentrum micans and Protoperidinium steinii and the silicoflagellate Dictyocha speculum were all significantly more abundant in the lough than in tidal inflow. The diatoms Thalassionema nitzshioides and Cylindrotheca closterium as well as copepods, nauplii and tintinnids were also more abundant in lough samples. Only Rhizosolenia hebetata, Gymnodinium spp. and Striatella unipunctata had significantly higher average adundances in tidal inflow than in lough samples. However, the average counts of these species were an order of magnitude smaller than the average counts for the most abundant dinoflagellates within the lough. Instead of reflecting the species composition of the adjacent coastal waters, the phytoplankton community in the lough is closer to the situation observed on the stratified side of fronts in the Celtic Sea. There are enhanced populations of dinoflagellates and cell counts of these species tend to increase following spring tides.

Effects of mean flow divergence on turbulent scalar flux and local flame structure in premixed turbulent combustion

Analysis of the balance equation for Favre-averaged progress variable variance developed previously areextended to open premixed flames in this paper. It is shown that both conditional mean dilatation and bulk-flow straining are linked to the turbulent scalar flux being countergradient (CGD) or gradient (GD) diffusion. The effect of bulk-flow straining is further investigated by using recent experimental data of joint particle image velocimetry/laser-induced OH fluorescence measurements in turbulent premixed Bunsenand stagnation-type flames. A diverging mean flow field is found for these open flames witheither CGD or GD transport, and the conditional mean dilatation always peaks near the burnt side of the local flame fronts. The coincidence of the conditional mean dilatation peaking at low values of progress variable and the gradient-type turbulent scalar flux is expected only for confined premixed flames without lateral mean flow divergence or for simulated flames in direct numerical simulations (DNS) having no bulk-flow straining. A positive correlation has been observed between scalar diffusion and scalar dissipation in both DNS data and in laser measurements of open premixed flames. For these flames at high turbulent levels where gradient transport prevails, the conflicting behavior of local flame-front thinning or thickening is reconciled.

The Kuroshio Fronts and Cold Eddies off Northeastern Taiwan Observed by NOAA-AVHRR Imageries

Over a hundred AVHRR-HRPT thermal imageries of NOAA satellites which covered dimensions 350Km (columns) x 280Km (rows) with center point at 26°N and 123°E, period from October 1989 to December 1990, had been derived to observe the surface features of waters off northeasern Taiwan. The patterns of Kuroshio fronts in this water (KEEP area) can be generalized to three types: (A) front extending away from northeastern Taiwan northeastwardly, (B) front extending away from northeastern Taiwan north- eastwardly, then turning into an anticlockwise frontal eddy, and (C) front extending westward along coast of northern Taiwan then away with an arcuate curve and turning to east. The locations of cold eddies, another major phenomenon in this area, can also be generalized to three types related to Kuroshio fronts: (A) eddy formed with no Kuroshio fronts around, (B) eddy located on cool (west) side of fronts, and (C) eddy located on warm (east) side of fronts. Front Type-A is the dominant and most stable pattern in summer. On the contrary, in other seasons, all three front types are variable and have different degrees of intrusion onto shelf waters. Regarding the cold eddies, the most common is Type-B, probably existing in each season. The next most common is Type-A, which prevails in summer. The least common is Type-C, which usually appears in winter. Regarding the relationship between front types and eddy types, the most prevailing composition is the cold eddy Type-B associated with the front Type-A.

Ocean frontal variability in the Frontal Air‐Sea Interaction Experiment

Oceanographic observations during the Frontal Air‐Sea Interaction Experiment (FASINEX) in the Sargasso Sea indicate that upper ocean fronts in the subtropical convergence are strongly surface‐intensified coherent flow structures which have a preferred orientation and translation speed. Fronts grow as they propagate in this region such that currents roughly double in strength in a time that they translate by twice their width. While currents are dominated by geostrophic momentum balance, the Rossby number of these flows can reach a few tenths. Mixed layer water from the cold side of fronts appears to subduct under warm‐side near‐surface waters, since isolated lenses of water with cold‐side temperature and salinity characteristics are found embedded in the seasonal thermocline on the warm side of fronts. Although plausible vertical speeds were observed, no actual rates of subduction could be estimated. Typical frontal widths, temperature contrasts, horizontal currents, vertical shears, vertical currents, and translation speeds of fronts in the region studied are 20 km, 1°C, 0.5 m s−1, 0.005 s−1, 0.0005 m s−1, and 0.2 m s−1, respectively.

Some analyses and numerical simulations of meiyu in east asia in 1983

In this paper the differences between Meiyu and Baiu front in 1983 have firstly been analysed, the trajectories of air on and to the north side of Meiyu and Baiu fronts during the Meiyu period have then been traced, and the forecasting and simulating of 4 sets of Meiyu onset of the year have finally been run utilizing the global model at UK Meteorological Office. The results show: 1) Meiyu fronts are different from Baiu ones in temperature, humidity and stratification fields in lower atmosphere; and the possibly reasons for it are explained. 2) The Bay of Bengal is the main moisture source for Meiyu front, the South China Sea and the Pacific, for Baiu ones; and some existed arguments on it are also discussed. 3) The onset of Meiyu and its rainfall and rain belts are sensitive to the Tibetan Plateau, and the water vapour conditions over the Bay of Bengal and the South China Sea, but not sensitive to the SST over the equatorial area or to the East of Japan.

Frontogenesis and Geostrophically Forced Secondary Circulations in the Vicinity of Jet Stream-Frontal Zone Systems

Abstract Frontogenesis and geostrophically forced secondary circulations in the vicinity of jet stream-frontal zone systems are treated, assuming that the motions conform to the geostrophic momentum approximation and that the secondary circulations are confined to the cross-front plane. Absolute momentum is used as a frontal defining parameter as its spatial discontinuities are shown to bound the tow hyper-gradient domain of upper jet-front systems. The solution of the Sawyer-Eliassen secondary circulation equation for simulated and observed frontal zones illustrates the importance of the geostrophic shearing deformation of an along-front thermal gradient in forcing strong subsidence within and to the warm side of upper fronts. The Miller frontogenesis equations are rederived with the frontal processes partitioned into those arising from geostrophic and ageostrophic motions. The transformation of the frontogenesis equations into geostrophic momentum, isentropic coordinates is introduced to further simplif...