Environmental Factors Controlling the Precipitation in California

Using observational data covering 1948–2020, the environmental factors controlling the winter precipitation in California were investigated. Empirical orthogonal function (EOF) analysis was applied to identify the dominant climate regimes contributing to the precipitation. The first EOF mode described a consistent change, with 70.1% variance contribution, and the second mode exhibited a south–east dipole change, with 11.7% contribution. For EOF1, the relationship was positive between PC1(principal component) and SST (sea surface temperature) in the central Pacific Ocean, while it was negative with SST in the southeast Indian Ocean. The Pacific–North America mode, induced by the positive SST and precipitation in the central Pacific Ocean, leads to California being occupied by southwesterlies, which would transport warm and wet flow from the ocean, beneficial for precipitation. As for the negative relationship, California is controlled by biotrophically high pressure, representing part of the Rossby wave train induced by the positive SST in the Indian ocean, which is unfavorable for the precipitation. For EOF2, California is controlled by positive vorticity at the upper level, whereas at the lower level, there is positive vorticity to the south and negative vorticity to the north, the combination of which leads to the dipole mode change in the precipitation.

Submesoscale Vorticity and Divergence in the Alboran Sea: Scale and Depth Dependence

The statistics of submesoscale divergence and vorticity (kinematic properties, KPs) in the Alboran Sea (Mediterranean Sea) are investigated, using data from drifters released during two experiments in June 2018 and April 2019 in the framework of the Coherent Lagrangian Pathways from the Surface Ocean to Interior (CALYPSO) project. Surface drifters sampling the first meter of water (CARTHE and CODE) and 15 m drifters (SVP) are considered. The area of interest is dominated by processes of strong frontogenesis and eddy formation as well as mixing, related to the high lateral gradients between Mediterranean and Atlantic waters. Drifter coverage and distribution allow to investigate the dependence of KPs on horizontal scales in a range between 1 and 16 km, that effectively bridges submesoscale and mesoscale processes, and at two depths, of 1 and 15 m. For both experiments, the surface flow is highly ageostrophic at 1 km scale, with positive vorticity skewness indicating the presence of submesoscale features. Surface divergence quickly decreases at increasing scales with a slope compatible with a turbulent process with broadband wavenumber spectrum, suggesting the influence of surface boundary layer processes such as wind effects, waves and Langmuir cells at the smaller scales. Vorticity, on the other hand, has a significantly slower decay, suggesting interaction between submesoscale and mesoscale dynamics. Results at 15 m are characterized by reduced ageostrophic dynamics with respect to the surface, especially for divergence. Submesoscale processes are present but appear attenuated in terms of KP magnitude and skewness. The results are generally consistent for the two experiments, despite the observed differences in the mixed layer stratification, suggesting that submesoscale instabilities occur mostly at surface fronts associated with filaments of Atlantic and Mediterranean waters that are present in both cases. The results are compared with previous literature results in other parts of the world ocean and a synthesis is provided. Good agreement with previous surface results is found, suggesting some general properties for divergence and vorticity scale dependence. The importance of further investigating very high resolution frontal processes at scales of tens of meters, as well as processes of interaction with high wind effects is highlighted.

Remarks on Stationary and Uniformly-rotating Vortex Sheets: Rigidity Results

In this paper, we show that the only solution of the vortex sheet equation, either stationary or uniformly rotating with negative angular velocity $$\Omega $$ Ω , such that it has positive vorticity and is concentrated in a finite disjoint union of smooth curves with finite length is the trivial one: constant vorticity amplitude supported on a union of nested, concentric circles. The proof follows a desingularization argument and a calculus of variations flavor.

Seasonal Variation of the Surface Kuroshio Intrusion into the South China Sea Evidenced by Satellite Geostrophic Streamlines

The long-term satellite altimeter and reanalysis data show that large seasonal variations are associated with geostrophic Kuroshio intrusion, but not with the current intensity, width and axis position east of Philippine. To address this issue, we examine the seasonal variability of surface intrusion patterns by a new streamline-based method. The along-streamline analysis reveals that the seasonality of geostrophic intrusion is only attributed to the cyclonic shear part of the flow, while the anticyclonic shear part always leaps across the Luzon Strait. A possible physical mechanism is proposed to accommodate these seasonal characteristics based on globally the vorticity (torque work) balance between the basin-wide negative wind stress curl and the positive vorticity fluxes induced by the lateral wall, as well as locally loss of balance between the torques of frictional stresses and normal stresses owing to the boundary gap. Through modifying the nearshore sea surface level, the northeasterly/southeasterly monsoon increases/decreases the positive vorticity fluxes in response to global vorticity balance, and simultaneously amplifies/alleviates the local imbalance by enhancing/reducing the positive frictional stress torque within the cyclonic shear layer. Therefore, in winter when the positive torque is large enough, the Kuroshio splits and the intrusion occurs, while in summer the stress torque is so weak that the entire current keeps flowing north.

Distinct Western North Pacific Tropical Cyclogenesis During Inactive Intraseasonal Phase of August 1996 and 2014

While intraseasonal oscillation was in the inactive phase over the western North Pacific (WNP) during August of 1996 and 2014, no tropical cyclone (TC) genesis occurred in August of 2014, whereas 9 TCs (average 5.7 TCs) formed in August of 1996 with 5 TCs in the northeastern part (the largest number since 1979) and 4 TCs in the southwestern part. The present analysis reveals an obvious southwest-northeast-oriented lower-level wave train over the WNP associated with anomalous convection around the Maritime Continent in August 1996. This wave train induced anomalous cyclone and enhanced convection over the northeastern WNP, which provided a favorable background for TC genesis. Over the southwestern WNP, although monthly mean anomalies were unfavorable, the intraseasonal variation contributed to positive vorticity anomalies at the time and location of TC genesis. In contrast, both monthly anomalies and daily variations of environment factors were hostile to TC genesis during August 2014.

Using reflection seismic method to estimate the fluid dynamics parameters related to wakes

<p>In this study, when using reflection seismic data to study the wakes of the Batan Islands, a method for estimating the fluid dynamics parameters such as the relative vorticity (relative Rossby number) and the relative potential vorticity is proposed. Although the relative Rossby number estimation method proposed in this study cannot guarantee absolute accuracy in the calculation value, this method is more accurate in describing the positive and negative vorticity distribution for the wakes, and the resolution of the positive and negative vorticity distribution described by this method is higher than the result of the reanalysis data. For the wakes developed in the Batan Islands, the reflection events in the wake development area have the larger inclination than the reflection events in the western Pacific water distribution area. It is also found that the negative vorticity wakes are mainly distributed on the west side of the island/ridge, and the positive vorticity wakes are mainly distributed on the east side of the island/ridge. This is consistent with the understanding of previous wakes simulations. The strong vorticity values in the study area are mainly distributed at depths above 300m, and the maximum impact depth of wakes can reach 600m. At the downstream position of the wake on the survey line 7, it can be seen that the bottom boundary layer has separated, and there is the negative vorticity wakes on the west side intruding into the positive vorticity wakes on the east side , which is presumed to be caused by the disturbance of the small anticyclone existing near the Batan Islands. For the survey line 7, the negative potential vorticity is mainly distributed on the west side of the island/ridge, and the influence range can reach the sea depth of 600m. In the negative potential vorticity region, there is strong energy dissipation and vertical shear. In this study, we don’t find the existence of submesoscale coherent vortices on the survey line 7, but find the reflection structure similar to filaments on the seismic section. Combined with the analysis of the balanced Richardson number angle of survey line 7, we speculate that the wake in the negative potential vorticity distribution area has the characteristics of symmetrical instability, and the symmetrical instability may destroy the process of filaments forming submesoscale coherent vortices.</p>

Particle-scale fluid-particle interactions in particle-laden gravity flows over the flat slope

<p>This study used the LES-DEM (Large-Eddy Simulation and Discrete Element Method) model to simulate the lock-exchange particle-laden gravity flow over a flat slope and studied its fluid-particle interactions. The following understandings are obtained. According to the longitudinal particle-fluid interaction force, the flat-slope lock-exchange PGF process can be divided into two stages: fluid conveying particles (Stage I) and particles pushing fluid (Stage II). In the early Stage I, due to the positive vorticity and the positive slip velocity, the lift force plays a leading role in the interaction force. And in the later Stage I, the drag force causes the fluid to push the particles when the lift force decreases and becomes negative due to the negative vorticity caused by the bottom resistance. In Stage II, the lift force hinders the particles’ advancement, which exceeds the drag force that transports the particles forward. The vertical suspension of particles mainly benefits from drag force and contact force, and the former is more prominent. In addition, the longitudinal transport of head particles is mainly controlled by the lift force caused by positive vorticity which is cause by the resistance from the ambient fluid at the current profile. Based on the interaction force, the study distinguishes two energy conversion modes. The final destination of the energy in the two modes is longitudinal particle kinetic energy and longitudinal fluid kinetic energy, respectively.</p>

Numerical Simulation of a Heavy Rainstorm in Northeast China Caused by the Residual Vortex of Typhoon 1909 (Lekima)

From 14 to 17 August 2019, a heavy rainstorm occurred in Northeast China due to the combined influence of the residual vortex of typhoon 1909 (Lekima) and cold air intrusion. Based on the precipitation data of China Meteorological observation stations, surface and upper charts, HMW-8 satellite images, NCEP/NCAR 0.25° × 0.25° reanalysis data and WRF4.0 numerical prediction model are used to carry out numerical simulations. According to the weather situation and numerical simulation results, the cause of 1 h severe precipitation is thoroughly studied. Results show that: (1) According to the weather situation, the precipitation process can be divided into two stages. The first stage is from 1412 to 1612 August 2019, which is caused by the interaction between the residual vortex, the inverted trough of typhoon 1909 (Lekima) and the upper trough. The rain belt lies from northeast to southwest, and the rainfall center has typical meso-β-scale characteristics. At the second stage from 1612 to 1712 August 2019, the residual vortex of typhoon reaches Heilongjiang Province, at the same time, 500 hPa cold vortex falls to the south; (2) Based on the 1 h rainfall of automatic weather stations, it can be seen that there are three rainfall peaks from 00 UTC 14 to 12 UTC 17, which are 53.2 mm in the Middle East of Jilin Province, 38.2 mm in the south of 1610 Liaoning Province, and 21.3 mm in the east of 1707 Heilongjiang Province respectively. (3) Before the occurrence of 1 h heavy rainfall, the water vapor is concentrated in the middle and lower troposphere. The residual vortex trough of typhoon 1909 extends northward, converges with the southwest airflow at the edge of the subtropical high, and transports water vapor and energy to the northeast. The convective cloud clusters generated ahead of the trough move southeast, then merge into the mesoscale convective system in the inverted trough; (4) In the Bohai Bay and North Korea, there is a vortex-like zone composed of several convergence centers, and the convergence zone in typhoon-inverted trough meets with the trough in Central Jilin. There exist a rising area and a positive vorticity belt in the typhoon-inverted trough, and the center of heavy rain lies in front of the positive vorticity center. At the west of the inverted trough, there is a large center of positive vertical wind shear, and a small center in the east. The center of heavy rainfall is located on the line between the maximum and minimum centers, which is close to the right of the maximum center; (5) The high energy tongue is transported from the center of the typhoon to the northeast along the inverted trough of the typhoon, and the southwest airflow at the edge of the subtropical high. There is a zone titled downward from northwest to southeast that contains dry and cold air, where there is convective instability; (6) The strong precipitation area is located on the lee in the northwest of Changbai Mountain. There is a convergence area in the middle of the troposphere, and a strong divergence area in the upper troposphere, with remarkable topographic effect, and the west divergence column inclines on the east convergence column.

Climatic analysis of effective jet streams frequency on extreme precipitations in west of Iran

In this study, the frequency of effective jet streams was analyzed in extreme and widespread precipitations in the west of Iran. For this purpose, the daily precipitation of 69 synoptic and climatic stations over 18,624 days (1961–2010) were selected. Then, 119 days of extreme and widespread precipitation in the study area were chosen based on generalized distribution for conducting related reviews and analyses. The frequency of jet streams in the geographical location from 0° to 120°E and −10° to 80°N were reviewed at four levels (250, 300, 400 and 500 hPa). Due to the large volume of information, only the highest and lowest levels (250 and 500 hPa) in relation to the surface were considered. According to the results, the highest frequency of jet stream was observed at 250 hPa. The second quarter of the jet stream core lay over the west of Iran (which is associated with increasing positive vorticity as well as upper-level divergence and lower-level convergence of the atmosphere). In general, the extension of jet stream up to 500 hPa indicated an unstable layer thickness, which can cause extreme and widespread precipitation in the west of Iran. The results of selected days based on cluster analysis and Lund correlation revealed that in rainy days, the wind speed was more than 50 m/s and the subtropical jet stream speed was over 40 m/s, leading to extreme precipitation in the west of Iran.

Numerical Investigation of the Turbulent Wake-Boundary Interaction in a Translational Cascade of Airfoils and Flat Plate

Rotor stator interaction (RSI) is an important phenomenon influencing performances in the pump, turbine, and compressor. In this paper, the correlation-based transition model is used to study the RSI phenomenon between a translational cascade of airfoils and a flat plat. A comparison was made between computational results and experimental results. The computational boundary layer velocity is in reasonable agreement with the experimental velocity. The thickness of boundary layer decreases as the RSI frequency increases and it increases as the fluid flows downstream. The spectral plots of velocity fluctuations at leading edge x/c = 2 under RSI partial flow condition f = 20 Hz and f = 30 Hz are dominated by a narrowband component. RSI frequency mainly affects the turbulence intensity in the freestream region. However, it has little influence on the turbulence intensity of boundary layer near the wall. A secondary vortex is induced by the wake–boundary layer interaction and it leads to the formation of a thickened laminar boundary layer. The negative-vorticity wake also facilitates the formation of a thickened boundary layer while the positive-vorticity wake has a similar effect, like a calmed region which makes the boundary layer thinner.