Research and control of water circulation and structures of the thermochaline field in Omega Bay (Crimea) in the summer-autumn period 2019

Based on the data of two expeditions carried out in August and November 2019, the water circulation and the structure of the thermohaline field in Omega Bay are analyzed. A well-known idea on the predominantly wind nature of currents in the Sevastopol bays is confirmed. In August, under the influence of the north wind, a south-west oriented coastal wind current was observed in the Sevastopol seaside adjacent to the bay. Its speed in the upper and middle layers varied from 15 to 40 cm /s and from 10 to 15 cm /sec – at the bottom. The branch of this stream directed to the bay, interacting with the bottom rising located in its central part, contributed to the formation of an anticyclone water circulation cell. A similar dynamic situation in Omega Bay, but under conditions of a weaker and oppositely directed alongshore current, was also observed in November. It is shown that the anticyclone topographic eddy, found in the central part of Omega Bay is quasi stationary and has a topographic nature. In the structure of the thermohaline field, signs have been revealed that confirm the anticyclone nature of the local water circulation in Omega Bay, as well as the typical coastal current system generated by the wind surge, which was recorded in the August survey. In November, under the influence of convective mixing, the thermohaline field in the studied water area was more uniform. It is shown that, in contrast to relatively homogeneous waters of the most part of the bay water area, vertical stratification and frontal horizontal structure of the thermohaline field are characteristic of its kut region.

Risk Prediction of Coastal Hazards Induced by Typhoon: A Case Study in the Coastal Region of Shenzhen, China

This study presents a risk prediction of coastal hazards induced by typhoons, which are a severe natural hazard that often occur in coastal regions. Taking the coastal hazards happened in Shenzhen as a case study, where is a southeast coastal city of China, we described a methodology to predict the typhoon wind-surge-wave hazard. A typhoon empirical tracking model was adopted to construct full-track typhoon events for 1000 years, based on the statistical characteristics of observed typhoons from satellite imageries. For each individual typhoon, a wind-field model is applied to compute the wind speeds, while the Simulating Waves Nearshore and Advanced Circulation (SWAN+ADCIRC) coupled model is applied to simulate the significant wave heights (SWHs) and storm surge heights. By frequency distribution histogram, it is noted that there exhibits a heavy tail in the probability distribution of maximum surge heights and a thin tail of the peak wind speeds and SWHs in the coastal area of Shenzhen, China. Using the Generalized Pareto Distribution (GPD) model, the extreme values of typhoon wind-surge-wave associated with various return periods can be predicted. Taking account into the combined effects of the wind, surge and wave, the joint hazard maps of typhoon wind-surge-wave can be produced for the study area. The methodology of this case study can provide a new reference for risk prediction of coastal hazards induced by typhoon in similar coastal regions like Shenzhen, China.

New results on equatorial thermospheric winds and temperatures from Ethiopia, Africa

Measurements of equatorial thermospheric winds, temperatures, and 630 nm relative intensities were obtained using an imaging Fabry–Perot interferometer (FPI), which was recently deployed at Bahir Dar University in Ethiopia (11.6° N, 37.4° E, 3.7° N magnetic). The results obtained in this study cover 6 months (53 nights of useable data) between November 2015 and April 2016. The monthly-averaged values, which include local winter and equinox seasons, show the magnitude of the maximum monthly-averaged zonal wind is typically within the range of 70 to 90 ms−1 and is eastward between 19:00 and 21:00 LT. Compared to prior studies of the equatorial thermospheric wind for this local time period, the magnitude is considerably weaker as compared to the maximum zonal wind speed observed in the Peruvian sector but comparable to Brazilian FPI results. During the early evening, the meridional wind speeds are 30 to 50 ms−1 poleward during the winter months and 10 to 25 ms−1 equatorward in the equinox months. The direction of the poleward wind during the winter months is believed to be mainly caused by the existence of the interhemispheric wind flow from the summer to winter hemispheres. An equatorial wind surge is observed later in the evening and is shifted to later local times during the winter months and to earlier local times during the equinox months. Significant night-to-night variations are also observed in the maximum speed of both zonal and meridional winds. The temperature observations show the midnight temperature maximum (MTM) to be generally present between 00:30 and 02:00 LT. The amplitude of the MTM was ∼ 110 K in January 2016 with values smaller than this in the other months. The local time difference between the appearance of the MTM and a pre-midnight equatorial wind was generally 60 to 180 min. A meridional wind reversal was also observed after the appearance of the MTM (after 02:00 LT). Climatological models, HWM14 and MSIS-00, were compared to the observations and the HWM14 model generally predicted the zonal wind observations well with the exception of higher model values by 25 ms−1 in the winter months. The HWM14 model meridional wind showed generally good agreement with the observations. Finally, the MSIS-00 model overestimated the temperature by 50 to 75 K during the early evening hours of local winter months. Otherwise, the agreement was generally good, although, in line with prior studies, the model failed to reproduce the MTM peak for any of the 6 months compared with the FPI data.

Modelling suspended sediment distribution in the Selenga River Delta using LandSat data

The Selenga River is the largest tributary of Baikal Lake and it's delta covers around 600 km2. Suspended sediment concentrations (SSC) in the Selenga river delta were modelled based on LandSat images data. The seasonal variability in suspended sediment retention during the period 1989 to 2015 was calculated. The results suggest that sediment storage in the Selenga delta is observed during high discharges (> 1500 m3 s−1), whereas sediment export increases under lower flow conditions (< 1500 m3 s−1). The changes in seasonal SSC patterns are explained by wetland inundation during floods and channel erosion or Baikal wind surge during low flow periods.

Correlating Storm Surge Heights with Tropical Cyclone Winds at and before Landfall

This paper investigates relationships between storm surge heights and tropical cyclone wind speeds at 3-h increments preceding landfall. A unique dataset containing hourly tropical cyclone position and wind speed is used in conjunction with a comprehensive storm surge dataset that provides maximum water levels for 189 surge events along the U.S. Gulf Coast from 1880 to 2011. A landfall/surge classification was developed for analyzing the relationship between surge magnitudes and prelandfall winds. Ten of the landfall/surge event types provided useable data, producing 117 wind–surge events that were incorporated into this study. Statistical analysis indicates that storm surge heights correlate better with prelandfall tropical cyclone winds than with wind speeds at landfall. Wind speeds 18 h before landfall correlated best with surge heights. Raising wind speeds to exponential powers produced the best wind–surge fit. Higher wind–surge correlations were found when testing a more recent sample of data that contained 63 wind–surge events since 1960. The highest correlation for these data was found when wind speeds 18 h before landfall were raised to a power of 2.2, which provided R2 values that approached 0.70. The R2 values at landfall for these same data were only 0.44. Such results will be useful to storm surge modelers, coastal scientists, and emergency management personnel, especially when tropical cyclones rapidly strengthen or weaken while approaching the coast.

The Role of Trade Wind Surges in Tropical Cyclone Formations in the Western North Pacific

A total of 40 out of 531 tropical cyclones that formed in the western North Pacific during 1986–2005 have accompanied trade wind surges located 5°–15° latitude to the north of the pretropical cyclone disturbance centers. Composite and empirical orthogonal function analyses indicate that the trade wind surges are related to a midlatitude eastward-moving high pressure system often found during the East Asian winter monsoon. Therefore, these trade wind surge tropical cyclones tend to occur in late season (with one-third of them in December), and at lower latitudes (7° latitude lower than the climatological average formation position). The evolution of mesoscale features during formation of trade wind surge tropical cyclones is examined. Various satellite datasets show similar mesoscale patterns during their formations. A few convective lines form by convergence between the trade wind surges and the strengthening cyclonic circulation associated with incipient vortex within the 24 h before formation. Some mesoscale convective systems are embedded in the convective line with lifetimes of about 5 h, and these are illustrated through case studies. Formations usually occur when the trade winds start to decrease in magnitude and a short period after the major episodes of convection in the convective lines and mesoscale convective systems. The relationships between the temporal variability of synoptic-scale trade wind surges, the mesoscale features, and associated tropical cyclone formations are discussed.