Classification and properties of coastal wind profiles with negative gradients – an observational study

Wind profiles with a negative gradient are frequently occurring over the Baltic Sea and are important to take into consideration for offshore wind power as they affect not only the power production, but also the loads on the structure and the behavior of the wake behind the turbine. In this study, we classified non-normal profiles as wind profiles having negative shear in at least one part of the profile between 28 and 300 m: low-level jets (with a local wind maximum in the profile), profiles with a local minimum, and negative profiles. Using observations spanning over 3 years, we show that the non-normal wind profiles are common over the Baltic Sea in late spring and summer, with a peak of 40 % relative occurrence in May. Negative profiles (in the 28–300 m layer) were mostly occurring during unstable conditions, in contrast to low-level jets that primarily occurred in stable stratification. There were indications that the the zone with strong shear during low-level jets could cause a relative suppression of the variance for large turbulent eddies compared to the peak of the velocity spectra, in the layer below the jet core. Swell conditions were found to be favourable for the occurrence of negative profiles and profiles with a local minimum, as the waves fed energy into the surface layer, resulting in an increase of the wind speed from below.

Transmission modes of COVID-19 disease pandemic in the light of ancient wisdom of Ayurveda medicine: a review

Human-to-human transmission of the Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) occurs most often when people are in the incubation stage of the disease or are carriers and have no symptoms. SARS-CoV-2 transmission on different levels showed that the cold and dry climate, hot wind and relative humid wind, maximum wind speed, turbulent wind, intensified recirculating flow were causing transmission higher than factors. The role of environmental factors described in Ayurveda like air, water, soil, season, frequent earthquakes, sunlight exposure, frequent thunderstorm with lightening, and factors from arthropods were revalidated in perspective of COVID-19 transmission. The authors searched the information regarding epidemic diseases in Ayurveda textbooks. Pubmed, Science Direct, Wikipedia, Elsevier, Lancet, and Springer were approached for the latest knowledge relating to SARS-CoV-2 and COVID-19. Google Scholar search engine was thoroughly checked upon for scientific evidence regarding the Ayurveda drugs. Various environmental factors like soil, air, water, frequent earthquake disasters, wildlife animals, aquatic birds, space, inevitable disastrous factors, weather or seasonal hazards, violent thunderstorm with lightning, intermediate hosts, sunlight exposure etc. were considered for their role in the genesis of the disease. The Ayurvedic concepts behind the etiology and development of epidemics are the same as modern epidemiological theories. The mysteries of many aspects of the current pandemic might be deciphered by traditional medicine knowledge and thus adding upon to the integrative medicine concept.

The Unconventional Eyewall Replacement Cycle of Hurricane Ophelia (2005)

Hurricane Ophelia (2005) underwent an unconventional eyewall replacement cycle (ERC) as it was a Category 1 storm located over cold sea surface temperatures near 23°C. The ERC was analyzed using airborne radar, flight-level, and dropsonde data collected during the Hurricane Rainband and Intensity Change Experiment (RAINEX) intensive observation period on 11 September 2005. Results showed that the spin-up of the secondary tangential wind maximum during the ERC can be attributed to the efficient convergence of absolute angular momentum by the mid-level inflow of Ophelia’s dominantly stratiform rainbands. This secondary tangential wind maximum strongly contributed to the azimuthal mean tangential wind field, which is conducive for increased low-level supergradient winds and corresponding outflow. The low-level supergradient forcing enhanced convergence to form a secondary eyewall. Ophelia provides a unique example of an ERC occurring in a weaker storm with predominantly stratiform rainbands, suggesting an important role of stratiform precipitation processes in the development of secondary eyewalls.

Beneficial role of diurnal smoothing for grid integration of wind power

<p>Smoothing of wind generation variability is important for grid integration of large-scale wind power plants. One approach to achieving smoothing is aggregating wind generation from plants that have uncorrelated or negatively correlated wind speed. It is well known that the wind speed correlation on average decays with increasing distance between plants, but the correlations may not be explained by distance alone. In India, the wind speed diurnal cycle plays a significant role in explaining the hourly correlation of wind speed between location pairs. This creates an opportunity of “diurnal smoothing”. At a given separation distance the hourly wind speeds correlation is reduced for those pairs that have a difference of +/- 12 hours in local time of wind maximum. This effect is more prominent for location pairs separated by 200 km or more and where the amplitude of the diurnal cycle is more than about  0.5 m/s. “Diurnal smoothing” also has a positive impact on the aggregate wind predictability and forecast error. “Diurnal smoothing” could also be important for other regions with diurnal wind speed cycles.</p>

The diminishing contribution of low-level jets to the U.S. Great Plains water budget

<p>In the semi-arid U.S. Great Plains, nocturnal southerly low-level jets (LLJs) serve critical roles as conveyors of remotely-sourced (i.e., Gulf of Mexico) water vapor and agents of atmospheric instability in the warm-season.  Defined by a diurnally oscillating wind maximum between 0–3 km above the surface, LLJs have been studied by meteorologists for over 60-years due to their role in severe weather outbreaks. It is only within the past decade that a subset of LLJs with especially high vertically integrated water vapor transport, termed atmospheric rivers, have drawn the attention of hydrologists.</p><p>In this study, changes in LLJ frequency and structure over the period from 1901–2010 are quantified using ECMWF’s Coupled Reanalysis of the Twentieth Century (CERA-20C). A new objective dynamical LLJ classification dataset is used to separately quantify changes in the two predominant LLJ types: synoptically coupled and uncoupled. The findings reveal that both the frequency of Great Plains LLJs and their associated precipitation have decreased significantly over the 20th century. Decreases in LLJ associated precipitation range between 10–14% of total present day May–September precipitation. The largest differences observed are attributable to uncoupled jet frequency and structural changes during July and August over the central and northern Great Plains. Overall, the results indicate the contribution of LLJs to the region’s water budget has diminished.</p>

Asymmetric Rainband Processes Leading to Secondary Eyewall Formation in a Model Simulation of Hurricane Matthew (2016)

The dynamics of an asymmetric rainband complex leading into secondary eyewall formation (SEF) are examined in a simulation of Hurricane Matthew (2016), with particular focus on the tangential wind field evolution. Prior to SEF, the storm experiences an axisymmetric broadening of the tangential wind field as a stationary rainband complex in the downshear quadrants intensifies. The axisymmetric acceleration pattern that causes this broadening is an inward-descending structure of positive acceleration nearly 100 km wide in radial extent and maximizes in the low levels near 50 km radius. Vertical advection from convective updrafts in the downshear-right quadrant largely contributes to the low-level acceleration maximum, while the broader inward-descending pattern is due to horizontal advection within stratiform precipitation in the downshear-left quadrant. This broad slantwise pattern of positive acceleration is due to a mesoscale descending inflow (MDI) that is driven by midlevel cooling within the stratiform regions and draws absolute angular momentum inward. The MDI is further revealed by examining the irrotational component of the radial velocity, which shows the MDI extending downwind into the upshear-left quadrant. Here, the MDI connects with the boundary layer, where new convective updrafts are triggered along its inner edge; these new upshear-left updrafts are found to be important to the subsequent axisymmetrization of the low-level tangential wind maximum within the incipient secondary eyewall.

Transition of the Hurricane Boundary Layer during the Landfall of Hurricane Irene (2011)

The hurricane boundary layer (HBL) has been observed in great detail through aircraft investigations of tropical cyclones over the open ocean, but the coastal transition of the HBL has been less frequently observed. During the landfall of Hurricane Irene (2011), research and operational aircraft over water sampled the open-ocean HBL simultaneously with ground-based research and operational Doppler radars onshore. The location of the radars afforded 13 h of dual-Doppler analysis over the coastal region. Thus, the HBL from the coastal waterways, through the coastal transition, and onshore was observed in great detail for the first time. Three regimes of HBL structure were found. The outer bands were characterized by temporal perturbations of the HBL structure with attendant low-level wind maxima in the vicinity of rainbands. The inner core, in contrast, did not produce such perturbations, but did see a reduction of the height of the maximum wind and a more jet-like HBL wind profile. In the eyewall, a tangential wind maximum was observed within the HBL over water as in past studies and above the HBL onshore. However, the transition of the tangential wind maximum through the coastal transition showed that the maximum continued to reside in the HBL through 5 km inland, which has not been observed previously. It is shown that the adjustment of the HBL to the coastal surface roughness discontinuity does not immediately mix out the residual high-momentum jet aloft. Thus, communities closest to the coast are likely to experience the strongest winds onshore prior to the complete adjustment of the HBL.

Outer Rainbands–Driven Secondary Eyewall Formation of Tropical Cyclones

Secondary eyewall formation (SEF) could be considered as the aggregation of a convective-ring coupling with a tangential wind maximum outside the primary eyewall of a tropical cyclone (TC). The dynamics of SEF are investigated using idealized simulations based on a set of triplet experiments, whose differences are only in the initial outer-core wind speed. The triplet experiments indicate that the unbalanced boundary layer (BL) process driven by outer rainbands (ORBs) is essential for the canonical SEF. The developments of a secondary tangential wind maximum and a secondary convective ring are governed by two different pathways, which are well coupled in the canonical SEF. Compared with inner/suppressed rainbands, the downwind stratiform sectors of ORBs drive significant stronger BL convergence at its radially inward side, which fastens up the SEF region and links the two pathways. In the wind-maximum formation pathway, the positive feedback among the BL convergence, supergradient force, and relative vorticity within the BL dominates the spinup of a secondary tangential wind maximum. In the convective-ring formation pathway, the BL convergence contributes to the ascending motion through the frictional-forced updraft and accelerated outflow associated with the supergradient force above the BL. Driven only by inner rainbands, the simulated vortex develops a fake SEF with only the secondary convective ring since the rainband-driven BL convergence is less enhanced and thus fails to maintain the BL positive feedback in the wind-maximum pathway. Therefore, only ORBs can promote the canonical SEF. It also infers that any environmental/physical conditions favorable for the development of ORBs will ultimately contribute to SEF.

Corridors of Mei-Yu-Season Rainfall over Eastern China

Successive mesoscale convective systems may develop for several days during the mei-yu season (June–July) over eastern China. They can yield excessive rainfall in a narrow latitudinal band (called a corridor), causing severe floods. The climatology of rainfall corridors and related environmental factors are examined using 20 yr of satellite rainfall and atmospheric data. A total of 93 corridors are observed over eastern China, with maximum occurrence at 27°–31°N. They typically last 2–3 days, but some persist ≥4 days, with an extreme event lasting 11 days. These multiday convective episodes exhibit primary and secondary peaks in the morning and afternoon, respectively, with a diurnal cycle that is in contrast to other afternoon-peak rain events. On average, the corridors occur in ~23% days of the mei-yu season, but they can contribute ~51% of the total rainfall. They also vary with years and explain ~70% of the interannual variance of mei-yu-season rainfall. Composite analyses show that most corridors develop along zonally oriented quasi-stationary mei-yu fronts over central China where monsoon southwesterlies converge with northerly anomalies from the midlatitudes. The monsoon flow accelerates at ~0200 LST and forms a regional wind maximum or low-level jet over South China, which induces moisture flux convergence in morning-peak corridors. The nocturnal acceleration is less evident for afternoon-peak corridors. The mei-yu front and monsoon southwesterlies also influence the corridor’s duration, which is regulated by a dipole of geopotential anomalies, with positive in the tropics and negative in the midlatitudes. The dipole expresses a joint influence of the blocking patterns in midlatitudes and the El Niño–related anomalous high over the western Pacific Ocean, and the dipole's intensity explains well the interannual variations of the corridors.

Diurnal Valley Winds in a Deep Alpine Valley: Observations

Diurnal valley winds frequently form over complex topography, particularly under fair weather conditions, and have a significant impact on the local weather and climate. Since diurnal valley winds result from complex and multi-scale interactions, their representation in numerical weather prediction models is challenging. Better understanding of these local winds based on observations is crucial to improve the accuracy of the forecasts. This study investigates the diurnal evolution of the three-dimensional mean wind structure in a deep Alpine valley, the Rhone valley at Sion, using data from a radar wind profiler and a surface weather station operated continuously from 1 September 2016 to 17 July 2017. In particular, the wind profiler data was analyzed for a subset of days on which fair weather conditions allowed for the full development of thermally driven winds. A pronounced diurnal cycle of the wind speed, as well as a reversal of the wind direction twice per day is documented for altitudes up to about 2 km above ground level (AGL) in the warm season and less than 1 km AGL in winter. The diurnal pattern undergoes significant changes during the course of the year. Particularly during the warm-weather months of May through to September, a low-level wind maximum occurs, where mean maximum up-valley velocities of 8–10 m s−1 are found between 15–16 UTC at altitudes around 200 m AGL. In addition, during nighttime, a down-valley jet with maximum wind speeds of 4–8 m s−1 around 1 km AGL is found. A case study of a three-day period in September 2016 illustrates the occurrence of an elevated layer of cross-valley flow around 1–1.5 km AGL.