Diurnal Wind Variations Research Articles

Passive Optical Observation of Mesosphere and Thermosphere Wind Over Three Stations in China

AbstractFabry Perot interferometer (FPI) is an essential ground‐based passive optical observation device to detect middle and upper atmospheric information. Three FPIs are located at Kunming (103.8°E, 25.6°N), Xinglong (117.4°E, 40.2°N), and Mohe (122.3°E, 53.5°N), China. The diurnal and annual variation of night wind at 87, 97, and 250 km are investigated from 2019 to 2021, compared to Horizontal Wind Model 14 (HWM14) to check the prediction accuracy for the local wind field. Our results are as follows: (a) At 87 km, the zonal winds are similar in Kunming and Xinglong, but the meridional winds are generally stronger in Kunming. The zonal and meridional winds in both locations are dominated by semidiurnal variations. (b) At 97 km, the duration of semidiurnal variation of zonal winds and diurnal variation of meridional winds in Kunming is long, which is opposite to that of Xinglong. (c) At 250 km, the wind speed increases with latitude for both zonal and meridional winds, which are both dominated by diurnal variations for all the three sites. Unlike 87 and 97 km, the entire wind field at 250 km is dominated by annual variation, except for a significant semiannual variation at midnight in Mohe. (d) Overall, the HWM14 predictions are stronger than the FPI measurements at peak wind speeds and similar at 87 and 97 km, which surpasses the performance at 250 km. Especially at 250 km, the model results are worse for the zonal winds of Kunming and the meridional winds of Mohe. These results will help to understand the wind field in the middle and upper atmosphere and improve the accuracy of the model.

Diurnal variation of monsoon winds during the onset, active-break and withdrawal phases observed at its gateway in India: A case study of 2021

Diurnal variation of the low-level jet (LLJ) and tropical easterly jet stream (TEJ) over Cochin – the gateway of Indian summer monsoon – was investigated during the monsoon onset over Kerala (MOK), active-break, and withdrawal phases in the year 2021. Horizontal wind profiles obtained from the 205 MHz wind profiling radar at Cochin University of Science and Technology, India, were analyzed to delineate the core speed and core heights of the LLJ and TEJ. Distinct diurnal variability in wind patterns was observed during various monsoon phases. The LLJ and TEJ core speed and heights exhibited significant diurnal variations, with the LLJ reaching maximum diurnal amplitudes between 2.5 m s−1 (onset phase) and 0.5 m s−1 (break phase). Notably, the LLJ core speed during the break phase was approximately half of that during the active phase. The ratios of the diurnal range to the time mean LLJ core speed varied between 0.19 (active phase) and 1.33 (onset phase). Moreover, the LLJ core height is higher during active compared to break. Similarly, the TEJ core height at Cochin is marginally higher during active compared to that during break. Interestingly, an enhanced TEJ core speed and weakened LLJ were observed during the break phases. The variation of moisture flux exhibited positive correlation with the core speed with 2–3 h lag, significantly influencing the diurnal variability of rainfall. The causative factors behind the diurnal oscillations during various phases of the monsoon are discussed.

Daily and Subdaily Wind and Divergence Variations Observed by a Shipborne Doppler Radar off the Southwestern Coast of Sumatra

Abstract This study investigated the daily cycle of the wind and divergence fields observed off the southwestern coast of Sumatra during a field campaign of the Years of the Maritime Continent pilot study. An algorithm was developed to retrieve kinematic variables from the single-Doppler data collected aboard the Research Vessel Mirai from 24 November to 13 December 2015. The observed daily cycles of the wind and divergence fields consisted of diurnal, semidiurnal, and short-term variations. Diurnal wind variation was characterized by deep and three-dimensional circulation. There was an approximate phase locking of the semidiurnal variation to the diurnal variation, both in the wind and divergence fields. The short-term wind variation occurred at a time scale of ∼1–3 h, and this pattern was associated with density currents or mesoscale gravity waves. Up to 73% of the daily vertical motion variance can be attributed to the diurnal and semidiurnal vertical motion variations with comparable strengths. Concurrently, precipitation propagated offshore in phase with density currents and mesoscale gravity waves. Our results suggest that diurnal and semidiurnal wind variations dominate the daily evolution of precipitation, whereas density currents and mesoscale gravity waves control offshore propagation. Additionally, it appears that the daily precipitation cycle is modulated by multiple-time-scale wind variabilities of less than a day, which is also responsible for the development of strong nocturnal convection off the southwestern coast of Sumatra. Significance Statement To improve our understanding of the daily wind and divergence cycle off the southwestern coast of Sumatra, we examined wind data collected by a shipborne Doppler radar. The observed daily cycles of the wind and divergence fields consisted of diurnal and semidiurnal variations, as well as a 1–3-h variation associated with a density current or mesoscale gravity wave. Our results suggest that diurnal and semidiurnal wind variations dominate the daily evolution of precipitation, whereas density currents and mesoscale gravity waves control offshore propagation. Thus, we highlight the role of multiple-time-scale wind variabilities of less than a day in modulating the daily precipitation cycle off the southwestern coast of Sumatra.

Strong Coupling in Diurnal Variations of Clouds, Radiation, Winds, and Precipitation during the East Asian Summer Monsoon

Abstract This study investigates the diurnal cycles of convective activities and low-level winds with emphasis on their interactions during the East Asian summer monsoon. We categorize atmospheric convection states using satellite-derived cloud regimes and group southerly monsoon flows with different characteristics of diurnal cycles, and found that they are closely related to each other. Over South China, deep convection and associated cirrus anvils produce a strongly negative shortwave cloud radiative effect and induce anomalous cooling in the ABL in the daytime, which reduces the diurnal amplitude of monsoon flows during the night that follows. Conversely, a fair-weather state in the daytime leads to an anomalously warm ABL that enhances the subsequent nighttime acceleration of the monsoon southerlies. The dominant cloud regimes in the daytime over South China are closely related to whether the organized deep convection over the Yun-Gui Plateau (southeast of the Tibetan Plateau) propagates eastward during the preceding night. Such a precursor of clouds/convection governing wind diurnal amplitude indicates a possible predictability of monsoon diurnal cycles over South China. Further analyses show that the cloud regimes and induced monsoon diurnal cycles over South China can regulate the moist convection over the downstream region (central China) through modulating moisture transport and convergence. Therefore, the observational statistics of this study reveal a strong coupling of clouds, radiation, winds, and precipitation at the diurnal time scale over the summer monsoon regions, in which the cloud radiative effects of atmospheric convection can strongly regulate the diurnal variations of low-level winds that further influence precipitation downstream.

The planetary boundary layer physical processes, the secondary thermal baroclinic circulation and inertial oscillation contribution to diurnal variation of the Etesian winds over the Aegean Sea

Etesian winds constitute an important climatological phenomenon, which does not only moderate the heat during the summer in the Aegean Sea, but provides a source of clean renewable energy as well. Even though several papers have attempted to explain their dynamical and physical characteristics, the respective processes that drive the diurnal variation of the wind speed are not fully understood. The objective of this paper is to identify the processes responsible for diurnal variation with observed maximum wind speed around noon and minimum around midnight. Analytical solutions of a primitive equation set in Eulerian form, after introducing suitable conditions and approximations, reveal an inertial oscillation over the Aegean Sea. Data based on direct observations, ECMWF IFS high resolution analyses and high-resolution simulations with the Weather Research and Forecasting (WRF) model are utilized to find out the type and structure of the planetary boundary layer (PBL) over the Aegean Sea. This PBL appears to be of a marine character and turbulent mostly during the day but less during the night. The direct impact of local and regional thermally-driven circulations is found to be the main cause of the diurnal variation of the observed wind and partly the inertial oscillation. Results from numerical simulations certify these findings. Furthermore, the momentum and Newtonian heating exchanges by the physical processes inside the PBL, where the gradient wind together with smaller scales of atmospheric motions exist, are also necessary for explaining the variability of the Etesian winds.

Planetary boundary layer climatological features over North China Plain: Derived from intensive experiment data

AbstractA field campaign was carried out in the North China Plain (NCP) spanning 4 years and formed a dataset of more than 2,700 planetary boundary layer (PBL) sounding samples (872 in summer and 1,841 in winter). Based on these data, fundamental aspects of the PBL climatology over this region were investigated. First, the ensemble mean features of the PBL were revealed. The maximum PBL height in the daytime reaches 1,079 versus 786 m (summer vs. winter, hereafter the same), while about 200 m at nighttime. The bulk mean temperature and specific humidity across the PBL in summer are significantly higher than those in winter (24.0 vs. −0.5°C; 8.9 vs. 1.7 g⋅kg−1, respectively). Mean wind speed within the PBL keeps almost the same in winter and summer (4.5 m⋅s−1) but with a seasonal difference in wind direction (southerly vs. northwesterly). Second, significant diurnal variations within/above the PBL were quantified. Day‐night temperature contrast is largest near the ground (7.9 vs. 5.1°C), and it decreases with height to almost a constant above the PBL. Diurnal variation amplitude of specific humidity is about 1.0 versus 0.3 g⋅kg−1. Diurnal variation of winds also supports the extended influence of the earth surface across the PBL. Third, the local influence on the PBL over the NCP is also presented. PBL height is higher in the plain centre; temperature is lower in coast than inland; winds in the PBL are strongly modulated by local or mesoscale circulations including the plain wind and sea breeze; low‐level jets are common at summer night over the central and eastern parts of the NCP. Analysis reveals possible energy and water vapour exchange across the PBL, and additional heating mechanism above the PBL (e.g., solar radiation absorption).

An Evaluation of Surface Wind and Gust Forecasts from the High-Resolution Rapid Refresh Model

Abstract We utilized high temporal resolution, near-surface observations of sustained winds and gusts from two networks, the primarily airport-based Automated Surface Observing System (ASOS) and the New York State Mesonet (NYSM), to evaluate forecasts from the operational High-Resolution Rapid Refresh (HRRR) model, versions 3 and 4. Consistent with past studies, we showed the model has a high degree of skill in reproducing the diurnal variation of network-averaged wind speed of ASOS stations, but also revealed several areas where improvements could be made. Forecasts were found to be underdispersive, deficient in both temporal and spatial variability, with significant errors occurring during local nighttime hours in all regions and in forested environments for all hours of the day. This explained why the model overpredicted the network-averaged wind in the NYSM because much of that network’s stations are in forested areas. A simple gust parameterization was shown not only to have skill in predicting gusts in both networks but also to mitigate systemic biases found in the sustained wind forecasts. Significance Statement Many users depend on forecasts from operational models and need to know their strengths, weaknesses, and limitations. We examined generally high-quality near-surface observations of sustained winds and gusts from the nationwide Automated Surface Observing System (ASOS) and the New York State Mesonet (NYSM) and used them to evaluate forecasts from the previous (version 3) and current (version 4) operational High-Resolution Rapid Refresh (HRRR) model for a selected month. Evidence indicated that the wind forecasts are excellent yet imperfect and areas for further improvement remain. In particular, we showed there is a high degree of skill in representing the diurnal variation of sustained wind at ASOS stations but insufficient spatial and temporal forecast variability and overprediction at night everywhere, in forested areas at all times of day, and at NYSM sites in particular, which are more likely to be sited in the forest. Gusts are subgrid even at the fine grid spacing of the HRRR (3 km) and thus must be parameterized. Our simple gust algorithm corrected for some of these systemic biases, resulting in very good predictions of the maximum hourly gust.

Summer Season Precipitation Biases in 4 km WRF Forecasts Over Southern China: Diagnoses of the Causes of Biases

AbstractIn a recent study, we evaluated the performance of real‐time convection‐permitting WRF model forecasting over China. Although the overall rainfall distribution and diurnal cycle are well simulated, the model greatly overestimates the rainfall intensity in southern China and the predicted rainfall shows a southeast location bias. In this study, possible factors contributing to the precipitation forecast bias are discussed. The precipitation over southern China is found to be greatly influenced by land‐sea circulations. While the model captures diurnal variations of winds well, the predicted amplitude and onset timing of the land/sea breezes differ from observations. Cold bias of land‐surface temperature is shown to partially cause wind bias. In the afternoon, the cold bias results in a weakened inland penetration of sea breeze, which in turn leads to more rain on the eastern south‐China coastline. During nighttime, excessive cooling over land results in a stronger land breeze, leading to location bias of early morning precipitation. While the observed coastal rain belt is mostly located over land during early morning, the predicted coastal rain belt is mostly located over ocean. In addition, the model overestimates moisture at low levels, especially over the southern slope of the coastal mountains and over ocean, which explains the excessive rainfall over southern China. This study emphasizes the importance of accurately simulating land‐surface‐related variables for precipitation forecast in coastal regions where land‐sea contrast plays an important role.

Large diurnal wind variability over the western and northern Campeche Bank caused by the low latitude of the Yucatan Peninsula and its interaction with Easterlies

The northern and western Campeche Bank have an intense diurnal cycle with a significant offshore extension. Wind observations along the coast of the southern Gulf of Mexico, confirm that the breeze phenomenon occurs with great intensity and extension. To analyze this mesoscale phenomenon, a numerical simulation of high spatial-temporal resolution was performed using the Weather Research and Forecasting model (WRF). Diurnal wind components were fitted to a sinusoidal function, and the resulting parameters were used to obtain monthly elliptical hodographs. The greatest amplitude of the diurnal signal occurred in April, at the cays and islands, whereas in the continental region the amplitude of the diurnal component was greatest on the northern and western coasts of the Yucatan Peninsula. The large amplitude of this diurnal wind component is determined by a combination of different factors: the latitude, the geographical configuration of the southern Gulf of Mexico, the orography, the dominant easterly winds, and the development of an intense thermal contrast. Regarding the analysis of the wind components, a strong asymmetry was found between the sea-breeze and the land-breeze. Sea breeze winds are intense over the ocean, exceeding 10 ms−1 at 100 km from the coastline. The friction effect over land is also perceptible, reducing the wind magnitude by more than 6 ms−1. The formation of a breeze cell circulation with opposing easterly winds creates a strong convergence zone. It was found that the Yucatan Peninsula location, due to its relatively low latitude and altitude, in combination with the predominant easterly winds, favors these dynamics.

Diurnal Variations in Ocean Wind Speeds Measured by CYGNSS and Other Satellites

Ocean wind speed and/or wind vector measurements from the cyclone global navigation satellite system (CYGNSS), advanced scatterometer (ASCAT), and WindSat satellites are used to investigate diurnal wind variations over the tropical ocean using a method previously reported for diurnal amplitude estimation. The resulting diurnal variability in wind speed is examined in terms of its spatial and temporal properties. The results also show that the remotely sensed diurnal amplitudes obtained are largely consistent with those predicted by the existing reanalysis products such as the European Centre for Medium-range Weather Forecasts (ECMWF) ERA5.

Examination of the Daily Cycle Wind Vector Modes of Variability from the Constellation of Microwave Scatterometers and Radiometers

Offshore of many coastal regions, the ocean surface wind varies in speed and direction throughout the day, owing to forcing from land/sea temperature differences and orographic effects. Far offshore, both diurnal and semidiurnal wind vector variability has been noted in the Tropical Atmosphere Ocean-TRIangle Trans-Ocean buoy Network (TAO-TRITON) mooring data in the tropical Pacific Ocean. In this manuscript, the tropical diurnal wind variability is examined with microwave radiometer-derived winds from the Tropical Rainfall Measuring Mission (TRMM) and the Global Precipitation Measurement (GPM), merged with RapidScat and other scatterometer data. Since the relationship between wind speed and its zonal and meridional components is nonlinear, this manuscript describes an observationally based methodology to merge the radiometer and scatterometer-based wind estimates as a function of observation time, to generate a multi-year dataset of diurnal wind variability. Compared to TAO-TRITON mooring array data, the merged satellite-derived wind components fairly well replicate the semidiurnal zonal wind variability over the tropical Pacific but generally show more variability in the meridional wind components. The meridional component agrees with the associated mooring location data in some locations better than others, or it shows no clear dominant diurnal or semidiurnal mode. Similar discrepancies are noted between two forecast model reanalysis products. It is hypothesized that the discrepancies amongst the meridional winds are due to interactions between surface convergence and convective precipitation over tropical ocean basins.

Surface Expressions of Atmospheric Thermal Tides in the Tropical Atlantic and Their Impact on Open‐Ocean Precipitation

AbstractDiurnal and semidiurnal variations of atmospheric pressure and surface winds are fundamental to the Earth‐Sun system. Past research in the tropical Pacific shows semidiurnal and diurnal patterns in the zonal and meridional wind anomalies, respectively. While the semidiurnal zonal wind pattern is consistent with atmospheric thermal tidal forcing, it is not yet certain what drives diurnal meridional wind variability. This study examines the diurnal cycle of meridional winds in the tropical Atlantic Ocean across four different seasons and the extent to which they impact the diurnal evolution of open‐ocean precipitation in boreal summer. Comparisons of direct observations from long‐term moored buoys to an atmospheric reanalysis (MERRA‐2) show that MERRA‐2 reproduces diurnal and semidiurnal pressure and wind variations. The MERRA‐2 atmospheric thermal tides are decomposed into migrating and nonmigrating tidal components, and the linear terms in the zonal and meridional momentum equations are calculated. There is an approximate balance between the acceleration and pressure gradient terms for the zonal migrating momentum budget in the open ocean, with larger residuals or imbalances between those terms for the other budgets. The meridional nonmigrating budget shows largest residuals near the African coastline, indicating that nonlinear processes like the evening initiation of convection are important over land. This evening convection drives the meridional component of surface convergence which is translated offshore by the nonmigrating tide in the form of gravity waves, potentially modified by the African Easterly Jet. During boreal summer, this convergence helps to induce a morning peak of precipitation over the open ocean.

The Role of Weather during the Greek–Persian “Naval Battle of Salamis” in 480 B.C.

The Battle of Salamis in 480 B.C. is one of the most important naval battles of all times. This work examines in detail the climatically prevailing weather conditions during the Persian invasion in Greece. We perform a climatological analysis of the wind regime in the narrow straits of Salamis, where this historic battle took place, based on available station measurements, reanalysis and modeling simulations (ERA5, WRF) spanning through the period of 1960–2019. Our results are compared to ancient sources before and during the course of the conflict and can be summarized as follows: (i) Our climatological station measurements and model runs describing the prevailing winds in the area of interest are consistent with the eyewitness descriptions reported by ancient historians and (ii) The ancient Greeks and particularly Themistocles must have been aware of the local wind climatology since their strategic plan was carefully designed and implemented to take advantage of the diurnal wind variation. The combination of northwest wind during the night and early morning, converging with a south sea breeze after 10:00 A.M., formed a “pincer” that aided the Greeks at the beginning of the clash in the morning, while it brought turmoil to the Persian fleet and prevented them to escape to the open sea in the early afternoon hours.

Kernel Density Estimation Based Time-Dependent Approach for Analyzing the Impact of Increasing Renewables on Generation System Adequacy

Integration of non-conventional renewables such as wind and solar to the power system may affect the system reliability, especially when the proportion of renewable power in the system is large. Therefore, with a significant level of renewable penetration, the intermittency and both diurnal and seasonal variations of renewable power generation should be deliberately modeled in order to accurately quantify the power system reliability. This paper presents a novel method based on Kernel Density Estimation (KDE) for modeling intermittency and both diurnal and seasonal variations of wind and solar power generation using historical renewable power generation data. The proposed KDE based renewable power models are used with non-sequential Monte Carlo simulation to evaluate the generation system adequacy. Several case studies are conducted on IEEE reliability test system to analyze the impact of increasing renewables on the generation system adequacy. The results show that the generation system adequacy tends to decay exponentially when the renewable integration is increased. It is shown that the reliability values obtained using the proposed approach are very close to those provided by the time-consuming sequential simulations. Importance of modeling seasonal variations of wind and solar is also investigated.

Diurnal Variations of Low-Level Winds and Precipitation Response to Large-Scale Circulations during a Heavy Rainfall Event

AbstractA succession of MCSs developed during the last week of October 2016 and produced extreme heavy rainfall in central China. The event underwent an evident shift from a mei-yu-like warm scenario to an autumn cold scenario. Diurnal cycles of rainfall and low-level winds may be modulated by the shifting of large-scale atmospheric conditions. We conducted observational analyses and numerical experiments to examine how large-scale circulations influenced rainfall systems through diurnally varying processes. The results show that, in the first half (warm) period of the event, intense rainfall mostly occurred in eastern-central China with an early morning peak. It was closely related to a nocturnal southwesterly low-level jet (NLLJ) on the flank of the western Pacific subtropical high. The NLLJ formed near midnight in southern China where ageostrophic wind rotated clockwise due to Blackadar’s inertial oscillation. The NLLJ extended downstream to central China during the predawn hours due to the horizontal advection of momentum. Both the formation and extension of the NLLJ were supported by an enhanced subtropical high that provided relatively warm conditions with surface heating for boundary layer inertial oscillation and strong background southwesterly winds for momentum transport. The NLLJ induced MCSs at its northern terminus where the low-level ascent, moisture flux convergence, and convective instability were enhanced during the predawn hours. In the second half period with an intrusion of cold air, the diurnal amplitude of low-level winds became small under relatively cold and cloudy conditions. Moderate rainfall tended to occur in western-central China with a peak after midnight, most likely due to frontogenetic processes, upslope lifting, and nighttime cloud-top cooling.

Improved representation of the diurnal variation of warm season precipitation by an atmospheric general circulation model at a 10\xa0km horizontal resolution

This study investigates the diurnal variation of the warm season precipitation simulated by the Goddard Earth Observing System version 5 atmospheric general circulation model for 2 years (2005–2006) at a horizontal resolution of 10 km. The simulation was validated with the satellite-derived Tropical Rainfall Measuring Mission (TRMM) 3B42 precipitation data and the Modern-Era Retrospective analysis for Research and Applications atmospheric reanalysis for atmospheric winds and moisture. The simulation is compared with the coarse-resolution run in 50 km to examine the impacts driven by resolution change. Overall, the 10 km model tends to reproduce the important features of the observed diurnal variation, such as the amplitude and phase at which precipitation peaks in the evening on land and in the morning over the ocean, despite an excessive amplitude bias over land. The model also reproduces the realistic propagation patterns of precipitation in the vicinity of ocean coasts and major mountains. The regional characteristics of the diurnal precipitation over two regions, the Bay of Bengal and the Great Plains in North America, are examined in detail, where the observed diurnal cycle exhibits a systematic transition in the peak phase due to the development and propagation of regional-scale convective systems. The model is able to reproduce this pattern as well as the diurnal variation of low-level wind and moisture convergence; however, it is less effective at representing the nocturnal peak of precipitation over the Great Plains. The model results suggest that increasing the horizontal resolution of the model to 10 km substantially improves the representation of the diurnal precipitation cycle. However, intrinsic model deficiencies in topographical precipitation and the accurate representation of mesoscale convective systems remain a challenge.

Evaluation of the impact of high-resolution winds on the coastal waves

This study discusses the impact of high-resolution winds on the coastal waves and analyses the effectiveness of the high-resolution winds in recreating the fine-scale features along the coastal regions during the pre-monsoon season (March–May). The influence of the diurnal variation of winds on waves is studied for the Tamil Nadu coastal region using wind fields from weather research and forecast (WRF) (3 km) and European Centre for Medium-Range Weather Forecasts (ECMWF) (27.5 km). The improvement in the coastal forecast is then quantified with wave rider buoy observations. The high-resolution wind fields simulated fine-scale features like land–sea breeze events and showed good agreement with observation results. The error in the wave height and period is reduced by 8% and 46%, respectively, with the use of high-resolution forcing winds WRF over ECMWF, although the overestimation of wave energy on high frequencies due to overestimated WRF winds remains as a challenge in forecasting. The analysis also shows the importance of accurate wave forecast during a short-duration sudden wind (~12 m/s) occurrence in southern Tamil Nadu near Rameswaram during the pre-monsoon period. Low pressure forms over Tamil Nadu due to the land surface heating, resulting in a sudden increase of winds. High winds and steep waves which cause damage to the property of the coastal community near Rameswaram also were well simulated in the high-resolution forecast system with WRF winds.

Physical mechanisms of surface wind variability at Incheon Airport

AbstractThe mechanisms of surface wind variability observed by Aerodrome Meteorological Observation System at Incheon International Airport (IIA), located near the western coast of the Korean Peninsula, are investigated using cyclostationary empirical orthogonal function (CSEOF) analysis. Despite the complex nature of surface wind, as various factors contribute to its variance, distinct features of surface wind variability are separated reasonably into CSEOF modes. The physical interpretation of each mode using the ERA‐Interim reanalysis data is accomplished through regression analysis in CSEOF space. In essence, three atmospheric features dominate the surface wind variability: East Asian monsoon, land–sea breeze, and baroclinic waves. East Asian monsoon, the first mode, explains ~12% of the total variance of the surface wind at IIA. The annual reversal of the monsoon circulation due to the thermal contrast between the Asian continent and the western Pacific is captured as the single most dominant source of surface wind variability at IIA. The second CSEOF mode reveals that zonal circulation induced by land–sea breeze dominates the diurnal variation of the surface wind at IIA, and explains ~8% of the total variance. The largest fraction of variability is associated with baroclinic waves, which is separated into many CSEOF modes due to their irregular phase and wavelength, and the locations of vortex centre. Modes 3–10 are all associated with baroclinic waves and they together explain ~42% of the total variance. Baroclinic waves are amplified as they move over the ocean due to strong turbulent flux from the ocean surface induced by the associated upper tropospheric vortices.

A Characterization of the Delaware Sea Breeze Using Observations and Modeling

AbstractSea-breeze circulations are a prominent source of diurnal wind variability along coastlines throughout the world. For Delaware, the sea breeze is the largest source of variability in the coastal wind field. We developed a detailed, year-round sea-breeze climatology for the Delaware coastline using 9 years of meteorological station data and an objective sea-breeze detection algorithm. Sea-breeze fronts were identified and characterized by timing, speed, and duration as well as the resulting temperature and humidity changes. The observed temperature change associated with the Delaware sea-breeze front varied spatially, as well as with season, time of day, location, and developmental stage of the front. The observed sea breeze also had some unique features because of the location of southern Delaware on the Delmarva Peninsula and the complicated shape of the local coastline. Details of the summertime sea breeze were further explored using simulations with the Weather Research and Forecasting Model for June–August of 2000–09. Model-simulated sea-breeze characteristics were then compared with the observed sea-breeze climatology whenever possible. Results suggest that the mesoscale atmospheric model is capable of simulating the complex, observed spatial and temporal characteristics of the Delaware Sea breeze. However, the sea breeze in the model was weaker than that observed and tended to dissipate earlier in the afternoon, making it a challenging phenomenon to detect and characterize in the model. Improved detection and simulation of the sea-breeze fronts will increase our understanding of the impact this regional phenomenal has on the local climate and on the populations living by the coast.

Diurnal, monthly and seasonal variation of mean winds in the MLT region observed over Kolhapur using MF radar

Medium Frequency (MF) radar located at Kolhapur (16.8°N, 74.2°E) has been upgraded in August 2013. Since then continuous measurements of zonal and meridional winds are obtained covering larger altitudes from the Mesosphere and Lower Thermosphere (MLT) region. Diurnal, monthly and seasonal variation of these mean winds is presented in this study using four years (2013–2017) of observations. The percentage occurrence of radar echoes show maximum between 80 and 105 km. The mean meridional wind shows Annual Oscillation (AO) between 80 and 90 km altitudes with pole-ward motion during December solstice and equatorial motion during June solstice. Quasi-biennial oscillation (QBO) with weaker amplitudes are also observed between 90 and 104 km. Zonal winds show semi-annual oscillation (SAO) with westward winds during equinoxes and eastward winds during solstices between 80 and 90 km. AO with eastward winds during December solstice and westward wind in the June solstice is also observed in the mean zonal wind between 100 and 110 km. These results match well with that reported from other latitudes within Indian region between 80 and 90 km. However, above 90 km the results presented here provide true mean background winds for the first time over Indian low latitude region as the present station is away from equatorial electro-jet and are not contaminated by ionospheric processes. Further, the results presented earlier with an old version of this radar are found contaminated due to unknown reasons and are corrected in the present work. This upgraded MF radar together with other MLT radars in the Indian region forms unique network to investigate the vertical and lateral coupling.