Micro pulse lidar observations of mineral dust layer in the lower troposphere over the southwest coast of Peninsular India during the Asian summer monsoon season

A large aerosol plume with optical depth exceeding 0.7 engulfs most parts of the Arabian Sea during the Asian summer monsoon season. Based on Micro Pulse Lidar observations during the June–September period of 2008 and 2009, the present study depicts, for the first time, the existence of an elevated dust layer occurring very frequently in the altitude band of 1–3.5 km over the west coast of peninsular India with relatively large values of linear depolarization ratio (δ_L). Large values of δ_L indicate the dominance of significantly non-spherical aerosols. The aerosol optical depth of this layer (0.2) is comparable to that of the entire atmospheric column during dust-free days. Back-trajectory analysis clearly shows the advection of airmass from the arid regions of Arabia and the west Arabian Sea, through the altitude region centered around 3 km. This is in contrast to the airmass below 1 km originating from the pristine Indian Ocean region which contains relatively spherical aerosols of marine origin with δ_L generally <0.05.     

Diatom community dynamics in a tropical,monsoon-influenced environment: West coast of India

Diatom communities are influenced by environmental perturbations, such as the monsoon system that impact the niche opportunities of species. To discern the influence of the monsoon system on diatom community structure, we sampled during two consecutive post-monsoons (2001 and 2002) and the intervening pre-monsoon at Mumbai and Jawaharlal Nehru ports along the central west coast of India. Characteristic temporal shifts in diatom community structure were observed across the sampling periods; these were mainly driven by temperature, salinity and dissolved oxygen saturation. The nutrient-poor pre-monsoon period supported low abundance yet high species richness and diversity of diatoms. Coscinodiscus, Cyclotella, Thalassiosira, Triceratium, Pleurosigma, Skeletonema and Surirella were the most dominant genera. Both the post-monsoon periods, following dissimilar monsoon events, were dominated by Skeletonema costatum, but differed in some of the residual species. Thalassiosira and Thalassionema spp. dominated mostly during post-monsoon I whereas Triceratium and Pleurosigma spp. dominated during post-monsoon II. To understand the underlying ecological mechanisms involved in such dynamics, we focus on the dominant diatom species in post-monsoon periods, S. costatum, that contributes up to 60% to total diatom cell numbers. This research is relevant in light of the fluctuating monsoon regimes over the Asian continent, the confounding effects of anthropogenic eutrophication and the resulting cascading effects on trophic web dynamics.     

Simulating the daily discharge of the Mandovi River,west coast of India

Abstract

A hydrological modelling framework was assembled to simulate the daily discharge of the Mandovi River on the Indian west coast. Approximately 90% of the west-coast rainfall, and therefore discharge, occurs during the summer monsoon (June–September), with a peak during July–August. The modelling framework consisted of a digital elevation model (DEM) called GLOBE, a hydrological routing algorithm, the Terrestrial Hydrological Model with Biogeochemistry (THMB), an algorithm to map the rainfall recorded by sparse raingauges to the model grid, and a modified Soil Conservation Service Curve Number (SCS-CN) method. A series of discharge simulations (with and without the SCS method) was carried out. The best simulation was obtained after incorporating spatio-temporal variability in the SCS parameters, which was achieved by an objective division of the season into five regimes: the lean season, monsoon onset, peak monsoon, end-monsoon, and post-monsoon. A novel attempt was made to incorporate objectively the different regimes encountered before, during and after the Indian monsoon, into a hydrological modelling framework. The strength of our method lies in the low demand it makes on hydrological data. Apart from information on the average soil type in a region, the entire parameterization is built on the basis of the rainfall that is used to force the model. That the model does not need to be calibrated separately for each river is important, because most of the Indian west-coast basins are ungauged. Hence, even though the model has been validated only for the Mandovi basin, its potential region of application is considerable. In the context of the Prediction in Ungauged Basins (PUB) framework, the potential of the proposed approach is significant, because the discharge of these (ungauged) rivers into the eastern Arabian Sea is not small, making them an important element of the local climate system.

Editor D. Koutsoyiannis; Associate editor S. Grimaldi

Citation Suprit, K., Shankar, D., Venugopal, V. and Bhatkar, N.V., 2012. Simulating the daily discharge of the Mandovi River, west coast of India. Hydrological Sciences Journal, 57 (4), 686–704.     

Connection of sea level variability between the tropical western Pacific and the southern Indian Ocean during recent two decades

Based on the merged satellite altimeter data and in-situ observations,as well as a diagnosis of linear baroclinic Rossby wave solutions,this study analyzed the rapidly rise of sea level/sea surface height(SSH)in the tropical Pacific and Indian Oceans during recent two decades.Results show that the sea level rise signals in the tropical west Pacific and the southeast Indian Ocean are closely linked to each other through the pathways of oceanic waveguide within the Indonesian Seas in the form of thermocline adjustment.The sea level changes in the southeast Indian Ocean are strongly influenced by the low-frequency westward-propagating waves originated in the tropical Pacific,whereas those in the southwest Indian Ocean respond mainly to the local wind forcing.Analyses of the lead-lag correlation further reveal the different origins of interannual and interdecadal variabilities in the tropical Pacific.The interannual wave signals are dominated by the wind variability along the equatorial Pacific,which is associated with the El Ni?o-Southern Oscillation;whereas the interdecadal signals are driven mainly by the wind curl off the equatorial Pacific,which is closely related to the Pacific Decadal Oscillation.     

Sea level changes induced by local winds on the west coast of India

The contribution of atmospheric pressure and local wind to sea level variability at Goa (West coast of India) for the period 2007–2008 is investigated. Sea level data from a tide gauge are compared with measured local surface meteorological as well as oceanographic data. Multilinear regression analysis is used to resolve the dependence of sea level on various forcing parameters. The multilinear regression analysis performed over approx. 2-year data shows that the local surface meteorological data and water temperature account for the sea level variability only up to 6%. The accounted sea level variability increases to 25%, when the local wind and the surface currents obtained from satellite altimetry in the near vicinity of the study area are incorporated in the regression analysis. The contribution of local wind increases substantially when the regression is performed over a 2-month duration, and it is variable within the year. During the summer monsoon season (May–September), the sea level variability attributable to wind is up to 47% and 75%, respectively, for 2007 and 2008; however, it reduces to <20% during the winter monsoon (November–February) season. A significant part of the variability observed in sea level remains unaccounted for and is attributed to remote forcing.     

Role of Ocean in the Variability of Indian Summer Monsoon Rainfall

Asian summer monsoon sets in over India after the Intertropical Convergence Zone moves across the equator to the northern hemisphere over the Indian Ocean. Sea surface temperature (SST) anomalies on either side of the equator in Indian and Pacific oceans are found related to the date of monsoon onset over Kerala (India). Droughts in the June to September monsoon rainfall of India are followed by warm SST anomalies over tropical Indian Ocean and cold SST anomalies over west Pacific Ocean. These anomalies persist till the following monsoon which gives normal or excess rainfall (tropospheric biennial oscillation). Thus, we do not get in India many successive drought years as in sub-Saharan Africa, thanks to the ocean. Monsoon rainfall of India has a decadal variability in the form of 30-year epochs of frequent (infrequent) drought monsoons occurring alternately. Decadal oscillations of monsoon rainfall and the well-known decadal oscillation in SST of the Atlantic Ocean (also of the Pacific Ocean) are found to run parallel with about the same period close to 60 years and the same phase. In the active–break cycle of the Asian summer monsoon, the ocean and the atmosphere are found to interact on the time scale of 30–60 days. Net heat flux at the ocean surface, monsoon low-level jetstream (LLJ) and the seasonally persisting shallow mixed layer of the ocean north of the LLJ axis play important roles in this interaction. In an El Niño year, the LLJ extends eastwards up to the date line creating an area of shallow ocean mixed layer there, which is hypothesised to lengthen the active–break (AB) cycle typically from 1 month in a La Niña to 2 months in an El Niño year. Indian monsoon droughts are known to be associated with El Niños, and long break monsoon spells are found to be a major cause of monsoon droughts. In the global warming scenario, the observed rapid warming of the equatorial Indian ocean SST has caused the weakening of both the monsoon Hadley circulation and the monsoon LLJ which has been related to the observed rapid decreasing trend in the seasonal number of monsoon depressions.     

Evolution of Vertical Moist Thermodynamic Structure Associated with the Indian Summer Monsoon 2010 in a Regional Climate Model

The 2010 boreal summer marked a worldwide abnormal climate. An unprecedented heat wave struck East Asia in July and August 2010. In addition to this, the tropical Indian Ocean was abnormally warm during the summer of 2010. Several heavy rainfall events and associated floods were also reported in the Indian monsoon region. During the season, the monsoon trough (an east–west elongated area of low pressure) was mostly located south of its normal position and monsoon low pressure systems moved south of their normal tracks. This resulted in an uneven spatial distribution with above-normal rainfall over peninsular and Northwest India, and deficient rainfall over central and northeastern parts of India, thus prediction (and simulation) of such anomalous climatic summer season is important. In this context, evolution of vertical moist thermodynamic structure associated with Indian summer monsoon 2010 is studied using regional climate model, reanalysis and satellite observations. This synergised approach is the first of its kind to the best of our knowledge. The model-simulated fields (pressure, temperature, winds and precipitation) are comparable with the respective in situ and reanalysis fields, both in intensity and geographical distribution. The correlation coefficient between model and observed precipitation is 0.5 and the root-mean-square error (RMSE) is 4.8 mm day^?1. Inter-comparison of model-simulated fields with satellite observations reveals that the midtropospheric temperature [Water vapour mixing ratio (WVMR)] has RMSE of 0.5 K (1.6 g kg^?1), whereas the surface temperature (WVMR) has RMSE of 3.4 K (2.2 g kg^?1). Similarly, temporal evolution of vertical structure of temperature with rainfall over central Indian region reveals that the baroclinic nature of monsoon is simulated by the model. The midtropospheric warming associated with rainfall is captured by the model, whereas the model failed to capture the surface response to high and low rainfall events. The model has strong water vapour loading in the whole troposphere, but weaker coherent response with rainfall compared to observations. Thus, strong water vapour loading and overestimation of rainfall are reported in the model. This study put forward that the discrepancy in the model-simulated structure may be reduced by assimilation of satellite observations.     

Future wave climate over the west-European shelf seas

In this paper, we investigate changes in the wave climate of the west-European shelf seas under global warming scenarios. In particular, climate change wind fields corresponding to the present (control) time-slice 1961–2000 and the future (scenario) time-slice 2061–2100 are used to drive a wave generation model to produce equivalent control and scenario wave climate. Yearly and seasonal statistics of the scenario wave climates are compared individually to the corresponding control wave climate to identify relative changes of statistical significance between present and future extreme and prevailing wave heights. Using global, regional and linked global–regional wind forcing over a set of nested computational domains, this paper further demonstrates the sensitivity of the results to the resolution and coverage of the forcing. It suggests that the use of combined forcing from linked global and regional climate models of typical resolution and coverage is a good option for the investigation of relative wave changes in the region of interest of this study. Coarse resolution global forcing alone leads to very similar results over regions that are highly exposed to the Atlantic Ocean. In contrast, fine resolution regional forcing alone is shown to be insufficient for exploring wave climate changes over the western European waters because of its limited coverage. Results obtained with the combined global–regional wind forcing showed some consistency between scenarios. In general, it was shown that mean and extreme wave heights will increase in the future only in winter and only in the southwest of UK and west of France, north of about 44–45° N. Otherwise, wave heights are projected to decrease, especially in summer. Nevertheless, this decrease is dominated by local wind waves whilst swell is found to increase. Only in spring do both swell and local wind waves decrease in average height.     

A cold pool formation in the Lakshadweep Sea during Indian summer monsoon

In Lakshadweep Sea, the distribution of observed sea surface temperature (SST) during summer monsoon season (June–September) shows the presence of a distinct cold pool (SST?<?27°C). Available satellite measurements and assimilated datasets are utilized to investigate the characteristics and mechanisms that govern the genesis and evolution of this cold pool. It is located in the grid 8° N–10° N/74° E–76° E, with a diameter of about 200?km, centered approximately at 9° N/75° E off the southwest coast of India. This cold pool, which we call as the Lakshadweep cold pool (LCP), forms invariably during the fifth pentad of June as a small cooling within the cold surface waters advected northward along the southwest coast of India from the Arabian Sea Mini Cold Pool. With the progress of the season, LCP intensifies, spread radially outwards and shows a westward spread during late July. Maximum intensity and radial spread are attained during July. By the end of August, LCP extends northward along the coast up to 13° N, and by September, it gets completely dissipated. Within the LCP, the thermocline exhibits pronounced shoaling compared to the adjacent regions. The intensity, duration, and spread of LCP showed annual variations in each summer monsoon during 1998–2005 and owes its origin to upwelling produced by uplift of poleward undercurrent induced by an elevated bathymetry in the presence of a seamount. The mechanism for the intensification is thought to be due to the combined action of Ekman pumping due to positive wind stress curl, eddy-induced upwelling due to the Lakshadweep low, and the intensification of the poleward undercurrent during the season. West- and northward spreads of LCP are attributed to the westward movement of Lakshadweep Low and the northerly spreading and intensification of positive wind stress curl, respectively. The mechanisms that govern this phenomenon are thoroughly examined.     

Role of the Himalayan Orography in Simulation of the Indian Summer Monsoon using RegCM3

In this study, sensitivity of the Indian summer monsoon simulation to the Himalayan orography representation in a regional climate model (RegCM) is examined. The prescribed height of the Himalayan orography is less in the RegCM model than the actual height of the Himalayas. Therefore, in order to understand the impact of the Himalayan orography representation on the Indian summer monsoon, the height of the Himalayan orography is increased (decreased) by 10 % from its control height in the RegCM model. Three distinct monsoon years such as deficit (1987), excess (1988) and normal rainfall years are considered for this study. The performance of the RegCM model is tested with the use of a driving force from the reanalysis data and a global model output. IMD gridded rainfall and the reanalysis-2 data are used as verification analysis to validate the model results. The RegCM model has the potential to represent mean rainfall distribution over India as well as the upper air circulation patterns and some of the semi-permanent features during the Indian summer monsoon season. The skill of RegCM is reasonable in representing the variation in circulation and precipitation pattern and intensity during two contrasting rainfall years. The simulated seasonal mean rainfall over many parts of India especially, the foothills of the Himalaya, west coast of India and over the north east India along with the whole of India are more when the orography height is increased. The low level southwesterly wind including the Somali jet stream as well as upper air circulation associated with the tropical easterly jet stream become stronger with the enhancement of the Himalayan orography. Statistical analysis suggests that the distribution and intensity of rainfall is represented better with the increased orography of RegCM by 10 % from its control height. Thus, representation of the Himalayan orography in the model is close to actual and may enhance the skill in seasonal scale simulation of the Indian summer monsoon.     

Evolution of an anticyclonic eddy southwest of Taiwan

Satellite images of sea-surface temperature, surface chlorophyll a concentration, and sea-level anomaly, together with ocean reanalysis data of Asia and Indian–Pacific Ocean (AIPOcean1.0), are utilized to study the three-dimensional characteristics and evolution of an anticyclonic warm eddy adjacent to the southwest coast of Taiwan during October and November 2006. Originated from the Kuroshio intrusion in the Luzon Strait, but unlike previously found westward moving anticyclonic eddies (AE) in the northeastern South China Sea, this AE was so close to the Taiwan coast and stayed where it was formed for over 1 month until it dissipated. Energy analysis is utilized to study the evolution process of the AE, and it shows that the barotropic instability (BTI) and baroclinic instability introduced by the Kuroshio intrusion flow appear to be the main energy sources for the AE. Periodical enhancement/relaxation of local northeasterly monsoon and its associated negative wind stress curl modify the current patterns in this region, reinforce the intraseasonal variability of the Kuroshio intrusion flow, and act together with Kuroshio to form the AE. Eddies detected from AIPOcean1.0 data also show that AEs are most likely to be generated southwest of Taiwan during the transition period of summer monsoon to winter monsoon, and generally, the BTI of Kuroshio intrusion contributes more than the direct wind stress work to the increase of the eddy kinetic energy for the generation and growth of the AEs.     

Spectral wave characteristics in the coastal waters of the central west coast of India during tropical cyclone Kyarr

Ocean Dynamics - The study presents the assessment of spectral wave conditions in the coastal waters of the central west coast of India based on data from waverider buoys at two locations during...     

Hydroclimatic influence of large‐scale circulation on the variability of reservoir inflow

In this study, the nature of basin‐scale hydroclimatic association for Indian subcontinent is investigated. It is found that, the large‐scale circulation information from Indian Ocean is also equally important in addition to the El Niño‐Southern Oscillation (ENSO), owing to the geographical location of Indian subcontinent. The hydroclimatic association of the variation of monsoon inflow into the Hirakud reservoir in India is investigated using ENSO and EQUatorial INdian Ocean Oscillation (EQUINOO, the atmospheric part of Indian Ocean Dipole mode) as the large‐scale circulation information from tropical Pacific Ocean and Indian Ocean regions respectively. Individual associations of ENSO & EQUINOO indices with inflow into Hirakud reservoir are also assessed and found to be weak. However, the association of inflows into Hirakud reservoir with the composite index (CI) of ENSO and EQUINOO is quite strong. Thus, the large‐scale circulation information from Indian Ocean is also important apart form the ENSO. The potential of the combined information of ENSO and EQUINOO for predicting the inflows during monsoon is also investigated with promising results. The results of this study will be helpful to water resources managers due to fact that the nature of monsoon inflow is becoming available as an early prediction. Copyright © 2009 John Wiley & Sons, Ltd.     

The 26 December 2004 Sumatra Tsunami: Analysis of Tide Gauge Data from the World Ocean Part 1. Indian Ocean and South Africa

The M_w = 9.3 megathrust earthquake of December 26, 2004 off the northwest coast of Sumatra in the Indian Ocean generated a catastrophic tsunami that was recorded by a large number of tide gauges throughout the World Ocean. Part 1 of our study of this event examines tide gauge measurements from the Indian Ocean region, at sites located from a few hundred to several thousand kilometers from the source area. Statistical characteristics of the tsunami waves, including wave height, duration, and arrival time, are determined, along with spectral properties of the tsunami records.     

Major moisture sources for Western and Southern India and their role on synoptic‐scale rainfall events

Although the identification of the moisture sources of a region is of prominent importance to characterize precipitation, the origin and amount of moisture towards the Indian Subcontinent and its relationship with the occurrence of precipitation are still not completely understood. In this article, the origin of the atmospheric water arriving to the Western and Southern India during a period of 5 years (1 January 2000–31 December 2004) is investigated by using a Lagrangian diagnosis method. This methodology computes budgets of evaporation minus precipitation by calculating changes in the specific humidity of thousands of air particles aimed to the study area following the observed winds. During the summer monsoon, the main supply of moisture is the Somali Jet, which crosses the equator by the West Indian Ocean. The recycling process is the main water vapour source in winter. Two additional moisture sources located over northwestern India and the Bay of Bengal are identified. A 30% increase in the moisture flux from the Indian Ocean has been related to the occurrence of strong precipitation in the area, and at the end of the monsoon, the recycling became a significant contribution to the last part of the wet season of Western and Southern India. Copyright © 2011 John Wiley & Sons, Ltd.     

Influence of southern oscillation index on the variability and predictability of Indian monsoon. A reappraisal

The relationship between the monsoon rainfall throughout all India, northwest India and peninsular India as well as the onset dates of the monsoon and two indices of southern oscillation (SOI), namely Isla de Pascua minus Darwin (I-D) and Tahiti minus Darwin (T-D) pressure anomaly have been studied for different periods. The study indicates that the monsoon rainfall shows a strong and significant direct relationship with SOI for the concurrent, succeeding autumn and succeeding winter seasons. The magnitude of the direct correlation coefficient for the SOI using (I-D) is enhanced over all India and peninsular India if the above seasons happen to be associated with an easterly phase of the QBO (Quasi-Biennial Oscillation) at 50 mb. The result indicates that the strength of the monsoon plays an important role in the following southern oscillation events in the Pacific Ocean. The premonsoon tendency of the SOI anomaly spring minus winter SOI shows a significant positive correlation with monsoon rainfall over all India, northwest India and peninsular India. The absolute value of the positive correlation coefficient becomes highly enhanced over all India, northwest India as well as peninsular India if the 6-month period from December to March is associated with the westerly phase of the QBO. Hence, the premonsoon SOI tendency parameter can be a useful predictor of Indian monsoon rainfall especially if it happens to be associated with the westerly QBO. Significant negative association is also found between the anomaly of monsoon onset dates and SOI of the previous spring season, the absolute value being higher for SOI (T-D) than for SOI (I-D). The negative correlation coefficient becomes enhanced if the previous springs are associated with a westerly phase of the QBO. It shows that the previous spring SOI has some predictive value for the onset date of Indian monsoon, a positive SOI followed by an early onset of monsoon, andvice versa, especially if it is associated with a westerly phase of the QBO.     

Impact of local and remote winds on the shelf circulation off the central west coast of India

A linear coastal-trapped-wave (CTW) model is used to examine the effects of large-scale winds, with time scale ranging from a few days to a few weeks, on the West India Coastal Current (WICC), particularly on the shelf off the central west coast of India. We show that unlike the seasonal cycle of WICC, which is primarily forced by the winds along the east coast of India, the high-frequency WICC is mostly driven by the west-coast winds. Nevertheless, the influence of winds as far as Sri Lanka and east coast of India cannot be neglected. Simple numerical experiments with the CTW model show that the strong current observed at Goa (15° N) compared to Bhatkal (13° N) and Jaigarh (17° N) is due to two factors: (1) the superposition of local and remote CTWs and (2) the widening of shelf width north of Goa, which decreases the amplitude of the currents poleward of Goa. If the local winds are weak, the amplitude of current decreases poleward due to friction, and the current at the south leads the north. We also note that the observed phase difference between sea level and alongshore current at Goa could be attributed to the propagation of remotely forced higher-order modes of CTWs.     

The effect of climate change on extreme waves in front of the Dutch coast

Coastal safety may be influenced by climate change, as changes in wave conditions (height, period, direction) may increase the vulnerability of dunes and other coastal defences. Dune erosion depends on mean water level, storm surge height and wave conditions. In this paper, we investigate the change in wave conditions in the North Sea in a changing climate. Until now, the effect of climate change on annual maximum wave conditions has been investigated, while events with higher return periods are actually most damaging for the coast (e.g. severe dune erosion). Here, we use the 17-member Ensemble SimulationS of Extreme weather under Non-linear Climate changeE (ESSENCE) change of climate change simulations, to analyse A1b-induced changes in the mean wave climate, the annual maxima and wave conditions with return periods of up to 1:10,000?years in front of the Dutch coast. The mean wave climate is not projected to differ between 1961–1990 and 2071–2100, with both wave height (H _s) and wave period (T _m) remaining unaltered. In the annual maximum conditions, a decrease is projected; especially, the annual T _m maximum decreases significantly by 0.3 to 0.6?s over the whole study area. Furthermore, we find that the direction of the annual maximum wave conditions shifts from north and north-west to west and south-west for both H _s and T _m. This is induced by a similar shift in the direction of the extreme wind speeds. Despite the decrease in annual maximum conditions, the return H _s and T _m are not projected to change significantly as a result of climate change in front of the Dutch coast for the period 2071–2100 relative to 1961–1990.     

Remote forcing of water levels by tropical cyclones in southwest Australia

Tropical cyclones (termed hurricanes and typhoons in other regions), are extreme events associated with strong winds, torrential rain and storm surges (in coastal areas) and cause extensive damage as a result of strong winds and flooding (caused by either heavy rainfall or ocean storm surges) in the immediate area of impact. The eastern Indian Ocean, particularly in the northwest region of Australia, is impacted by up to 10 tropical cyclones during the cyclone season, although direct impact of cyclones along the west and southwest coastlines is rare. However, the sub-tidal frequency component of sea level records along the west and south coasts of Western Australia indicates lagged correspondence with the occurrence of tropical cyclones. It is demonstrated that the tropical cyclones generate a continental shelf wave which travels along the west and south coasts of Australia up to 3500 km with speeds of 450–500 km day⁻¹ (5.2–5.8 ms⁻¹) with maximum trough to crest wave height of 0.63 m, comparable with the mean daily tidal range in the region. The shelf wave is identified in the coastal sea level records, initially as a decrease in water level, 1–2 days after the passage of the cyclone and has a period of influence up to 10 days. Amplitude of the shelf wave was strongly affected by the path of the tropical cyclone, with cyclones travelling parallel to the west coast typically producing the most significant signal due to resonance and superposition with local forcing. Analysis of water levels from Port Hedland, Geraldton, Fremantle and Albany together with cyclone paths over a ten year period (1988–1998) indicated that the tropical cyclones paths may be classified into 6 different types based on the amplitude of the wave.     

Inversion of swell frequency from a 1-year HF radar dataset collected in Brittany (France)

This article presents long period ocean wave (swell) frequencies inverted from a 13-month dataset of high-frequency (HF) phased array radars and an assessment of these estimates by comparison with WAVEWATCH III model data. The method of swell frequency inversion from high-frequency radar sea echo Doppler spectra is described. Radar data were collected from a two-site HF Wellen Radar (WERA) radar system on the west coast of Brittany (France) operating at 12 MHz. A standard beam-forming processing technique has been used to obtain Doppler spectra of processed radar cells. Swell frequencies are obtained from the frequencies of particular spectral peaks of the second-order continuum in hourly averaged Doppler spectra. The data coverage of effective Doppler spectra considered for swell frequency estimates shows the influence of islands and shallow water effects. Swell estimates from both radar stations are in good agreement. The comparison of radar-derived results to WAVEWATCH III (WW3) estimates shows that radar measurements agree quite well with model results. The bias and standard deviation between two estimates are very small for swells with frequency less than 0.09 Hz (period >11 s), whereas radar estimates are generally lower than model estimates for shorter swells, along with higher standard deviation. Statistical analysis suggests that radar measurement uncertainty explains most of the difference between radar and model estimates. For each swell event, time series of frequency exhibits a quasi-linear frequency increase which is associated with the dispersive property of wave phase velocity. The use of swell frequency estimates from both radars on common radar cells only slightly increases the accuracy of swell frequency measurement.