Response to Reviewer 1

Kudryavtseva, N. A.

doi:10.5194/esd-2016-68-ac2

Cited by 2 publications

(3 citation statements)

References 28 publications

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“…Most of the cells have a similar number of measurements with a median of 301 observations per pixel, except for a few lines going from southeast to The presented estimates of average wave heights are most likely overestimated because we have removed a large part of relatively calm sea conditions from the analysis. However, the resulting map is expected to reflect well the spatial variations in the wave climate (Kudryavtseva & Soomere 2017). The obtained spatial structure of the Caspian Sea wave climate is consistent with the previous studies (e.g., Boukhanovsky et al 2011;Lopatoukhin et al 2003).…”

Section: Resultssupporting

confidence: 90%

“…To assess such changes, we consider the satellite-derived significant wave heights separately for the five regions of this water body (Fig. 1, left), following Rakisheva et al (2019) and the perception that in some similar seas such as the Baltic Sea a large part of changes in the wave properties are caused by changes in the directional structure of winds (e.g., Kudryavtseva & Soomere 2017). This partitioning includes a division of the central and the southern basin of the sea into two subbasins (II and III, and IV and V, respectively).…”

Section: Resultsmentioning

confidence: 99%

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Spatial variations in the Caspian Sea wave climate in 2002–2013 from satellite altimetry

Kudryavtseva¹,

Kussembayeva²,

Rakisheva³

et al. 2019

Estonian J. Earth Sci.

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The core properties of the wave climate and its changes in the Caspian Sea are established in terms of the annual mean significant wave height and its regional changes in 2002-2013 based on the outcome of the satellite altimetry mission JASON-1. Remotely estimated wave heights are validated against properties of the empirical distribution of instrumentally measured wave heights in the southern Caspian Sea and monthly averages of visually observed wave heights at three locations. A correction for systematic differences leads to very good correspondence between monthly averaged in situ and satellite data with a typical root mean square difference of 0.06 m.The average significant wave height in the Caspian Sea is 0.5-0.7 m in the northern basin of the sea, around 1.2 m in large parts of the central and southern basins and reaches up to 1.8 m in the northern segment of the central basin. The basin-wide average wave intensity varied insignificantly in the range of 1.02-1.14 m in 2002-2013. These estimates overestimate the wave heights by about 30% because low wave conditions are ignored. Substantial and statistically significant changes in the wave height occurred in certain areas. The wave height decreased by 0.019 ± 0.007 m/yr in the eastern segment of the central basin and by 0.04 ± 0.04 m/yr in the western segment of the southern basin. These changes can be explained by an increase in the frequency of westerly winds at the expence of southerly winds. Both basin-wide and regional extreme wave heights exhibit large interannual variations but do not show any significant trend. The patterns of changes in mean and extreme wave height are different. The average wave height has increased while the extreme wave height has decreased in the eastern segment of the southern basin.

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Section: Resultssupporting

confidence: 90%

Section: Resultsmentioning

confidence: 99%

Spatial variations in the Caspian Sea wave climate in 2002–2013 from satellite altimetry

Kudryavtseva¹,

Kussembayeva²,

Rakisheva³

et al. 2019

Estonian J. Earth Sci.

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“…At this level of reasoning, we may wonder if the visual pattern produced by waves was textured enough during the gulls’ flights for an optic flow field to be perceived. To investigate this, knowing that the average significant wave height of the Baltic Sea in 1991–2015 was in the range 0.44–1.94 m [34], which corresponds to a Beaufort number of 3 (gentle breeze, mean wind speed equivalent from 3.4 m/s to 5.4 m/s) to 4 (moderate breeze, mean wind speed equivalent from 5.5 m/s to 7.9 m/s) [6]. We deduce that gulls could see scattered or fairly frequent white-crested waves at an effective height of 10 m above the sea level.…”

Section: Discussionmentioning

confidence: 99%

Optic flow helps explain gulls’ altitude control over seas

Serres

Åkesson

et al. 2019

Preprint

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For studies of how flying animals control their flight, seabirds are of particular interest to track with a biologger because they forage offshore where the visual environment can be simply modeled by a flat world textured by waves. This study suggests that optic flow can explain gull's altitude control over seas. In particular, a new flight model that includes both energy and optical invariants (called the ventral optic flow regulation) explain the dynamics of gulls' altitude control during offshore takeoff and cruising flight. A linear statistical model applied to 352 flights from 16 individual lesser black backed gulls (Larus fuscus) gave a strong correlation between wind assistance and gulls' altitude. Thereafter, an optic flow-based flight model was applied to 18 offshore takeoff flights from 9 individual gulls. By introducing an upper limit in climb rate in a non-linear first order parametric model on the gull's elevation dynamics, coupled with an optic-flow set-point, the predicted altitude gives an optimized fit factor value of 63% on average (min value: 30%, max value: 83%) with respect to GPS data. We conclude that the optic-flow regulation principle (here running close to 25 • /s) allows gulls to adjust their altitude over sea without having to directly measure their current altitude.

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Response to Reviewer 1

Cited by 2 publications

References 28 publications

Spatial variations in the Caspian Sea wave climate in 2002–2013 from satellite altimetry

Spatial variations in the Caspian Sea wave climate in 2002–2013 from satellite altimetry

Optic flow helps explain gulls’ altitude control over seas

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