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Differences Between ASCAT, OSCAT and QuikSCAT Derived Vorticity Mark A. Bourassa and Heather Holbach Center for Ocean-Atmospheric Prediction Studies & Department of Earth, Ocean and Atmospheric Science The Florida State University

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Motivations & Approach <ul><li>Spatial derivatives (curl and divergence) of wind or stress are very important in regards to ocean forcing and convection in the atmosphere. </li></ul><ul><ul><li>The ocean transport and mixing are both critical to climate </li></ul></ul><ul><ul><li>The atmospheric convection and subsidence are important for mixing between the free atmosphere and the boundary-layer </li></ul></ul><ul><li>Goal: estimate the spatial resolution of winds needed for these processes </li></ul><ul><li>For this study, we will focus on a short period where we have data from ASCAT, QuikSCAT, and the OceanSat-2 scatterometer (OSCAT) </li></ul><ul><li>The technique for calculating area averaged vorticity at different spatial scales is published (Bourassa and McBeth-Ford, 2010, JTECH) </li></ul><ul><ul><li>This technique allows the influences of spatial smoothing to be examined </li></ul></ul><ul><ul><li>Bourassa and McBeth-Ford (2010) showed that determining vorticity from swath data was noisy for relatively fine scale resolutions </li></ul></ul>Heather Holbach Mark Bourassa

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Examples from the same day IGARSS 2009 IGARSS 2009 Vorticity (10 -5 s -1 ) <ul><li>QSCAT </li></ul><ul><ul><li>25km product </li></ul></ul><ul><ul><li>Single wide swath </li></ul></ul><ul><ul><li>Rain flag </li></ul></ul><ul><li>OSCAT </li></ul><ul><ul><li>50km product </li></ul></ul><ul><ul><li>Single wide swath </li></ul></ul><ul><ul><li>No rain flag </li></ul></ul><ul><li>ASCAT </li></ul><ul><ul><li>25km product </li></ul></ul><ul><ul><li>Two swaths </li></ul></ul><ul><ul><li>No rain flag, </li></ul></ul><ul><ul><li>but has quality flag </li></ul></ul>OSCAT 01Z ASCAT 12Z QSCAT 06Z

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Methodology <ul><li>Relative vorticity is determined by dividing the circulation by the area. </li></ul><ul><ul><li> = C / A </li></ul></ul>Heather Holbach Mark Bourassa <ul><li>Only data on the perimeter of the shape contributes to the calculation. </li></ul><ul><li>As the shape becomes larger, it becomes more circular. </li></ul><ul><li>Rain flagged data are not used in the calculation </li></ul><ul><li>Random errors are much larger for ringsize 1 and 2 than for averages over more data (ringsize 3 or greater) </li></ul><ul><ul><li>Random errors for ringsize 1 and 2 are similar is magnitude </li></ul></ul>Ringsize 1 Ringsize 4 Ringsize 2

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Methodology Heather Holbach Mark Bourassa <ul><li>Circulation is calculated as a line integral about a “shape” using the circulation theorem. </li></ul><ul><ul><li>“ shape” is dependent on available data: polygon (triangle, square …). </li></ul></ul><ul><ul><li>Linearly interpolate wind vectors between adjacent good observations. </li></ul></ul><ul><ul><ul><li>Spline fit results in a slight improvement </li></ul></ul></ul><ul><ul><li>Too many missing points on the “shape” perimeter (<80%; except 75% for square shapes) and the vorticity isn’t calculated. </li></ul></ul><ul><li>Individual dot products along circumference of the “shape” using wind speed components and distance between data points. </li></ul><ul><ul><li>Circulation is the sum of the dot products. </li></ul></ul>

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Scatterometer Data <ul><li>ASCAT </li></ul><ul><ul><li>25km product </li></ul></ul><ul><ul><li>Two swaths </li></ul></ul><ul><ul><li>No rain flag, but has quality flag </li></ul></ul><ul><li>QSCAT </li></ul><ul><ul><li>25km product </li></ul></ul><ul><ul><li>Single wide swath </li></ul></ul><ul><ul><li>Rain flag </li></ul></ul><ul><li>OSCAT </li></ul><ul><ul><li>50km grid spacing within a swath that’s 1800 km wide </li></ul></ul><ul><ul><ul><li>The smallest area to calculate vorticity is 50km  50km. </li></ul></ul></ul><ul><ul><li>No rain flag </li></ul></ul>Heather Holbach Mark Bourassa

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Example of Smoothing: ASCAT 50km averaging 100km averaging Vorticity (10 -5 s -1 ) Heather Holbach Mark Bourassa

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Example of Smoothing: QSCAT 50km averaging 100km averaging Vorticity (10 -5 s -1 ) Heather Holbach Mark Bourassa

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Example of Smoothing: OSCAT Vorticity (10 -5 s -1 ) Heather Holbach Mark Bourassa 100km averaging 200km averaging

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Probability Density of Vorticity (Atlantic, 40-60N) Vorticity (10 -4 s -1 ) d Probability / d Vorticity (10 4 s) Heather Holbach Mark Bourassa

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Vorticity Spectra (50km averages; 40-60N, 52-10W) Wavelength (km) 2000 1000 750 500 250 150 100 75 50 10 -9 10 -10 10 -11 10 -12 Spectra Amplitude (s -2 ) Heather Holbach Mark Bourassa

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Probability Density of Vorticity (Atlantic, 40-60N) d Probability / d Vorticity (10 4 s) Vorticity (10 -4 s -1 ) Heather Holbach Mark Bourassa The data density is very important for determining accurate vorticity <ul><li>Truncation error vastly dominates for finer resolutions </li></ul><ul><ul><li>Error ~ diameter -1.5 * grid spacing 1.5 (Bourassa and McBeth-Ford) </li></ul></ul>

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Tropics (10S-10N and 25W-17W d Probability / d Vorticity (10 4 s) Vorticity (10 -4 s -1 ) Heather Holbach Mark Bourassa

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Conclusions <ul><li>Vorticity probability distributions and spectra were examined for ASCAT, QuikSCAT, and the OceanSat-2 scatterometer </li></ul><ul><li>ASCAT wind vector data are clearly less noisy in comparison to the Ku-band retreivals </li></ul><ul><ul><li>Likely also related to (1) less severe rain problems & (2) greater smoothing </li></ul></ul><ul><ul><li>Contributes to (1) shifting the probability of vorticity towards more cyclonic vorticity, and (2) a tighter vorticity distribution </li></ul></ul><ul><li>For spatial scales less than three grid cells, the noise has a relatively huge impact </li></ul><ul><li>A finer resolution is very useful for producing less noisy area averaged vorticity </li></ul><ul><ul><li>If the curl of the stress on a 25km scales is important to ocean forcing, then winds (stress) should be resolved on a 8.3 km scale or finer . </li></ul></ul><ul><ul><li>For interactions between the boundary-layer and the free atmosphere, we will need much finer resolution retrievals (2 km winds?) </li></ul></ul><ul><ul><li>Assuming similar accuracy in winds and proportionally finer accuracy in position </li></ul></ul>Heather Holbach Mark Bourassa

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Differences Between ASCAT, OSCAT and QuikSCAT Derived Vorticity IGARSS 2009 Heather Holbach and Mark A. Bourassa Center for Ocean-Atmospheric Prediction Studies & Department of Earth, Ocean and Atmospheric Science The Florida State University

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Vorticity Spectra (100km averages) 10 -9 10 -10 10 -11 10 -12 Spectra Amplitude (s -2 ) Wavelength (km) 2000 1000 750 500 250 150 100 75 50 10 -13 Heather Holbach Mark Bourassa