Development and Validation

Analysis of surface return

Aeolus return signal is affected by many processes in the atmosphere, which could prevent the interpretation of the surface return. To select Aeolus surface returns with sufficient signal to noise and and free of clouds or other interfering elements, an elaborated filtering needs to be applied to the Aeolus signal. The particular signal considered here is the mie signal intensity range corrected. An example is shown in Figure (2), which gives the Aeolus observations along the track presented in Figure (1). This a Goes RGB image collected on 26 Jan. 2021, with the Aeolus satellite track is indicated by the dashed line. The crosses indicate the (lon, lat) coordinates of selected points. The Aeolus return as a function of height bin (on y-axis), along the satellite track (on the x-axis) is shown in Figure (2). Of interest of current study is the Aeolus return in bins closest to the surface, which are the bins with the with highest indices, in particular bin no 22 and 23.

Note that many observations contain clouds, as indicated by the large values of the Mie signal higher in atmosphere. Only a few observations were collected over cloud free field of views. In fact our initial analysis indicated that less than 20% of the Aeolus observations are free of clouds or other interfering species.

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(1)\[\textbf{Goes Image of Region of Interest}\]
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(2)\[\textbf{Intensity Range Corrected Mie Signal}\]

The mie signal intensity range corrected is collected for a two month period (September-October 2019) as the Aeolus team indicated that during the period july 2019 - dec 2020 the Aeolus data is of very good quality. A histrogram of all surface returns (land and ocean) after removing some outliers, is shown in Figure (3). The figure shows a single modal structure, with a single maximum around zero. The figure also shows some returns have negative (i.e. non-physical) values.

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(3)\[\textbf{Raw Intensity Range Corrected Mie Signal}\]

After removing non-phyiscal observations (with negative values), observations which contain clouds, and selecting only the observations over ocean, the histogram presented in Figure (4) is obtained.

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(4)\[\textbf{Filtered Intensity Range Corrected Mie Signal over Ocean Surfaces}\]

This histogram shows a typical Weibull distribution which is very characteristics for ocean windspeed distributions [Pavia86], and is a first indication of a wind induced surface return signal. This build confidence that indeed the Aeolus surface return can be used to inter sea surface properties.

To investigate any interannual variation, results over 2019 can be compared to results obtained for same period in 2020, presented in Figure (5) and Figure (6) respectively.

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(5)\[\textbf{Raw Intensity Range Corrected Mie Signal}\]
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(6)\[\textbf{Filtered Intensity Range Corrected Mie Signal over Ocean Surfaces}\]

There are very distinct similarities in the raw and final distribution of the Aeolus surface return. What is especially interesting is that the distribution of the filtered surface returns follows a Weibull distribution as well, although the width and location of the maximum of the distribution are different from the 2019 results. Also when the distribution of the unfiltered surface returns over the 2020 period is compared to the 2019 period some remarkable correspondances and differences are found.

The results will be discussed in more detail, after completion of the ongoing analysis.