Method for Retrieving Aerosol Optical Thickness above Clouds Based on Polarization Remote Sensing Data

Abstract

The present invention provides a method for retrieving aerosol optical thickness above clouds based on polarized remote sensing data. Based on the aerosol optical thickness retrieval algorithm above clouds, the cloud optical thickness and the effective particle radius are retrieved using the intensity radiation information of different channels; according to the retrieval results of the cloud optical thickness and the effective particle radius and the polarization reflectivity, the corresponding aerosol optical thickness above clouds and the reflectivity data of different channels are found; the obtained reflection data is corrected, and the cloud optical thickness, the effective particle radius and the aerosol optical thickness are retrieved again, and the iteration is continuously carried out until the "cloud-gas separation" is completed, so as to realize the retrieval of the aerosol optical thickness above clouds. The present invention can be used for retrieving aerosol parameters above polarized remote sensing clouds, and provides support for realizing more accurate global aerosol characteristic evaluation.

Description

(1) Technical Field

[0001] The present invention relates to a method for retrieving aerosol optical depth above clouds based on polarization remote sensing data, which can be used for retrieving aerosol parameters above clouds in polarization remote sensing and provides support for more accurate global aerosol property assessment. (2) Background Art

[0002] Atmospheric aerosols refer to a relatively stable scattering system formed by solid or liquid particles dispersed in a gas-phase medium. In satellite remote sensing, the electromagnetic radiation received by the sensor is the information of atmospheric path radiation that the target electromagnetic wave is scattered and absorbed by atmospheric molecules, atmospheric aerosols, cloud particles, etc., and returned by atmospheric scattering before reaching the ground. In imaging quantitative remote sensing, the atmospheric influence will cause image blurring, loss of image details, etc., resulting in errors in the quantitative inversion results of surface parameters such as surface reflectance, chlorophyll concentration, and sea color. Atmospheric correction (i.e., removing the atmospheric influence) is a necessary step to achieve high-precision quantitative remote sensing. The effect of atmospheric correction depends on the accuracy of atmospheric parameters, and atmospheric aerosols are one of the important influencing factors. Real-time and effective monitoring of atmospheric aerosols plays an important role in climate prediction, quantitative remote sensing, and improvement of human living quality. Currently, the retrieval of aerosol parameters is limited to cloud-free polluted areas, and there is little research on the retrieval of aerosol parameters in cloud-polluted areas, which greatly reduces the ability to monitor global aerosol properties and evaluate their radiative forcing.

[0003] Aerosols suspended above the clouds play a role in positive radiative forcing (i.e., atmospheric warming) through their radiative interaction with the clouds. An active lidar sensor can obtain the aerosol backscatter profile above the clouds, and under certain assumptions, the extinction information including the aerosol optical depth above the clouds can be retrieved. Chand et al. retrieved the aerosol optical depth above the clouds based on the backscatter attenuation ratio information of the 1064 nm and 532 nm channels of the CALIOP sensor. Passive remote sensing detectors also have the ability to retrieve aerosol parameters above the clouds. Polarization information can be used to retrieve aerosol parameters above the clouds. For example, Waquet et al. retrieved the optical depth of carbonaceous aerosols above the clouds based on the 865 nm channel of the POLDER detector on the PARASOL satellite; Waquet et al. used the polarization information of the 670 nm and 865 nm channels of POLDER to retrieve the microphysical and optical properties of biomass burning aerosols and mineral dust aerosols above water clouds; Knobelspiesse et al. retrieved the aerosol and cloud particle size distribution parameters above the clouds in the Gulf of Mexico region based on the observational data of the polarization scanner (RSP). Torres et al. developed a new algorithm for simultaneously retrieving the optical depths of clouds and aerosols based on the observational data of the Ozone Monitoring Instrument (OMI). Jethva et al. simultaneously retrieved the cloud optical depth and the aerosol optical depth above the clouds based on the reflectance ratio of the 470 nm and 860 nm channels of MODIS.

[0004] The present invention discloses a method for retrieving the aerosol optical depth above the clouds based on polarization remote sensing data. This method deducts the influence of clouds on the aerosol layer through an iterative method, making the "cloud-aerosol separation" more thorough and improving the retrieval accuracy of the aerosol optical depth above the clouds. (III) Summary of the Invention

[0005] The purpose of the present invention is to provide a method for retrieving the aerosol optical depth above the clouds based on polarization remote sensing data, which can improve the retrieval accuracy of the aerosol optical depth above the clouds.

[0006] The purpose of the present invention is achieved by the following technical means:

[0007] A method for retrieving the aerosol optical depth above the clouds based on polarization remote sensing data includes:

[0008] Step 1, obtaining the optical radiation and observation geometry data of the polarization remote sensing cloud pixels;

[0009] Step 2, retrieving the cloud optical depth and the effective particle radius by using the look-up table method according to the reflectance data of the 670 nm, 1610 nm, and 2250 nm channels;

[0010] Step 3: Based on the cloud optical thickness and effective particle radius obtained in Step 2, combined with the polarized reflectance in the 670 nm band, the aerosol optical thickness above the cloud layer is retrieved by using a look-up table method to obtain the aerosol optical thickness above the cloud layer;

[0011] Step 4: Based on the reflectance in the 670 nm, 1610 nm, and 2250 nm channels of the cloud layer and aerosol layer obtained in Step 3;

[0012] Step 5: Compare the actual reflectance observed in the 670 nm channel with the reflectance obtained in Step 4. If the error between the two values exceeds Δ, correct the measured reflectance values in the 670 nm, 1610 nm, and 2250 nm channels, and repeat Step 2 until the reflectance error in the 670 nm channel is less than or equal to Δ;

[0013] Step 6: Use the aerosol optical thickness obtained from the last execution of Step 3 as the aerosol optical thickness above the cloud layer.

[0014] Furthermore, for the acquisition of cloud pixel data in Step 1, its flowchart is as Figure 2 shown. Cloud detection and cloud phase identification are performed using the downsampled data to obtain cloud pixel data.

[0015] Furthermore, for the look-up table of the aerosol optical thickness above the cloud layer in Step 3, its calculation flowchart is as Figure 3 shown. The look-up table of the aerosol optical thickness above the cloud layer is calculated according to the vector radiative transfer model.

[0016] The beneficial effects of the present invention: By using an iterative method to deduct the influence of the cloud layer on the aerosol layer, the "cloud-aerosol separation" is made more thorough, and the inversion accuracy of the aerosol optical thickness above the cloud layer is improved. It provides a reference for the field of polarized remote sensing of aerosol parameters above the cloud layer, and makes a preliminary exploration for obtaining more comprehensive aerosol characteristic parameters based on polarized remote sensing, realizing global aerosol monitoring and its radiative forcing assessment. (IV) BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is the flowchart of the method for retrieving the aerosol optical thickness above the cloud layer based on polarized remote sensing data.

[0018] Figure 2 is the flowchart for obtaining cloud pixel data.

[0019] Figure 3 is the flowchart for calculating the look-up table of the aerosol optical thickness above the cloud layer. (V) DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be further described below in conjunction with specific embodiments.

[0021] AsFigure 1 As shown in the figure, the method for inverting the aerosol optical depth above clouds based on polarized remote sensing data of the present invention includes:

[0022] Step 1: Obtain the optical radiation and observation geometry data of cloud pixels that have undergone cloud detection and cloud phase identification;

[0023] Step 2: The radiation amount in the 670 nm band is sensitive to the change of cloud optical depth, and the radiation amounts in the 1610 nm and 2250 nm bands are sensitive to the change of cloud effective particle radius. According to the measured reflectivities in the 670 nm, 1610 nm, and 2250 nm channels, the lookup table method is used to invert the cloud optical depth and effective particle radius as the input parameters for Step 3;

[0024] Step 3: According to the cloud optical depth and effective particle radius obtained in Step 2, combined with the measured polarized reflectivity in the 670 nm band, the lookup table method is used to invert the aerosol optical depth above clouds to obtain the aerosol optical depth above clouds;

[0025] Step 4: According to the aerosol optical depth obtained in Step 3, obtain the reflectivities in the 670 nm, 1610 nm, and 2250 nm channels corresponding to the cloud optical depth, effective particle radius, and aerosol optical depth from the lookup table of aerosol optical depth above clouds;

[0026] Step 5: Compare the actual reflectivity observed by the satellite in the 670 nm channel with the reflectivity in the 670 nm channel obtained in Step 4. If the error between the two values exceeds Δ, correct the measured reflectivity values in the 670 nm, 1610 nm, and 2250 nm channels, and return to Step 2 to re-invert the cloud optical depth and effective particle radius, re-invert the aerosol optical depth above clouds, and correct the reflectivity data, and iterate continuously until the reflectivity error in the 670 nm channel is less than or equal to Δ;

[0027] Step 6: Take the aerosol optical depth obtained by the last execution of Step 3 as the aerosol optical depth above clouds.

[0028] In this example, the aerosol optical depth above clouds is inverted based on the polarized reflectivity in the 670 nm channel. The polarized reflectivity in the 670 nm channel is not sensitive to the changes of cloud optical depth and effective particle radius, but is sensitive to the change of aerosol optical depth. After inverting the cloud optical depth and effective particle radius through the reflectivities in the 670 nm, 1610 nm, and 2250 nm channels, the "cloud-aerosol separation" can be effectively realized based on the polarized reflectivity in the 670 nm channel, so as to realize the inversion of the aerosol optical depth above clouds.

[0029] Vector radiative transfer equation of the plane-parallel atmosphere model:

[0030]

[0031] In the formula, I is the Stokes vector, the second term on the right side of the equation is the contribution of multiple scattering, the third term is the contribution of single scattering, ω is the single-scattering albedo, τ is the optical thickness, μ is the cosine of the zenith angle of the outgoing light, φ is the azimuth angle relative to the solar outgoing light, μ0 and φ0 are the solar zenith angle and azimuth angle respectively, F0 is the solar radiation flux at the top of the atmosphere, B is the Planck function, and M is the matrix after the reference plane conversion of the single-scattering phase matrix P.

[0032] The total radiation information and polarized radiation information can be obtained by solving the vector radiative transfer equation. The reflectance R and polarized reflectance Rp are used to characterize the total radiation information and polarized radiation information received by the satellite, and their definitions are:

[0033]

[0034]

[0035] In the formula, I, Q, and U are Stokes parameters, μ s is the cosine of the solar zenith angle, and E0 is the solar radiation flux at the top of the atmosphere.

[0036] The contributions to the radiation intensity at the top of the atmosphere mainly come from the following aspects: scattering by clouds, aerosol scattering, atmospheric molecular scattering, and reflection from the surface, etc. The apparent reflectance R TOA observed by the satellite can be expressed as:

[0037] R TOA = R m + R a + R c + R s

[0038] In the formula, R m , R a , R c and R s are the contributions of atmospheric molecules to the reflectance at the top of the atmosphere, the contributions of aerosols to the reflectance at the top of the atmosphere, the contributions of clouds to the reflectance at the top of the atmosphere, and the contributions of the surface to the reflectance at the top of the atmosphere, respectively.

[0039] For the parallel atmosphere model, the reflection function of the cloud is:

[0040]

[0041] In the formula, μ0 and μ are the cosines of the solar zenith angle and the outgoing zenith angle respectively, φ is the relative azimuth angle; F0(λ) is the incident solar radiation, and I λ (0, -μ, φ) is the reflected radiance; τ c is the cloud optical thickness, and rc is the effective radius of cloud particles and can be expressed as:

[0042]

[0043] where n(r) is the particle size distribution and r is the particle radius. For a waveband with a finite width, the reflectivity function can be written as:

[0044]

[0045] where f(λ) is the spectral response function of the instrument.

[0046] When the cloud optical thickness is large, ignoring the contribution of the surface to the polarized reflectance at the top of the atmosphere, the polarized reflectance at the top of the atmosphere is:

[0047]

[0048] where, R m is the contribution of atmospheric molecules, Rp a is the contribution of aerosols above the cloud layer, Rp c is the polarized reflectance of the cloud layer; M is the atmospheric mass factor; τ m 、τ a are the optical thicknesses of atmospheric molecules and aerosol optical thicknesses respectively; c is an empirical coefficient.

[0049] Based on polarized remote sensing inversion of aerosols above the cloud layer, the present invention separates the cloud contribution from the atmospheric contribution (the contributions of aerosols and atmospheric molecules), and establishes a look-up table through a radiative transfer model to determine the most suitable aerosol optical thickness corresponding to the atmospheric contribution.

[0050] The above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to specific embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention. The technologies, shapes, and structures not described in detail in the present invention are all well-known technologies.

Claims

1. A method for retrieving aerosol optical depth above clouds based on polarization remote sensing data, characterized in that The specific process of this method includes the following steps: Step 1: Obtain the optical radiation and observation geometry data of the polarized remote sensing cloud pixels; Step 2, since the radiation amount in the 670 nm band is sensitive to the change of cloud optical thickness, and the radiation amounts in the 1610 nm and 2250 nm bands are sensitive to the change of cloud effective particle radius, select the intensity radiation information of the 670 nm, 1610 nm, and 2250 nm channels to retrieve the cloud optical thickness and the effective particle radius. According to the measured reflectivities R means,670 , R means,1610 , R means,2250 of the cloud pixel, retrieve the cloud optical thickness and the cloud effective particle radius, which are used as the input parameters for Step 3; Step 3: Based on the cloud optical thickness and the inversion results of the effective particle radius in Step 2, combined with the measured polarized reflectance R of the 670 nm channel p670 , the lookup table method is used to invert the aerosol optical thickness above the cloud layer to obtain the aerosol optical thickness value above the cloud layer; Step 4, obtain the reflectances R calc,670 , R calc,1610 , and R calc,2250 of the cloud layer and aerosol layer in the 670nm, 1610nm, and 2250nm channels according to Step 3; calc,670 、R calc,1610 、R calc,2250 ; Step 5, compare the 670nm channel reflectance data R obtained in Step 4 calc,670 with the measured reflectance data R means,670 , if the error between the two exceeds Δ, correct the measured reflectance values of the 670nm, 1610nm, and 2250nm channels, and return to Step 2 until the error between the 670nm channel reflectance R calc,670 and the measured reflectance R means,670 is less than or equal to Δ; Step 6: Use the aerosol optical depth obtained from the last execution of Step 3 as the aerosol optical depth above the cloud layer; When the cloud optical depth is large, the contribution of the surface to the polarized reflectance at the top of the atmosphere is ignored, and the polarized reflectance at the top of the atmosphere is: R PTOA = R Pa + R Pm + R Pc .e (-M τ m - Mcτ a ) where R pm is the contribution of atmospheric molecules, R pa is the contribution of aerosols above the cloud layer, and R pc is the polarized reflectance of the cloud layer; M is the air mass factor; τ m , τ a are the optical thicknesses of atmospheric molecules and aerosol optical thickness, respectively; c is an empirical coefficient.

2. The method for retrieving aerosol optical thickness above clouds based on polarized remote sensing data according to claim 1, wherein In Step 3, after obtaining the cloud parameters based on the radiation simulation calculation of the coupling between the cloud layer and the aerosol layer, use the radiation transfer model to simulate and analyze the influence of the cloud layer on the aerosol, and effectively deduct the influence of the cloud layer on the inversion of the aerosol optical depth.

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