A method for estimating seawater absorption coefficient based on lidar sea-air molecular signals

Abstract

The present invention discloses a method for estimating the seawater absorption coefficient based on the laser radar sea-air molecular signal, comprising: (1) quality control and denoising of the profile data of the high spectral resolution laser radar; (2) calculating the average value of the sea surface atmospheric molecular signal and the column integral of the seawater molecular signal; (3) matching the auxiliary data such as the atmospheric temperature, atmospheric pressure, seawater temperature, and seawater salinity according to the time and position information in the profile data; (4) calculating the volume scattering function of the atmosphere and seawater molecules according to the auxiliary data, and calculating the iodine pool transmittance of the atmosphere and seawater molecular signals at the same time; (5) calculating the two-way atmospheric transmittance from the laser radar system to the sea surface; (6) based on the laser radar equation in the seawater, substituting the column integral of the seawater molecular signal, the iodine pool transmittance, the volume scattering function of the seawater molecules, and the two-way atmospheric transmittance, to estimate the seawater absorption coefficient. By using the present invention, the global seawater absorption coefficient can be estimated and the accuracy can be improved.

Description

Technical Field

[0001] The invention belongs to the technical field of ocean laser radar, and in particular relates to a method for estimating seawater absorption coefficient based on laser radar sea-air molecule signals. Background Art

[0002] The absorption coefficient is an important intrinsic optical property of water, which characterizes the degree to which light is absorbed when propagating in seawater. For a specific wavelength, it is usually related to phytoplankton, colored soluble organic matter and non-algae particles in the water, and can be used to evaluate phytoplankton photosynthesis and estimate primary productivity of water bodies. In addition, the absorption coefficient can also be used to assess the turbidity of water bodies, distinguish water types, and is also a key parameter for heat transfer in upper water bodies.

[0003] Passive water color remote sensing can be used to estimate the absorption coefficient and has advantages such as large range and high efficiency. However, passive water color remote sensing also has some inherent limitations, such as the error that may be introduced into the remote sensing reflectivity during atmospheric correction, which will lead to errors in the results. In addition, since passive water color remote sensing relies on sunlight, it cannot be observed at night and in winter in polar regions. LiDAR is an active detection method that can eliminate the above-mentioned atmospheric and light limitations. Chinese patent publication number CN105486664A discloses a laser radar device and method for detecting marine phytoplankton biomass and POC; Chinese patent publication number CN117194876A discloses a method for extracting the diffuse attenuation coefficient of a water body based on a laser radar water body echo; Chinese patent publication number CN116794678A discloses a Raman laser radar and a method for obtaining the beam attenuation coefficient of particles in each section of the water body.

[0004] However, these lidar methods all use the characteristic of signal attenuation with depth to calculate the attenuation coefficient, and cannot estimate the absorption coefficient. In addition, some lidars are limited by the depth resolution of the detection module and the system response bandwidth, and cannot obtain effective depth-resolved signals. The signal equation contains many unknowns such as atmospheric transmittance, seawater particle backscattering coefficient and attenuation coefficient, which limits the accuracy of attenuation coefficient detection. Summary of the invention

[0005] The present invention provides a method for estimating the seawater absorption coefficient based on the sea-air molecular signal of a lidar. By filtering out the Mie scattering signal with the help of a high spectral resolution lidar and utilizing the molecular scattering signals of the atmosphere and the ocean, the global seawater absorption coefficient can be estimated by calculating the atmospheric transmittance, thereby improving the accuracy.

[0006] A method for estimating seawater absorption coefficient based on laser radar sea-air molecular signals comprises the following steps:

[0007] (1) Quality control and denoising of high spectral resolution lidar profile data;

[0008] (2) Using the denoised profile data, calculate the average value of the atmospheric molecular signal at the sea surface and the column integral of the seawater molecular signal;

[0009] (3) Match auxiliary data including atmospheric temperature, atmospheric pressure, seawater temperature, and seawater salinity based on the time and location information in the profile data;

[0010] (4) Based on the auxiliary data, calculate the volume scattering function of the atmospheric and seawater molecules, and simultaneously calculate the iodine cell transmittance of the atmospheric and seawater molecular signals;

[0011] (5) Calculate the two-way atmospheric transmittance from the lidar system to the sea surface by combining the average value of the atmospheric molecular signal on the sea surface, the iodine pool transmittance, and the atmospheric molecular volume scattering function;

[0012] (6) Based on the lidar equation in seawater, the seawater molecular signal column integral, iodine cell transmittance, seawater molecular volume scattering function and two-way atmospheric transmittance are substituted to estimate the seawater absorption coefficient.

[0013] The specific process of step (1) is as follows:

[0014] Find the maximum value of the molecular channel signal and filter according to the altitude corresponding to the maximum value; exclude the profile data with saturated vertical polarization channel signals; perform sliding average on adjacent lidar profile data; and perform Savitzky-Golay filtering on the part above the sea surface for each molecular channel profile data after sliding average.

[0015] The specific process of step (2) is as follows:

[0016] First, the high spectral resolution lidar molecular channel signal, i.e., attenuated backscatter, is defined as follows:

[0017] ;

[0018] ;

[0019] In the formula, is the attenuated backscattering of the molecular channel, obtained by high spectral resolution lidar; is the two-way atmospheric transmittance, is the iodine cell transmittance of the molecular signal, is the backscattering function, is the absorption coefficient of seawater; is the working height of the laser radar, is the altitude corresponding to the signal sampling point, Indicates the water penetration depth, air and waterIn air and sea water respectively;

[0020] Calculate the average value of the atmospheric molecular signal close to the sea surface according to the altitude range corresponding to the sea surface ;

[0021] Calculate molecular signal column integrals for seawater , as follows:

[0022] .

[0023] Furthermore, the average value of the atmospheric molecular signal close to the sea surface is calculated in the range of 60 to 200 meters above sea level. .

[0024] The specific process of step (3) is as follows:

[0025] According to the longitude, latitude and time information of the high-spectral resolution lidar system, we look for the global meteorological data with the shortest spatial distance and the smallest time difference as auxiliary data, including the atmospheric temperature and atmospheric pressure on the sea surface, and the seawater temperature and salinity on the sea surface.

[0026] The specific process of step (4) is as follows:

[0027] According to the Rayleigh scattering theory and the atmospheric temperature and atmospheric pressure data, the average value of the backscattering function of the Rayleigh scattering of the sea surface atmosphere is calculated. ; Calculate the backscattering function of seawater Brillouin scattering based on seawater temperature and salinity data ;

[0028] According to the laboratory calibration results, the spectral transmittance curve of the iodine cell is obtained;

[0029] According to the Tenti S6 model and atmospheric temperature and atmospheric pressure data, the Rayleigh scattering spectrum distribution of the sea surface atmosphere is calculated; the ratio of the signal intensity after passing through the iodine cell to the signal intensity before passing through the iodine cell is calculated to obtain the iodine cell transmittance of the atmospheric molecular signal ;

[0030] According to the sea surface temperature and salinity data, the frequency shift and line width of Brillouin scattering are calculated; the ratio of the signal intensity after passing through the iodine cell to the signal intensity before passing through the iodine cell is calculated to obtain the iodine cell transmittance of the seawater molecular signal .

[0031] In step (5), the two-way atmospheric transmittance from the lidar system to the sea surface is calculated using the following formula:

[0032] ;

[0033] In the formula, is the average signal of atmospheric molecules on the sea surface, is the iodine cell transmittance of atmospheric molecular signals, is the average value of the backscattering function of Rayleigh scattering of the sea surface atmosphere.

[0034] In step (6), the formula for estimating the seawater absorption coefficient is as follows:

[0035] ;

[0036] In the formula, is the seawater absorption coefficient, is the two-way atmospheric transmittance, is the iodine cell transmittance of seawater molecular signals, is the backscattering function of seawater Brillouin scattering, is the molecular signal column integral of seawater, is the average value of the atmospheric molecular signal at the sea surface, is the iodine cell transmittance of atmospheric molecular signals, is the average value of the backscattering function of Rayleigh scattering of the sea surface atmosphere.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] The present invention provides a new method for inverting seawater absorption coefficient by high spectral resolution lidar, which does not require exponential fitting using high vertical resolution lidar data, thus reducing the requirements for lidar system indicators. Atmospheric transmittance is calculated using atmospheric molecular signals, and the estimation of seawater absorption coefficient is achieved based on the relationship between the column integral of seawater molecular signals and the absorption coefficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a flow chart of a method for estimating seawater absorption coefficient based on laser radar sea-air molecule signals of the present invention.

[0040] Figure 2 This is a flow chart of profile data quality control and denoising of the high spectral resolution lidar in the present invention. DETAILED DESCRIPTION

[0041] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be pointed out that the embodiments described below are intended to facilitate the understanding of the present invention and do not have any limiting effect on the present invention.

[0042] This embodiment of the present invention takes the satellite-borne high spectral resolution lidar ACDL data in September 2022 as an example.

[0043] like Figure 1 As shown, a method for estimating seawater absorption coefficient based on laser radar sea-air molecule signals includes the following steps:

[0044] Step 1: Quality control and denoising of high spectral resolution lidar data, such as Figure 2 shown.

[0045] Read the ACDL data of the high spectral resolution lidar and find the corresponding altitude of the maximum value of the molecular channel signal. If the difference with the digital surface elevation is greater than 120m, exclude this profile data. Exclude the profiles with vertical channel saturation. Perform sliding average on the 31 adjacent profiles. For each molecular channel profile data after sliding average, take the part above 60m above sea level and perform Savitzky-Golay filtering with a window size of 11 and an order of 3.

[0046] Step 2: Calculate the average value of the atmospheric molecular signal on the sea surface and the column integral of the seawater molecular signal using the denoised profile data.

[0047] The high spectral resolution lidar molecular channel signal, i.e. attenuated backscatter, is defined as:

[0048] (1)

[0049] (2)

[0050] In the formula, is the attenuated backscattering of the molecular channel, obtained by high spectral resolution lidar; is the two-way atmospheric transmittance, is the iodine cell transmittance of the molecular signal, is the backscattering function, is the absorption coefficient of seawater; is the working height of the laser radar, is the altitude corresponding to the signal sampling point, Indicates the water penetration depth, air and water In air and sea water respectively.

[0051] Take the altitude range of 60 meters to 200 meters and calculate the average value of the atmospheric molecule signal within this range .

[0052] Calculate the molecular signal column integral of seawater as follows:

[0053] (3)

[0054] Specifically, for ACDL data, the distance between adjacent bins is 24m. The atmospheric molecular signal is the average value of the signals from the 3rd bin to the 8th bin above the sea surface. Due to the limited penetration depth of the laser in water, the signals of a few bins below the sea surface account for the vast majority of the seawater echo signal. The molecular signal column integral of seawater is the signal integral value from the sea surface to the 4th bin below the sea surface:

[0055] (4)

[0056] In the formula, Indicates the The depth corresponding to the bin is Represents the distance between two adjacent bins. For ACDL, It is 24m.

[0057] Step 3: Match auxiliary data such as atmospheric temperature, atmospheric pressure, seawater temperature, and seawater salinity according to the time and location information in the profile data.

[0058] Based on the longitude, latitude and time information of the high-spectral resolution lidar system, we search for the global meteorological data with the shortest spatial distance and the smallest time difference, including the temperature and pressure of the sea surface atmosphere, and the temperature and salinity of the sea surface. In this example, the auxiliary data is obtained from the ERA-5 and CMEMS datasets.

[0059] Step 4, based on the auxiliary data, calculate the volume scattering function of the atmosphere and seawater molecules, and calculate the iodine cell transmittance of the atmosphere and seawater molecule signals.

[0060] According to the Rayleigh scattering theory and the temperature and pressure data, the average backscattering function of the atmospheric Rayleigh scattering in the altitude range of 60m-200m is calculated. . Based on the sea surface temperature and salinity data, the backscattering function of the Brillouin scattering of seawater is calculated. .

[0061] According to the laboratory calibration results, the spectral transmittance curve of the iodine cell in the ACDL system was obtained.

[0062] According to the Tenti S6 model and temperature and pressure data, the atmospheric Rayleigh scattering spectrum distribution is calculated, and the iodine cell transmittance of atmospheric molecular signals in the altitude range of 60m-200m is calculated. .

[0063] According to the sea surface temperature and salinity data, the frequency shift and line width of Brillouin scattering are calculated, and the iodine cell transmittance of seawater molecular signals is calculated. .

[0064] Step 5: Calculate the two-way atmospheric transmittance from the spaceborne lidar ACDL to the sea surface by combining the average atmospheric molecular signal on the sea surface, the iodine pool transmittance and the atmospheric molecular volume scattering function.

[0065] Calculate the two-way atmospheric transmittance as follows:

[0066] (5)

[0067] Step 6, based on the lidar equation in seawater, substitute the seawater molecular signal column integral, iodine cell transmittance, seawater molecular volume scattering function and two-way atmospheric transmittance to estimate the seawater absorption coefficient:

[0068] According to the molecular channel lidar equation in seawater expressed by formula (2), the molecular signal column integral expressed by formula (3), and the two-way atmospheric transmittance expressed by formula (5), the seawater absorption coefficient can be expressed as:

[0069] (6)

[0070] The embodiments described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for estimating seawater absorption coefficient based on laser radar sea-air molecular signals, characterized in that: The following steps are involved: (1) Quality control and denoising of high spectral resolution lidar profile data; (2) Using the denoised profile data, calculate the average value of the atmospheric molecular signal at the sea surface and the column integral of the seawater molecular signal; (3) Match auxiliary data including atmospheric temperature, atmospheric pressure, seawater temperature, and seawater salinity based on the time and location information in the profile data; (4) Based on the auxiliary data, calculate the volume scattering function of the atmospheric and seawater molecules, and simultaneously calculate the iodine cell transmittance of the atmospheric and seawater molecular signals; (5) Calculate the two-way atmospheric transmittance from the lidar system to the sea surface by combining the average value of the atmospheric molecular signal on the sea surface, the iodine pool transmittance, and the atmospheric molecular volume scattering function; (6) Based on the lidar equation in seawater, the seawater molecular signal column integral, iodine cell transmittance, seawater molecular volume scattering function and two-way atmospheric transmittance are substituted to estimate the seawater absorption coefficient.

2. The method for estimating seawater absorption coefficient based on laser radar sea-air molecular signals according to claim 1 is characterized in that: The specific process of step (1) is as follows: Find the maximum value of the molecular channel signal and filter according to the altitude corresponding to the maximum value; exclude the profile data with saturated vertical polarization channel signals; perform sliding average on adjacent lidar profile data; and perform Savitzky-Golay filtering on the part above the sea surface for each molecular channel profile data after sliding average.

3. The method for estimating seawater absorption coefficient based on laser radar sea-air molecular signals according to claim 1 is characterized in that: The specific process of step (2) is as follows: First, the high spectral resolution lidar molecular channel signal, i.e., attenuated backscatter, is defined as follows: ； ； In the formula, is the attenuated backscattering of the molecular channel, obtained by high spectral resolution lidar; is the two-way atmospheric transmittance, is the iodine cell transmittance of the molecular signal, is the backscattering function, is the absorption coefficient of seawater; is the working height of the laser radar, is the altitude corresponding to the signal sampling point, Indicates the water penetration depth, air and water In air and sea water respectively; Calculate the average value of the atmospheric molecular signal close to the sea surface according to the altitude range corresponding to the sea surface ; Calculate molecular signal column integrals for seawater , as follows: 。 4. The method for estimating seawater absorption coefficient based on laser radar sea-air molecular signals according to claim 3 is characterized in that: Take the altitude range of 60 meters to 200 meters and calculate the average value of the atmospheric molecular signal close to the sea surface .

5. The method for estimating seawater absorption coefficient based on laser radar sea-air molecular signals according to claim 3 is characterized in that: The specific process of step (3) is as follows: According to the longitude, latitude and time information of the high-spectral resolution lidar system, we look for the global meteorological data with the shortest spatial distance and the smallest time difference as auxiliary data, including the atmospheric temperature and atmospheric pressure on the sea surface, and the seawater temperature and salinity on the sea surface.

6. The method for estimating seawater absorption coefficient based on laser radar sea-air molecular signals according to claim 5 is characterized in that: The specific process of step (4) is as follows: According to the Rayleigh scattering theory and the atmospheric temperature and atmospheric pressure data, the average value of the backscattering function of the Rayleigh scattering of the sea surface atmosphere is calculated. ; Calculate the backscattering function of seawater Brillouin scattering based on seawater temperature and salinity data ; According to the laboratory calibration results, the spectral transmittance curve of the iodine cell is obtained; According to the Tenti S6 model and atmospheric temperature and atmospheric pressure data, the Rayleigh scattering spectrum distribution of the sea surface atmosphere is calculated; the ratio of the signal intensity after passing through the iodine cell to the signal intensity before passing through the iodine cell is calculated to obtain the iodine cell transmittance of the atmospheric molecular signal ; According to the sea surface temperature and salinity data, the frequency shift and line width of Brillouin scattering are calculated; the ratio of the signal intensity after passing through the iodine cell to the signal intensity before passing through the iodine cell is calculated to obtain the iodine cell transmittance of the seawater molecular signal .

7. The method for estimating seawater absorption coefficient based on laser radar sea-air molecular signals according to claim 6 is characterized in that: In step (5), the two-way atmospheric transmittance from the lidar system to the sea surface is calculated using the following formula: ； In the formula, is the average value of the atmospheric molecular signal near the sea surface, is the iodine cell transmittance of atmospheric molecular signals, is the average value of the backscattering function of Rayleigh scattering of the sea surface atmosphere.

8. The method for estimating seawater absorption coefficient based on laser radar sea-air molecular signals according to claim 7 is characterized in that: In step (6), the formula for estimating the seawater absorption coefficient is as follows: ； In the formula, is the seawater absorption coefficient, is the two-way atmospheric transmittance, is the iodine cell transmittance of seawater molecular signals, is the backscattering function of seawater Brillouin scattering, is the molecular signal column integral of seawater, is the average value of the atmospheric molecular signal near the sea surface, is the iodine cell transmittance of atmospheric molecular signals, is the average value of the backscattering function of Rayleigh scattering of the sea surface atmosphere.

Images