A method for retrieving typhoon wind field from SAR remote sensing images by removing rainfall rate

Abstract

The present invention provides a method for inverting a typhoon wind field in a SAR remote sensing image with a rainfall rate removed, the method comprising: obtaining the original satellite-borne SAR image and the wind field and rainfall rate data observed by SFMR; calculating the wind speed of the VH of the SAR by a formula, and calculating the NRCS value affected by the rainfall rate; exploring the relationship between the NRCS difference and the rainfall rate according to the difference between the NRCS affected by the rainfall rate and the NRCS observed by the SAR image; identifying the typhoon wind field and the rainfall rate based on the relationship between the NRCS difference and the rainfall rate, eliminating the influence of the rainfall rate on the typhoon wind field, and reconstructing the typhoon wind field; verifying the wind field result, and forming a typhoon wind field inversion algorithm with the rainfall rate removed. The present invention solves the problem of interference of the rainfall rate on the inversion result of the typhoon wind field with high wind speed.

Description

Technical Field

[0001] The invention relates to the technical field of synthetic aperture radar signal processing, and in particular to a method for inverting a typhoon wind field in a SAR remote sensing image by removing a rainfall rate. Background Art

[0002] The global ocean area accounts for 71% of the earth's surface area and is an important part of global climate change. The sea surface wind speed and direction are one of the important elements of the ocean and atmosphere, and are the main driving factors of the upper ocean movement. For the dynamics of the ocean environment, the sea surface wind field is not only the direct driving force of sea surface waves, but also the driving force of regional and global ocean circulation. At the same time, the sea surface wind field regulates the interaction between the sea and the air, and plays an important role in global climate change. In addition, the sea surface wind field also directly affects marine operations such as marine engineering, marine fisheries and marine shipping.

[0003] Hurricane disasters are generally accompanied by strong winds and heavy rains, which seriously threaten people's lives and property, and have a great impact on people's livelihood, agriculture, economy, etc. It is a natural disaster with a large impact and serious harm. From the perspective of classification, typhoons with wind force above level 12 are divided into three levels, while hurricanes have more levels and a higher upper limit. A level 1 hurricane is equivalent to a typhoon or a strong typhoon, a level 2 hurricane is equivalent to a strong typhoon, a level 3 hurricane is equivalent to a strong typhoon or a super typhoon, and a level 4 and level 5 hurricane is equivalent to a super typhoon. From ancient times to the present, hurricanes have brought countless devastating blows to human production and life, and humans lack effective means to observe hurricanes, cannot identify rainstorm areas and reconstruct wind speeds, and the wind recovery methods to correct the rain-polluted areas in the core of the hurricane are not perfect. Synthetic aperture radar (SAR) is an active microwave imaging radar. By emitting microwaves of a certain frequency and measuring the amplitude and phase information of the backscattered signal, an image of the backscattering intensity of the sea surface can be obtained. This image has a very high resolution, which can reach the order of several meters. In addition, SAR is very sensitive to changes in the roughness of the sea surface, and can provide more ocean dynamic information for subtle spatial changes in the sea surface, such as sea surface wind fields, sea surface waves, internal waves, ocean currents, sea ice, wakes of ships on the sea surface, and oil slicks on the sea surface. At the same time, as a microwave imaging radar, SAR can observe the sea surface at any time and in any weather conditions. Therefore, SAR is an imaging radar that can observe the ocean at all times and in all weather conditions with high resolution.

[0004] In the prior art, SAR can continuously obtain high-resolution images of a large area of ​​the ocean surface all day and all night on its orbit, which is very beneficial for monitoring the sea surface wind field. The sea surface wind field inversion technology CMODs of C-band SAR co-polarization, i.e. VV polarization and HH polarization, has been fully mature, and the geophysical model of CMODs has been widely used in the past few decades, such as CMOD4, CMOD-IFR2, CMOD5, neutral wind inversion algorithm CMOD5N and the new GMF C-SARMOD, etc. So far, this CMODs that describes the complex relationship between the SAR normalized backscatter cross section value (NRCS value) and the wind speed at 10 meters above the sea surface has been successfully used for the sea surface wind field inversion of C-band SAR remote sensing images. However, the existing typhoon wind speed inversion algorithms have not taken into account the influence of rainfall rate on the wind speed inversion results. The high wind speed inversion results obtained are usually interfered by the rainfall rate, resulting in errors in the inversion results. Summary of the invention

[0005] To solve the above problems, the purpose of the present invention is to provide a typhoon wind field inversion method in SAR remote sensing images by removing the rainfall rate, thereby solving the interference problem of rainfall on the typhoon wind field inversion algorithm of SAR remote sensing images in the prior art.

[0006] To solve the above problems, the technical solution of the present invention is:

[0007] A method for inverting typhoon wind field in SAR remote sensing images by removing rainfall rate comprises the following steps:

[0008] Obtaining raw spaceborne SAR images and wind field and rainfall rate data observed by SFMR;

[0009] The wind speed of VH of SAR is calculated by the formula, and the NRCS value affected by rainfall rate is calculated;

[0010] Based on the difference between the NRCS affected by rainfall rate and the NRCS observed by SAR images, the relationship between the NRCS difference and rainfall rate is explored;

[0011] Based on the relationship between the NRCS difference and the rainfall rate, the typhoon wind field and the rainfall rate are identified, the influence of the rainfall rate on the typhoon wind field is eliminated, and the typhoon wind field is reconstructed;

[0012] Verify the wind field results and form a typhoon wind field inversion algorithm that removes the rainfall rate.

[0013] Optionally, the step of acquiring original satellite-borne SAR images and wind field and rainfall rate data observed by SFMR specifically includes: collecting 4 SAR remote sensing images, and simultaneously collecting observed rainfall rate and wind speed data from SFMR in the four SAR images in time.

[0014] Optionally, the step of calculating the wind speed of the SAR VH by a formula and calculating the NRCS value affected by the rainfall rate specifically includes: after the SAR remote sensing image to be inverted is calibrated by radiation, a cross-polarization wind field quick view is inverted, the wind speed of the SAR VH is calculated by a formula, and the co-polarization GMF formula CMOD5.N is used to calculate the co-polarization NRCS value affected by the rainfall rate.

[0015] Optionally, the step of exploring the relationship between the NRCS difference and the rainfall rate based on the difference between the NRCS affected by the rainfall rate and the NRCS observed by the SAR image specifically includes: calculating the difference between the NRCS affected by the rainfall rate and the NRCS observed by the SAR image, and analyzing the relationship between the NRCS and the rainfall rate and the distance from the typhoon eye.

[0016] Optionally, the analyzing the relationship between NRCS, rainfall rate, and typhoon eye distance specifically includes the following steps:

[0017] The difference between the observed co-polarization NRCS on the SAR image obtained by radiometric calibration and the simulated co-polarization NRCS with rainfall rate interference is processed;

[0018] Calculate the distance between each point in the SAR and the center of the typhoon eye;

[0019] Find the location points where rainfall rate exists in SFMR and match them with the difference results;

[0020] Construct the relationship between distance, co-polarization NRCS difference, and rainfall rate.

[0021] Optionally, the typhoon wind field and rainfall rate are identified based on the relationship between the NRCS difference and the rainfall rate, the influence of the rainfall rate on the typhoon wind field is eliminated, and the steps of reconstructing the typhoon wind field specifically include: analyzing the relationship between the co-polarization NRCS difference and the rainfall rate, identifying the rainfall rate area, fixing the area affected by the rainfall rate from the SAR image, simulating the wind field in the area with rainfall rate using the Holland model, combining the wind speed in the rainfall rate area simulated by the model with the inverted wind field obtained from the SAR image by an empirical formula, to form a typhoon wind field inversion result without the rainfall rate.

[0022] Optionally, the specific formula of the Holland model is: Where Vg represents the wind speed at a distance r from the center eye, pc represents the central air pressure, pn represents the standard atmospheric pressure, f represents the latitudinal Coriolis force, ρ represents the atmospheric density, and A and B represent fixed parameters of the model setting.

[0023] Optionally, the step of verifying the wind field results and forming a typhoon wind field inversion algorithm that removes the rainfall rate specifically includes: comparing the wind speed inversion results with rainfall rate interference with the wind speed inversion results without the rainfall rate to verify the accuracy of the typhoon wind field inversion algorithm for wind field reconstruction.

[0024] Compared with the prior art, the typhoon wind field inversion method in SAR remote sensing images with rainfall rate removed of the present invention uses spaceborne SAR to obtain cross-polarization remote sensing images, calculates the VH wind speed of SAR through a formula and calculates the co-polarization NRCS value affected by the rainfall rate, explores the relationship between the NRCS difference and the rainfall rate according to the difference between the NRCS affected by the rainfall rate and the NRCS observed in the SAR image, fully analyzes the specific relationship between the NRCS difference and the rainfall rate and the distance from the typhoon eye, thereby constructing a typhoon wind field inversion algorithm for SAR remote sensing images with rainfall rate removed. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0026] Figure 1 A flowchart of a method for inverting a typhoon wind field from a SAR remote sensing image with rainfall rate removed provided in an embodiment of the present invention.

[0027] Figure 2 The cross-polarization Sentinel-1SAR remote sensing image provided by the embodiment of the present invention;

[0028] Figure 3 A cross-polarization wind field inversion result diagram of a Sentinel-1SAR IW mode remote sensing image provided by an embodiment of the present invention;

[0029] Figure 4 A relationship diagram between the difference of NRCS and the rainfall rate provided in an embodiment of the present invention;

[0030] Figure 5 A wind field reconstruction result diagram of a remote sensing image in Sentinel-1SAR IW mode provided in an embodiment of the present invention;

[0031] Figure 6a A comparison result diagram of a typhoon wind field affected by rainfall rate and SFMR data provided by an embodiment of the present invention;

[0032] Figure 6b The comparison results of the typhoon wind field with the rainfall rate effect removed provided in the embodiment of the present invention and the SFMR data are shown. DETAILED DESCRIPTION

[0033] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention and will make the technical solutions and beneficial effects of the present invention apparent.

[0034] Figure 1 The present invention provides a flowchart of a typhoon wind field inversion method for a synthetic aperture radar remote sensing image with rainfall rate removed, which is based on the development of a typhoon wind field inversion algorithm for Sentinel-1SAR with rainfall rate removed. Figure 1 As shown, the method comprises the following steps:

[0035] S1: Obtain the original spaceborne SAR images and the wind field and rainfall rate data observed by SFMR;

[0036] Specifically, a total of 4 Sentinel-1GRD IW mode SAR remote sensing images were collected, with a single pixel width of 10 meters in each image and a resolution of 0.125°×0.125°, which is about 12.5km×12.5km of ECMWF reanalysis wind field data. There are very few image deformations caused by other ocean phenomena such as upwelling, rainfall or oil spills in the collected SAR remote sensing images. In the hurricane remote sensing images of the east and west coasts of the United States, there is at least one obvious typhoon eye in each image. At the same time, the observed rainfall rate and wind speed data from SFMR were collected in four SAR images.

[0037] S2: Calculate the wind speed of SAR VH by formula, and calculate the NRCS value affected by rainfall rate;

[0038] Specifically, we first used the existing empirical typhoon wind speed inversion algorithm in Sentinel-1IW mode, divided the NRCS obtained by radiometric calibration and the incident angle into three intervals of 31° to 35.9°, 35.9° to 41.3°, and 41.3° to 46°, and calculated the cross-polarization wind field inversion results on the SAR image. And using the co-polarization GMF formula CMOD5.N, based on the cross-polarization typhoon wind speed (hereinafter referred to as VH wind speed) inverted on SAR, we simulated the co-polarization NRCS affected by the rainfall rate, and used it as a quality indicator to compare with the NRCS data observed on the SAR image.

[0039] Figure 2 The figure shows two VH polarization snap-view images of Sentinel-1SAR images, from which the obvious location of the typhoon eye can be found. Figure 3 The wind field inversion results obtained from the cross-polarized Sentinel-1SAR image are shown. From the image, we can see that there are obvious striping phenomena and interference from rainfall bands in the inversion results.

[0040] S3: Based on the difference between the NRCS affected by rainfall rate and the NRCS observed by SAR images, explore the relationship between the NRCS difference and rainfall rate;

[0041] Specifically, the difference between the NRCS affected by the rainfall rate and the NRCS observed by the SAR image is calculated, and the relationship between the NRCS and the rainfall rate and the distance to the typhoon eye is analyzed. The relationship between the difference between rainfall and NRCS and the distance to the typhoon eye is explored in the following steps:

[0042] Step 1: Perform difference processing between the co-polarization NRCS observed on the SAR image obtained by radiometric calibration and the simulated co-polarization NRCS with rainfall rate interference;

[0043] Step 2: Calculate the distance between each point in the SAR and the center of the typhoon eye;

[0044] Step 3: Find the location points with rainfall rate in SFMR and match them with the difference results;

[0045] Step 4: Construct the relationship between distance, co-polarization NRCS difference, and rainfall rate.

[0046] from Figure 4 It is found that the difference that satisfies NRCS is 1-2 (db) and the distance is less than 100km; the difference of NRCS is 3.5-4.5 (db) and the distance is less than 200km. For those above 100km, we consider that the area is affected by rainfall.

[0047] S4: Identify the typhoon wind field and the rainfall rate based on the relationship between the NRCS difference and the rainfall rate, eliminate the influence of the rainfall rate on the typhoon wind field, and reconstruct the typhoon wind field;

[0048] Specifically, the area considered to be affected by the rainfall rate in step S3 is fixed from the SAR image, and the typhoon wind speed in the rainfall area is simulated using the Holland model based on the central air pressure, maximum wind speed radius, latitudinal Coriolis force parameter, and atmospheric density of the typhoon. The wind speed in the rainfall rate area simulated by the model is combined with the inverted wind field obtained from the SAR image by the empirical formula to form the typhoon wind field inversion result without the rainfall rate. The specific formula of the Holland model is as follows:

[0049] Where Vg represents the wind speed at a distance r from the center eye, pc represents the central air pressure, pn represents the standard atmospheric pressure, f represents the latitudinal Coriolis force, ρ represents the atmospheric density, and A and B represent fixed parameters of the model setting.

[0050] Figure 5This is the typhoon wind field inversion result obtained by using the above inversion algorithm. It can be clearly seen from the figure that Figure 3 The existing striping phenomenon has been removed, and the inversion result is more continuous.

[0051] S5: Verify the wind field results and form a typhoon wind field inversion algorithm that removes the rainfall rate.

[0052] Specifically, the wind speed observed in SFMR is compared with the SAR inversion wind speed results accompanied by rainfall and the wind speed of the reconstructed wind field. From the comparison chart, we can clearly see that the reconstructed wind field has a better correlation, which verifies the accuracy of our algorithm. Figure 6a and Figure 6b As shown, Figure 6a The comparison results of typhoon wind field affected by rainfall rate and SFMR data are described. The root mean square error of wind speed inversion result is 5.99m / s. Figure 6b The comparison results between the typhoon wind field without the influence of rainfall rate and the SFMR data are described, and the root mean square error of the wind speed inversion result is 3.79m / s. Therefore, the inversion results show the feasibility of reconstructing the typhoon wind field without the influence of rainfall rate, and solve the shortcomings in the inversion of the cross-polarization sea surface wind field of the hurricane wind field in the rainfall rate.

[0053] Compared with the prior art, the typhoon wind field inversion method in SAR remote sensing images with rainfall rate removed of the present invention uses spaceborne SAR to obtain cross-polarization remote sensing images, calculates the VH wind speed of SAR through a formula and calculates the co-polarization NRCS value affected by the rainfall rate, explores the relationship between the NRCS difference and the rainfall rate according to the difference between the NRCS affected by the rainfall rate and the NRCS observed in the SAR image, fully analyzes the specific relationship between the NRCS difference and the rainfall rate and the distance from the typhoon eye, thereby constructing a typhoon wind field inversion algorithm for SAR remote sensing images with rainfall rate removed.

[0054] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A method for inverting typhoon wind field in SAR remote sensing images by removing rainfall rate, characterized in that: The method comprises the following steps: Obtaining raw spaceborne SAR images and wind field and rainfall rate data observed by SFMR; The wind speed of VH of SAR is calculated by the formula, and the NRCS value affected by rainfall rate is calculated; According to the difference between the NRCS affected by the rainfall rate and the NRCS observed by the SAR image, the relationship between the NRCS difference and the rainfall rate is explored, including: calculating the difference between the NRCS affected by the rainfall rate and the NRCS observed by the SAR image, and analyzing the relationship between the NRCS and the rainfall rate and the distance from the typhoon eye; The analysis of the relationship between NRCS, rainfall rate, and typhoon eye distance specifically includes the following steps: The difference between the observed co-polarization NRCS on the SAR image obtained by radiometric calibration and the simulated co-polarization NRCS with rainfall rate interference is processed; Calculate the distance between each point in the SAR and the center of the typhoon eye; Find the location points where rainfall rate exists in SFMR and match them with the difference results; Construct the relationship between distance, co-polarization NRCS difference, and rainfall rate; Based on the relationship between the NRCS difference and the rainfall rate, the typhoon wind field and the rainfall rate are identified, the influence of the rainfall rate on the typhoon wind field is eliminated, and the typhoon wind field is reconstructed, specifically including: analyzing the relationship between the co-polarization NRCS difference and the rainfall rate, identifying the rainfall rate area, fixing the area affected by the rainfall rate from the SAR image, using the Holland model to simulate the wind field in the area with the rainfall rate, combining the wind speed of the rainfall rate area simulated by the model with the inverted wind field obtained from the SAR image through the empirical formula, and forming a typhoon wind field inversion result without the rainfall rate; Verify the wind field results and form a typhoon wind field inversion algorithm that removes the rainfall rate.

2. The method for inverting typhoon wind field in SAR remote sensing images by removing rainfall rate according to claim 1, characterized in that: The step of obtaining the original satellite-borne SAR images and the wind field and rainfall rate data observed by SFMR specifically includes: collecting four SAR remote sensing images, and simultaneously collecting the observed rainfall rate and wind speed data from SFMR in the four SAR images in time.

3. The method for inverting typhoon wind field in SAR remote sensing images by removing rainfall rate according to claim 1, characterized in that: The steps of calculating the wind speed of the SAR VH by a formula and calculating the NRCS value affected by the rainfall rate specifically include: after the SAR remote sensing image to be inverted is calibrated by radiation, a cross-polarized wind field quick view is inverted, the wind speed of the SAR VH is calculated by a formula, and the co-polarization GMF formula CMOD5.N is used to calculate the co-polarization NRCS value affected by the rainfall rate.

4. The method for inverting typhoon wind field in SAR remote sensing images by removing rainfall rate according to claim 1, characterized in that: The specific formula of the Holland model is: Where Vg represents the wind speed at a distance r from the center eye, pc represents the central air pressure, pn represents the standard atmospheric pressure, f represents the latitudinal Coriolis force, ρ represents the atmospheric density, and A and B represent fixed parameters of the model setting.

5. The method for inverting typhoon wind field in SAR remote sensing images by removing rainfall rate according to claim 1, characterized in that: The step of verifying the wind field results and forming a typhoon wind field inversion algorithm without rainfall rate specifically includes: comparing the wind speed inversion results with rainfall rate interference with the wind speed inversion results without rainfall rate to verify the accuracy of the typhoon wind field inversion algorithm for wind field reconstruction.

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