Metr 51: Scientific Computing II

Lecture 10: Lidar Plotting Techniques 2

Allison Charland10 April 2012

• One week of Matlab left!• Matlab Final will be on April 19th

• Then on to HTML and creating a personal webpage

Reminder

Emitted infrared light

Infrared light reflected from aerosols

Movement of aerosols by wind

The shift in frequency of the return echo is related to the movement of aerosols. The faster the aerosols move, the larger the shift in frequency. From this, the wind speed relative to the light beam can be measured.

Doppler Lidar Theory

Doppler wind lidar

• Halo Photonics, Ltd. Stream Line 75 • 1.5 micron• Eye-safe• 75 mm aperture all-sky optical scanner • Min Range: 80 m• Max Range: 10km• 550 user defined range gates (24 m)• Temporal resolution: 0.1-30 s• Measurements:

• Backscatter Intensity • Doppler Radial Velocity

Lidar Scanning Techniques

• Multiple elevation and azimuth angles can be adjusted to create different types of scans.• DBS (Doppler Beam Swinging):

• Wind Profile

• Stare: Vertically pointing beam• RHI (Range Height Indicator):

• Fixed azimuth angle with varying elevation angles

• PPI (Plan Position Indicator):• Fixed elevation angle with varying

azimuth angles

95o

30o

70o

dlmread

• M = dlmread(filename, delimiter, R, C)• dlmread will read a file with any type of delimiter from any row or

column.• M = dlmread(filename, delimiter, range)

• It can also be used to specifically read a range of different rows and columns.

• The delimiter can be expressed with a character string that responds to the type of delimiter.

• Use '\t' to specify a tab delimiter• If you want to specify an R, C, or range input, but not a delimiter, set the

delimiter argument to the empty string, (two consecutive single quotes with no spaces in between, ''). For example,

• M = dlmread('myfile.dat', '', 5, 2)

0 500 1000 1500 2000 2500 30000

500

1000

1500

X (m)

Z (

m)

Doppler Radial Velocity (m/s)

-8

-6

-4

-2

0

2

4

6

8

dlmread

• M = dlmread(filename, delimiter, R, C)• Reads data whose upper left corner is at row R and column C in the file.

Values R and C are zero-based, so that R=0, C=0 specifies the first value in the file.

• M = dlmread(filename, delimiter, range)• Reads the range specified by range = [R1 C1 R2 C2] where (R1,C1) is the

upper left corner of the data to read and (R2,C2) is the lower right corner

RHI DataDate & Time(UTC)

dist = 0:129; % Zero to number of gates - 1dist = (dist + 0.5)*24; %Create distance arraydist=dist'; %TransposeNote: This distance refers to the distance along the lidar beam at any angle. For the stare data, this corresponded to the height. It will be used later.

Create your distance array

dist

dist

RHI DataDate & Time(UTC)

• This file has a fixed azimuth angle of 145o and elevation angles ranging from 0o to 80o at intervals of 5o

• There will be 16 sets of data for each elevation angle. (for i = 1:16)

R_t = 17;for i = 1:16

time_range = [R_t 0 R_t 2];T(:,i) = dlmread(‘filename’,'',time_range);R_t = R_t + 131; %Set row range to the next%time row = num of gates +1

end %forSo now T will be a 3x16 array with the first row corresponding to the times (will not be needed), the second row should be the same azimuth values, and the third row is the changing elevation angles.

Read the time and angle row

Within the same loop as the time rangeR1_d = 18; %Row where velocity beginsR2_d = 147; %Row where velocity ends

for i = 1:16data_range = [R1_d 1 R2_d 1];

V(:,i) = dlmread(‘filename’,'',data_range);R1_d = R1_d + 131;

R2_d = R2_d + 131;end %forSo now V will be a 130x16 array with the rows corresponding to velocity at range gates and columns corresponding to changing elevation angles.

Read in velocity

Calculating x and z distances

%Calculate horizontal distances elev = T(3,:)*(pi/180); %Calculate x and y distances [a,b] = size(V); for j = 1:a for p = 1:b x(j,p) = dist(j)*cos(elev(p)); z(j,p) = dist(j)*sin(elev(p)); end end

lidar

Lidar sca

n- dist

Elev. angle

x

z

figure(1);clf; pcolor(x,z,V) shading flat xlabel('X (m AGL)'); ylabel('Z (m AGL)'); title('Doppler Radial Velocity (m/s)') colorbar

Plotting with pcolor

figure(1)

500 1000 1500 2000 2500 30000

500

1000

1500

2000

2500

3000

X (m AGL)

Z (

m A

GL)

Doppler Radial Velocity (m/s)

-15

-10

-5

0

5

10

15

Set any data within the first 100 m to be NaN

Filter the data so that if there is a velocity change of ±7 ms-1 between any two range gates, then any data after that is NaN.

Adjust the colorbar axis to show the range of velocities for your data.