Metr 51: Scientific Computing II

Lecture 9: Lidar Plotting Techniques

Allison Charland3 April 2012

• Two weeks 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

DBS DataDate & Time(UTC)

Height (m AGL) Wind Direction & Wind Speed (ms-1)

• Remove 225 heading• Use load to read in the data then

create wind speed and direction profiles.

• What if you have multiple files that you want to plot using one code?

• You have three files that you want to plot: wind_profile_0900PST.txt, wind_profile_1200PST.txt, and wind_profile_1500PST.txt

• Create a character string array called filenamesfilenames = [‘wind_profile_0900PST.txt’;…

‘wind_profile_1200PST.txt’;‘wind_profile_1500PST.txt’];

Hint: filenames must all be the same character length

• Create a variable for the number of files you haveNum = 3;

Plotting multiple profiles

• Now you want to create a for loop that will go from 1 to the number of files you have and load and plot each file.

• Each time you loop through you want to reset the filename that is being read.

• Set your values of height, wind direction and wind speed.• Plot your variables.• Clear variables so that the next time through the loop they will be

replaced.

Plotting multiple profiles

for i =1:Num

f = filenames(i,:);

load f;

data = f;

clear f;

height = data(:,1);

wind_dir = data(:,2);

wind_speed = data(:,3);

figure(1);clf;

plot(wind_dir, height, ‘+k’); hold on;

plot(wind_speed, height, ‘k’); hold on;

clear data; clear height; clear wind_dir; clear wind_speed;

end %for

Plotting multiple profiles

• To plot each profile as a separate figure replace figure(1);clf; with figure(i);clf;

• To save each figure as a jpeg, use saveas after your figure is complete

figure_name = [‘profile_’i’.jpg’];saveas(gcf,figure_name);

• Each time through the loop the figure_name will be reset and saved with the new number i.

Plotting multiple profiles

Stare Lidar DataDate & Time(UTC) There is one data file per hour

Stare Data

• The load command cannot be used for stare data because the number of columns changes throughout the data file from 3 to 4 after each scan.

• Fastest way to plot stare data is using xlsread• This reads in an excel spreadsheet then later using loops

the data can be separated into specific variables.• The first step is to load the stare data file into excel

space delimited then save it as an excel file with the extension .xlsx (for Microsoft 2010).

Stare Data

• Before loading the data, set up the height array.• Heights will always be the same throughout a stare file.height = 0:225;height = (height + 0.5)*24;height = height';

Stare Data• Read in the datafile data = xlsread(filename.xlsx); j = hour; %The hour of the particular data file fprintf('Completed %2.0f \n', j)

• Set conditions for start of data columns c1=2; if j > 1 k=17; p=18; elseif j==1 k=9; p=10; end if j >= 4 c1=1; end %if if j >= 18 c1=2; end %if

Splitting the data

count = 0; n = 2100;for i=1:n count = count +1; time(count) = data(k,c1); k=k+226; V = data(p:p+100,3)'; V_2 = data(p+100:p+224,2)'; V_new(count,:) = [V V_2]; B = data(p:p+100,4)'; B_2 = data(p+100:p+224,3)'; B_new(count,:) = [B B_2]; p=p+226;end %forfprintf('Completed split \n')

Stare Data• Change time from UTC to PST• Depends on Daylight Savingsfor w = 1:length(time) if time(w) < 7; time(w) = 24 - (7-time(w)); else time(w) = time(w)-7; end %ifend %for

Transpose the data

Transpose the data

B_new = B_new'; V_new = V_new'; time =time' ;

figure(1);clf;

pcolor(time,height,V_new);hold on;

shading interp;

ylim([0 2400])

xlabel('Time (PST) ');

ylabel('Height (m AGL)');

title('Vertical Velocity (m s^-^1)')

caxis([-5 4])

colorbar

Show Example

Plotting Stare Data

Stare Data

• The example shown is the raw data.• A filter algorithm can be used to eliminate invalid data

including near the surface and aloft where the signal is no longer retrieved.

• The algorithm code will be available on D2L.• Multiple files could be read in and plotting using the

same technique as earlier to plot multiple wind profiles.• The problem is that the figures created will have so

much detail that there may be issues copying them.