MISR Plume Height Project
David Nelson, Cecelia Lawshe, David Diner, Ralph Kahn
The MISR and MODIS instruments aboard the Terra satellite produce a number of parameters relevant to the study of wildfire smoke plumes as well as dust plumes and volcanic plumes. This product is a publicly-available repository for the data acquired by MISR and MODIS and processed under the auspices of a project to develop an aerosol injection height climatology for North America to support wild fire, climate change and air quality studies. The project was originally funded by the EPA and NASA and included investigators from JPL, Harvard University, University of California at Irvine, University of Michigan and Washington State University. After the original project requirements were satisfied, the project was extended by NASA to include other regions of the world as well as volcanic plumes. Dust plumes may eventually be added as well.
One of the MISR team's contributions to this project is to provide a database of information on smoke plumes extracted from satellite data sets that includes the location and time of plume observations, plume height measurements (from which injection heights may be deduced), the approximate radiative power of their associated fires, the direction of transport of plumes, approximate areas of individual smoke plumes, and aerosol properties and top-of-atmosphere albedo estimates. In this context, smoke plumes are considered to be discrete regions of smoke that can be traced to their fire sources at ground level and that have a distinctive shape determined by the direction the smoke is driven by the wind. Similar information is retrieved for selected regions of dense smoke not clearly associated with specific fire sources (smoke clouds), and whose direction of transport is not easily determined.
The acquisition of these data is accomplished using an interactive visualization program named MINX which allows users to manually digitize plumes and other features (which together are termed regions) and to compute smoke height and wind velocity using a new stereo height algorithm. The program automatically creates the imagery and raw data files described in this document. The MINX program and its documentation can be freely downloaded from the Open Channel Foundation website at http://www.openchannelsoftware.org/projects/MINX.
Specific MISR products used by MINX in constructing this data collection are the ancillary AGP product containing the MISR digital elevation data (DEM), and the following standard products: level 1 terrain-referenced radiances, level 1 geometric parameters, and level 2 aerosol. The single MODIS product used is the Level 2 MOD14 Thermal Anomalies product from the TERRA satellite at a resolution of 1 km.
At the top level, the main page is organized first by plume aerosol type ("Wildfire smoke plumes" or "Volcanic plumes") and then by project area. Buttons near the top of the page allow selection of the aerosol plume type, and a list below them details the project areas for the selected plume type. Some project areas have multiple sub-projects, representing different time periods, different areas within the larger area or combinations of both. Clicking on any project area with multiple sub-projects will open the sub-project list, like a directory tree. It can be collapsed again with another click.
To go to the data in a project area or sub-project, click a "View/Download" link in the "Images and Data" column. The "Project Area" page that appears contains an index map showing the location of all the plumes in the project, a list containing basic plume statistics for the project area, three links that are described in greater detail below and a table of MISR orbits and block ranges where smoke plumes were found, together with the date the MISR data were acquired and the number of plumes found in each orbit and block range.
Ten-day Plume Statistics
When you click on this link, you will see a table that summarizes important plume characteristics for the project in histogram fashion by appropriate time intervals. For each interval, these parameters are enumerated: number of plumes, median height of plumes above sea level, mean area and mean power.
Project Summary Files
This link takes you to a page from which you can select to view the summary file either for digitized plumes or for digitized aerosol clouds. These files are constructed during a post-digitizing pass that reads aerosol plume and cloud raw data files for all digitized regions and compiles the summary information.
Clicking on either link brings up an ASCII text file containing almost 60 columns of data with one plume or aerosol cloud per line. There is an extensive header at the top of these files that describes each data column in detail. The plume or cloud heights in these files (column 16) are median values estimated from the many raw data points in each plume. You may decide that using the raw data points for each plume or cloud instead gives you better control over the application of these data to your work.
If you wish to download one of these summary files, simply right-click in the data page and select the "Save Page As..." option from the mouse menu. In the "Save As" dialog box that appears, specify the location to which the file should be saved and click "Save."
Raw Data Files
If you click on this link, you are shown a page containing a large table consisting of eleven columns of information in which each row represents a single plume or aerosol cloud. This table enables you to select those regions for which raw data files will be downloaded to your computer and is the preferred way to obtain large numbers of raw data files.
The first three columns of the table contain the components of the name of a digitized region: orbit number, block number and region identifier (region type code plus unique number). For older plume data, the region type code is a single letter with 3 alternatives: smoke plume (P), smoke cloud (S) and clear land (L). For plume data digitized in late 2011 and forward, a new version of MINX was used that expanded the type code from 1 to 4 letters as summarized here:
|Letter 1: region
|Letter 2: region
|Letter 3: was a wind
|Letter 4: what MISR band
was used in ht retrieval
|D = dust||L = line||N = no wind provided||R = red band
|S = smoke||P = polygon||W = wind provided||G = green band
|V = volcanic ash|| || ||B = blue band
|W = water|| || ||N = near IR band
C = red + blue bands
The wind direction code (Letter 3) replaces the old distinction between clouds (no wind direction provided) and plumes (wind direction provided). The retrieval band (Letter 4) is a new feature in MINX 2.0 that allows the user to select the MISR color band to use in retrievals. All digitizing prior to the end of 2011 used the red band, because only it has high resolution data in all cameras. For low optical thickness smoke over bright backgrounds, the red band performs poorly. The blue band returns superior heights and winds in these cases with an acceptable reduction in vertical precision.
In the "Raw Data Files" table, the "Type/Number" column has additional functionality - if you click the colored "Type/Number" link in any row, you are taken directly to the images and data plots for that region (see descriptions below). Similarly, the "View" links in the "Raw Data File" column can be clicked to take you to the ASCII file containing raw data for that region.
You can click on the column name in the header of any of the first nine columns in the table, and the table will be sorted in ascending order by that parameter. This is helpful for selecting a subset of raw data files for downloading. The final column named "Download File" contains checkboxes that you should use to indicate which plumes you want to download. Alternatively, you can click the "Check All" button at the bottom of the table to select all rows.
An "Advanced Search" link is also available above the right edge of the table. Clicking this opens a filter panel that allows you to specify the minimum and maximum values you want to include in your search for each of five parameters. Once you have done this and have clicked the "Submit Advanced Search" button, the table is updated to include only those rows that satisfy your search request. Next check the boxes in those rows corresponding to regions you want to download or click the "Check All" button at the bottom of the table to select all rows.
Once you have made your final selections, click the "Review File Selections" button to review them. This page also has a box at the bottom for entering your email address. Enter your address here, and press the "Email List of Selections" button, and a file will be sent to you that contains information you will need for pulling the files down to your computer.
When the email arrives, click on the link in the email that describes the "curl" application and follow the directions it provides. You can see the same information by clicking the "Download Help" link on the page where you enter your email address.
Orbits/Block Ranges Table
Clicking on any colored link in the "Orbits/Block Ranges" column of this table on the project page brings you to the "Plume" page for the appropriate MISR orbit and block range in the project. This page gives you access to the images, plots and raw data captured during the digitizing process. An index map in the upper left corner displays the location of all the digitized regions as well as the MISR block outlines. The table at the bottom lists those regions together with four additional parameters for each. The columns are:
- Region Name - The unique name assigned to the region. This includes the orbit number, the block number, 1 or 4 letters designating the type of region (see table above) and a unique number in the orbit and block.
- Region Type - The long word description of the region type.
- Height - The estimated median height in kilometers above mean sea level of points representing the top surface of the smoke plume or cloud.
- Direction - The angle in degrees measured clock-wise from north corresponding to the direction in which the plume smoke is being blown.
- Total Power - The total radiative power in megawatts of the plume as estimated from the MODIS MOD14 product (this is the sum of the radiative power in all MODIS pixels captured for this plume during digitizing).
Clicking a colored link in the "Region Name" column on the "Plume Page" displays the individual region data page for the selected region. Each of these pages contains 8 images and plots for the region as well as a link at the top labeled "View/Download Raw Data." Using this link, you can obtain the raw digitized data used in constructing the plots for any plume or cloud region. Alternatively, you could use the "Raw Data Files" link on the project page. There are also "Previous Region" and "Next Region" links at the top left and top right of this page that allow you to navigate directly between the images for different plumes in the same orbit.
Index maps are provided at each level of the product below the main page. At the individual region level, the map shows only the current region and the MISR block outline. On all the maps, plumes are indicated in red and clouds (aerosols for which no wind direction was provided and therefore lacking a wind-corrected height) are indicated in blue.
Two MISR nadir camera images (An) are shown on each page with the same scene but different overlays. The first image has overlays of the digitized region outline and name. An arrow indicating the wind direction, as interpreted by the person who digitized the scene, is drawn in yellow. MODIS thermal anomaly pixels are marked as red dots.
The second nadir image shows a color-coded dot posted at each successful height-retrieval point inside the current region. For plumes (which by definition have wind direction specified), the color indicates wind-corrected height. For clouds (which by definition have no wind direction specified), the color indicates uncorrected height. The color key with which to convert the colors to heights is shown on the image.
The MPEG camera animation of the region allows you to view MISR's nine camera images in succession in order to visualize the 3D nature of the plumes and to view the differences in scene reflectance and the effect of differences in atmospheric travel path through aerosols at different viewing angles.
The best way to view the nine camera images is by grabbing the manual control at the bottom of the image using your mouse and moving the control back-and-forth. Since these most often are MISR terrain-referenced images, you will see patches of black pixels appearing on the more oblique-looking camera images in mountainous regions. These are effectively no-data, shadow zones created when images are re-projected from the ellipsoid to the terrain.
Height profiles are constructed by sorting heights successfully retrieved for a region by increasing distance from the first point digitized. For plumes, the first point digitized is always near the plume source on the ground. For cloud regions, the first point digitized is selected at random. The wider the region digitized, the more points there will be at a similar distance from the source. These will be represented on the profile as different heights at the same distance coordinate creating the appearance of a poorly defined plume top. It is important to study the color-coded An height image to understand the 2D spatial distribution of heights as well as the 1D distribution that the profile represents. For the terrain height profile, the saw-tooth pattern that this 1D representation sometimes causes can be confusing.
The data represented on a height profile always includes terrain heights (always a green line) and zero-wind heights. Wind-corrected heights are also present if a wind-direction was specified during digitizing. The identity of each of these data types is indicated by color-coded text near the top of the profile.
When there are at least five successful wind-corrected heights for a plume, then two estimates of height are attempted. Both are based on fitting a plane to the heights, then removing all points more than 1.5 standard deviations from the plane. The "Maximum Height Estimate" is the maximum height value that remains, and the "Median Height Estimate" is the median value of the remaining point heights. Raw data files for each plume are accessible from the product so independent calculations of standard heights can be made.
For cloud regions, there is no wind retrieval, so there can be no wind-corrected heights. The Maximum and Median heights are calculated as above but represent zero-wind heights.
Wind profiles share a distance axis with height profiles, and they split the wind-speed vector into across-track and along-track components. Positive wind-speeds are defined to be directed toward the north (i.e. up-track) and toward the east (i.e. toward the right of the track). Unless there is a successful wind retrieval for a point, there cannot be a wind-corrected height.
Note that if there is a component of wind in the along-track direction toward the south (negative), then the corrected height will be greater than the zero-wind height. If there is a component of wind in the along-track direction toward the north (positive), then the corrected height will be less than the zero-wind height.
Wind retrievals and wind-corrected heights are less reliable the more the wind direction is aligned with the MISR along-track direction, because the along-track component of satellite motion eventually becomes inseparable from the along-track motion of the smoke. For plumes that are closely aligned with the along-track direction and have wind-corrected heights with much greater scatter than the zero-wind heights, the wind-corrected heights should be used with caution.
Height and Wind Histograms
For smoke plumes, four histograms summarize the height and wind data, one each for zero-wind and wind-corrected heights and one each for cross-track and along-track winds. These histograms are computed from the points plotted on the height and wind plots. For aerosol clouds, there is only a zero-wind histogram, since no wind retrieval is performed.
Albedo profiles also share a distance axis with height profiles. Spectral albedos are calculated at the Top of Atmosphere (TOA) during plume processing by MINX rather than read from the MISR level 2 products, because of resolution and algorithm considerations.
The spectral albedo algorithm is similar to that used in the MISR TOA local albedo product, but it differs in several respects. First, rather than re-projecting the radiances for each camera to the tops and sides of rectangular prisms that represent cloud heights, the disparities from the camera-matching step are used to co-register pixels from the same part of the scene for each camera. Second, the total radiances at these pixels are used in the solid-angle weighting calculation, whereas in the local albedo product, only the radiances projected to the tops of prisms are used. Finally, plume albedos are calculated at 275-meter resolution, whereas the local albedo product computes albedos at 2200 meters resolution. MINX TOA albedos are not expected to be as accurate as those from the local albedo product.
Aerosol data are read from the MISR standard aerosol product. They are reported at 17.6 km resolution and are typically not available when the optical depth is very high or when there is cloud contamination. When aerosol results are retrieved by MINX, it is usually because a low optical depth region of aerosol present near the plume has been captured during digitizing.
Because the low resolution of the aerosol product sometimes results in an entire plume being contained in a single aerosol pixel, histograms are not the best way to represent these data. A different method for representing them may be implemented in the future.
Two of the histograms, "Optical Depth" and "Single-Scatter Albedo", contain spectral data. The colors of their bars correspond to the spectral bands they represent. On these histograms, when more than one band occupies the same Tau or SSA interval, the bars are drawn with the largest value in the back and the smallest in the front to make all the bars visible.
The "Tau Fraction by Particle-Type" histogram depicts the fraction of the green-band optical depth value that is attributable to small, medium and large particle sizes. The weighted sum of these fractions must equal 1. The histogram also shows the fraction of the green-band optical depth value that is attributable to spherical particles. The non-spherical fraction is not shown and is simply 1 - spherical fraction. This histogram also has a key that identifies the meaning of its colors.