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3.4.2 Data Access.
3.4.2.1 Radar Data Access.
Weather radar data at NCEI are stored on a tape archive system and are accessed by placing orders through the NCEI website. There are several ordering systems available, optimized for different situations, including small orders, multisite and multiday orders, and large bulk orders. Currently, TDWR-SPG data are only available from the “Select by Site” and “Select by File” ordering systems. Small orders are typically completed in less than 15 minutes.
There are occasional gaps and missing data for each site in the archive. These gaps are caused by a number of reasons, including scheduled maintenance at the radar sites, unplanned downtime due to severe weather, communications problems, or archival problems. The data access web pages include lists or visualizations of file availability.
3.4.3 Products.
3.4.3.1 TDWR-SPG Products.
This complete list of all available data products (called Level-III products) includes descriptions and possible uses. There are 26 TDWR-SPG Level-III products routinely available from NCEI, including precipitation estimates, storm relative velocity, and echo tops.
3.4.3.2 Comparison of the TDWR to the WSR-88D.
The range resolution of the TDWR is finer than what is available in the WSR-88D or any other FAA radar that has weather channel capability. The TDWR utilizes a range gate resolution of 150 m for Doppler data. It has a resolution of 150 m for reflectivity data within 135 kilometers (km) and 300 m from beyond 135 km to 460 km. By contrast, the WSR-88D employed by the NWS, FAA, and DOD has a maximum range gate resolution of 250 m for Doppler and 1 km for surveillance data.
The angular (azimuth) resolution of the TDWR is nearly twice what is available in the WSR-88D. Each radial in the TDWR has a beam width of
0.55 degrees. The average beam width for the WSR-88D is 0.95 degrees. The following table shows a comparison of technical specifications between the TDWR and the WSR-88D.
3-59 Table 3-8. Technical Specifications: WSR-88D vs. TDWR WSR-88D TDWR Wavelength 10 cm 5 cm Volume Scan Time 4 minutes in Volume Coverage Pattern (VCP) 12 1 min 0.2 degree, Hazard (HAZ) Bean Width 1.25 degrees 0.5 degrees Range Gate 0.13 NM in velocity
0.54 NM in reflectivity
0.067 NM Max Unambiguous Velocity Up to 62 kts 20–30 kts Max Doppler Range 230 km 90 km The National Oceanic and Atmospheric Administration’s (NOAA) Radar Operations Center (ROC) has detailed information (http://www.roc.noaa.gov/).
3.4.4 How Does the Radar Collect Data?
The FAA designed the TDWR to look for low-altitude phenomena, such as wind shifts over the runways, windshear along the immediate approach and departure corridors, and downbursts. These radars are located close to major airports with the scanning strategy optimized to sample the atmosphere over its associated airport. The TDWR employs a scanning strategy called monitor mode, which is similar to the WSR-88D clear-air mode.
The TDWR remains in monitor mode until one of the two following conditions are recognized:
1. A region of 20 dBZ echoes located within 24.3 NM from the associated airport with a nominal areal extent of 1.3 NM and an altitude of at least
1.3 NM AGL, or
2. The radar detects windshear or a microburst.
This is a departure from the WSR-88D operations that can switch from clear-air mode to precipitation mode if the areal coverage of precipitation exceeds an adaptable parameter anywhere on the scope. The TDWR ties its decision on the area of influence directly to its associated airport.
3.4.5 Volume Coverage Pattern (VCP) 90.
VCP 90 is the TDWR monitor mode (clear air) and consists of 17 scans in about 6 minutes. The first cut is always a low Pulse Repetition Frequency (PRF), long-range scan (276 km). All remaining scans are short range (90 km). There is a small variation in the elevation angles of the lower scans, but all scans above 5 degrees are the same in all systems. Cuts two and three employ a split cut strategy. Cut two contains no dealiased Doppler data. Cut three (at the same elevation as cut two) is the first cut to contain dealiased Doppler velocity data. The first three cuts are used to initialize wind field models and velocity unfolding algorithms.
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3.4.6 Volume Coverage Pattern (VCP) 80.
VCP 80 is the TDWR hazardous mode (precipitation mode). The first cut is always a low PRF, long-range scan (276 km). Cuts two and three are the short-range, split cut scans used to collect information for unfolding and clutter migration algorithms. The first cut with dealiased Doppler data is cut three. There are two sub-volumes or aloft scans contained in each full volume, as denoted by the use of red and orange. Every fourth scan, the TDWR provides a short-range elevation scan at the same elevation as cuts two and three.
3.4.7 Where Can I Get Details on TDWR-SPG?
1. NOAA’s ROC maintains papers, presentations, and detailed information on the TDWR-SPG system (https://www.roc.noaa.gov/spg/default.aspx).
2. Details on a subset of the TDWR-SPG products are available in the Federal Meteorological Handbook No. 11 (FMH-11), Doppler Radar Meteorological Observations (WSR-88D). The handbooks are availabl e for download from the Office of the Federal Coordinator for Meteorology (OFCM) (http://www.ofcm.gov/).
3. NCEI provides descriptions and possible uses of radar products generated from the TDWR network operated by the FAA. All data are available digitally at NCEI for free (visit http://www.ncdc.noaa.gov/nexradinv/) and paper copies are available for purchase. Certification of the paper data is al so available at https://www.ncdc.noaa.gov/customer-support/certifica tion-data.
3.4.8 TDWR Level-II (Base) Data.
The TDWR Level-II Base Data is not available from NCEI.
3.4.9 TDWR Level-III Products.
There are 26 TDWR Level-III products routinely available from NCEI. Most Level-III products are available as digital images, color hard copy, grayscale hard copy, or acetate overlay copies. Each copy will include state, county, and city background maps.
The Certification of Data page (https://www.ncdc.noaa.gov/customersupport/certification-data) has further information on hard copy radar products, pricing, and certification information. A detailed list of Lev el-III product codes is also available at http://www1.ncdc.noaa.gov/pub/data/radar/RadarProductsDetailedTable.pdf.
Brief descriptions and possible uses of these products are included below.
3.4.9.1 General Products.
General products include Base Reflectivity and Base Velocity, as well as graphical products derived from algorithms, including Spectrum Width, Vertically Integrated Liquid, and the Velocity Azimuth Display (VAD) Wind Profile.
3.4.9.1.1 Digital Base Reflectivity (TR0, TR1, TR2/181).
A display of echo intensity measured in decibels. These products are used to detect precipitation, evaluate storm structure, locate boundaries, and
3-61 determine hail potential. The three lowest elevation angles are available, with a maximum range of 48 NM.
3.4.9.1.2 Digital Base Velocity (TV0, TV1, TV2/182).
A measure of the radial component of the wind either toward the radar (negative values) or away from the radar (positive values). Cool colors (green) represent negative values and warm colors (red) represent positive values.
These products are used to estimate wind speed and direction, locate boundaries, locate severe weather signatures, and identify suspected areas of turbulence. The three lowest elevation angles are available, with a maximum range of 48 NM.
3.4.9.1.3 Long Range Digital Base Reflectivity (TZL/186).
Digital Reflectivity data are measured at an elevation angle of 0.6 degrees with a maximum range of 225 NM.
3.4.9.1.4 Composite Reflectivity (NCR/37).
A display of maximum reflectivity for the total volume within the range of the radar. This product is used to reveal the highest reflectivity in all echoes, examine storm structure features, and determine intensity of storms.
3.4.9.1.5 Vertically Integrated Liquid (NVL/57).
The water content of a 2.2 x 2.2 NM column of air, which is color-coded and plotted on a 124-NM map. This product is used as an effective hail indicator, to locate most significant storms, and to identify areas of heavy rainfall.
3.4.9.1.6 Echo Tops (NET/41).
An image of the echo top heights color-coded in user-defined increments. This product is used for a quick estimation of the most intense Convection and higher echo tops, as an aid in identification of storm structure features, and for pilot briefing purposes.
3.4.9.1.7 VAD Wind Profile (NVW/48).
A graphic display of windbarbs plotted on a height staff in 500-ft or 1,000-ft increments. The current (far right) and up to 10 previous plots may be displayed simultaneously. This product is an excellent tool for meteorologists in weather forecasting, severe weather, and aviation.
3.4.9.2 Precipitation Products.
Precipitation products are estimated ground accumulated rainfall. Estimates are based on a reflectivity and rainfall rate relationship called Z-R (https://www.weather.gov/tae/research-zrpaper).
3.4.9.2.1 One-Hour Precipitation (N1P/78).
A display of estimated 1 hour precipitation accumulation on a 1.1-NM x 1-degree grid using the Precipitation Processing System (PPS) algorithm. This product is used to assess rainfall intensities for flash flood warnings, urban flood statements, and special weather statements.
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3.4.9.2.2 Storm Total Precipitation (NTP/80).
The estimated storm total precipitation accumulation on a 1.1-NM x 1-degree grid continuously updated since the precipitation event began. This product uses the PPS algorithm. This product is used to locate flood potential over urban or rural areas, estimate total basin runoff, and provide rainfall data 24 hours a day.
3.4.9.2.3 Digital Precipitation Array (DPA/81).
An array format of estimated 1-hour precipitation accumulations on the 1/4 Lyon Federal Maybery (LFM) (4.7625 km Hydrologic Rainfall Analysis Project (HRAP)) grid. This is an 8-bit product with 255 possible precipitation values. This product is used to assess rainfall intensities for flash flood warnings, urban flood statements, and special weather statements.
3.4.9.3 Overlay Products.
Overlay products provide detailed information for identified storm cells.
3.4.9.3.1 Storm Structure (NSS/62).
A table displaying information on storm attributes, which include maximum reflectivity, maximum velocity at lowest elevation angle, storm overhang, mass weighted storm volume, storm area base and top, storm position, and storm tilt.
3.4.9.3.2 Hail Index (NHI/59).
A product designed to locate storms that have the potential to produce hail.
Hail potential is labeled as either probable (hollow green triangle) or positive (filled green triangle). Probable means the storm is probably producing hail and positive means the storm is producing hail.
3.4.9.3.3 Digital Mesocyclone Detection Algorithm (NMD/141).
This product is designed to display information regarding the existence and nature of rotations associated with thunderstorms. Numerical output includes azimuth, range, and height of the mesocyclone.
3.4.9.3.4 Tornadic Vortex Signature (NTV/61).
A product that shows an intense gate-to-gate azimuthal shear associated with tornadic-scale rotation. It is depicted by a red triangle with numerical output of location and height.
3.4.9.3.5 Storm Tracking Information (NST/58).
A product that shows a plot of the past hour’s movement, current location, and forecast movement for the next hour or less for each identified thunderstorm cell. This product is used to determine reliable storm movement.
3.4.9.4 Radar Messages.
Radar messages provide information about the radar status and special product data.
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3.4.9.4.1 General Status Message (GSM/2).
A text message describing the status of the radar site. A reason may be provided if the radar is not transmitting.
3.4.9.4.2 Free Text Message (FTM/5).
A text message describing an upcoming, ongoing, or completed event at the radar site, including downtime, maintenance, and more.
3.4.9.4.3 Radar Status Log (RSL/152).
A running daily log of status, errors, and messages from the Radar Product Generator (RPG) and Radar Data Acquisition (RDA) processing systems.
Discussion Credit: NOAA/NCEI/ROC.
Figure 3-27. WSR-88D vs. TDWR (Image Credit: NWS)
3-64 Figure 3-28. TDWR Site Locations (Image Credit: MIT Lincoln Labs)
The image below is a classic thunderstorm producing a microburst. The TDWR radar picture on the left is a vertical cross section of a storm’s core. The image on the right is a visual photograph of a similar storm that produced a microburst.
Figure 3-29. Thunderstorm Producing a Microburst (Image Credit: MIT Lincoln Labs)
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3.5 Satellite.
3.5.1 Description.
Satellite is perhaps the single most important source of weather data worldwide, particularly over data-sparse regions, such as countries without organized weather data collection and the oceans.
The NOAA’s Geostationary Operational Environmental Satellite (GOES) imagery can be found on the AWC’s website, as well as all NWS WFO websites. Additional satellite imagery for Alaska can be found on the NWS Alaska Aviation Weather Unit (AAWU) website.
3.5.2 Imagery Types.
Three types of satellite imagery are commonly used: visible, infrared (IR), and water vapor. Visible imagery is only available during daylight hours. IR and water vapor imagery are available day or night.
3.5.2.1 Visible Imagery.
Visible imagery (see Figure 3-30, Visible Satellite Image—U.S. Example) displays reflected sunlight from the Earth’s surface, clouds, and particulate matter in the atmosphere. Visible satellite images, which look like black and white photographs, are derived from the satellite signals. Clouds usually appear white, while land and water surfaces appear in shades of gray or black.
The visible channel senses reflected solar radiation. Clouds, the Earth’s atmosphere, and the Earth’s surface all absorb and reflect incoming solar radiation. Since visible imagery is produced by reflected sunlight (radiation), it is only available during daylight.
Figure 3-30. Visible Satellite Image-U.S. Example http://adds.aviationwe 9 12 15 18 42 45 51 54 57 60 63 66 72252 4 7 0
3.5.2.1.1 Visible Image Data Legend.
The data legend (see Figure 3-31, Visible Satellite Data Legend) on a visible image displays albedo, or reflectance, expressed as a percentage. For example, an albedo of 12 means 72 percent of the sunlight that struck a feature was reflected back to space.
Figure 3-31. Visible Satellite Data Legend 0 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 02 05 78 01 84 87 00 03 The gray shades (values) represent albedo, or reflectance, expressed as a percentage.
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3.5.2.2 Inf rared (IR) Imagery.
IR images (see Figure 3-32, Infrared (Color) Satellite Image—U.S.
Example, and Figure 3-33, Unenhanced Infrared (Black and White) Satellite Image—U.S. Example) display temperatures of the Earth’s surface, clouds, and p articulate matter. Generally speaking, the warmer an object, the more IR energy it emits. The satellite sensor measures this energy and calibrates it to temperature using a very simple physical relationship.
Clouds that are very high in the atmosphere are generally quite cold
(e.g., -50 ºC), whereas clouds very near the Earth’s surface are generally quite warm (e.g., +5 ºC). Likewise, land may be even warmer than the lower clouds
(e.g., +20 ºC). Those colder clouds emit much less IR energy than the warmer clouds and the land emits more than those warm clouds.
The data measured by satellite is calibrated and colorized according to the temperature. If the temperature of the atmosphere decreases with height (which is typical), cloud-top temperature can be used to roughly determine which clouds are high-level and which are low-level.
When clouds are present, the temperature displayed on the IR images is that of the tops of clouds. When clouds are not present, the temperature is that of the ground or the ocean. A major advantage of the IR channel is that it can sense energy at night, so this imagery is available 24 hours a day.
Figure 3-32. Infrared (Color) Satellite Image—U.S. Example
The scale is in degrees Celsius. Blue/purple colors indicate colder temperatures, while orange/red colors indicate warmer temperatures.
3-68 Figure 3-33. Unenhanced Infrared (Black and White) Satellite Image—U.S. Example
The scale is in degrees Celsius. Lighter gray shades indicate colder temperatures, while darker gray shades indicate warmer temperatures.
3.5.2.2.1 Infrared Image Data Legends.
The data legend (see Figure 3-34, Infrared (Color) Satellite Image Data Legend, and Figure 3-35, Unenhanced Infrared (Black and White) Satellite Image Data Legend) on an IR image is calibrated to temperature expressed in degrees Celsius. The legend may vary based on the satellite image provider.
Figure 3-34. Infrared (Color) Satellite Image Data Legend The colors (values) represent temperature in degrees Celsius.
Figure 3-35. Unenhanced Infrared (Black and White) Satellite Image Data Legend The gray shades (values) represent temperature in degrees Celsius.
3.5.2.3 Water Vapor Imagery.
The water vapor imagery (see Figure 3-36, Water Vapor Satellite Image— U.S. Example) displays the quantity of water vapor generally located in the middle and upper troposphere within the layer between 700 mb (approximately 10,000 ft MSL) and 200 mb (approximately FL390). The actual numbers displayed on the water vapor images correspond to temperature in degrees Celsius. No direct relationship exists between these values and the temperatures of clouds, unlike IR imagery. Water vapor
3-69 imagery does not really “see” clouds, but “sees” high-level water vapor instead.
The most useful information to be gained from the water vapor images is the locations and movements of weather systems, jet streams, and thunderstorms.
Another useful tidbit is aided by the color scale used on the images. In general, regions displayed in shades of red are very dry in the upper atmosphere and may correlate to crisp, blue skies from a ground perspective.
On the contrary, regions displayed in shades of blue or green are indicative of a lot of high-level moisture and may also indicate cloudiness. This cloudiness could simply be high-level cirrus types or thunderstorms. That determination cannot be ascertained from this image by itself, but could easily be determined when used in conjunction with corresponding visible and IR satellite images.
A major advantage of the water vapor channel is that it can sense energy at night, so this imagery is available 24 hours a day.
Figure 3-36. Water Vapor Satellite Image—U.S. Example
The scale is in degrees Celsius. Blue/green colors indicate moisture and/or clouds in the mid/upper troposphere, while dark gray/orange/red colors indicate dry air in the mid/upper troposphere.
3.5.2.3.1 Water Vapor Image Data Legend.
The data legend (see Figure 3-37, Water Vapor Satellite Image Data Legend) on water vapor images is calibrated to temperature expressed in degrees Cels ius. The actual data values on the water vapor images are not particularly