DYCOMS-II Satellite: NOAA POES GAC Data (level 1b) 1.0 General Information The NOAA POES GAC Data is one of several satellite data sets collected by the University Corporation for Atmospheric Research/Joint Office for Science Support (UCAR/JOSS) as part of the Dynamics and Chemistry of Marine Stratocumulus Phase II: Entrainment Studies (DYCOMS-II) project. Included in the data set are Global Area Coverage (4 km resolution) measurements from the Advanced Very High Resolution Radiometer (AVHRR) aboard the National Oceanic and Atmospheric Administration (NOAA) Polar Orbiting Environmental Satellites (POES). The The data cover the period from 7 - 28 July 2001. Data are available at POES satellite overpass times. These data were acquired from the NOAA Satellite Active Archive (http://www.saa.noaa.gov) and are in level1B format. Any passes that had any portion over the DYCOMS-II region (26-37N and 115-127W) are included within this data set. 2.0 Data Contact Scot Loehrer (loehrer@ucar.edu) 3.0 Data and Format Information 3.1 AVHRR Introduction The Advanced Very High Resolution Radiometer (AVHRR) sensor is carried on NOAA's Polar Orbiting Environmental Satellites (POES) starting with TIROS-N in 1978. Onboard the TIROS-N, NOAA-6, 8 and 10 POES Satellites, the AVHRR Sensor measures in four spectral bands, while on the NOAA-7, 9, 11, 12 and 14 POES Satellites, the sensor measures in five bands. The AVHRR/3 sensor on NOAA-15 and 16 measures in six bands though only five are transmitted to the ground at any time. The visible data values may be converted into albedos and the IR data into radiances or temperatures using the calibration information which is appended but not applied. Latitudes and longitudes of 51 benchmark data points along each scan are included. Other parameters appended are: time codes, quality indicators, solar zenith angles, and telemetry. 3.2 AVHRR Applications The objective of the AVHRR instrument is to provide radiance data for investigation of clouds, land-water boundaries, snow and ice extent, ice or snow melt inception, day and night cloud distribution, temperatures of radiating surfaces, and sea surface temperature, through passively measured visible, near infrared and thermal infrared spectral radiation bands. The Advanced Very High Resolution Radiometer for TIROS-N and the follow-on satellites is a scanning radiometer with either four or five channels, which is sensitive to visible/near IR and infrared radiation. The instrument channelization has been chosen to permit multispectral analyses which provide improved determination of hydrologic, oceanographic, and meteorological parameters. The visible (0.5 micron) and visible/near IR (0.9 micron) channels are used to discern clouds, land-water boundaries, snow and ice extent, and, when the data from the two channels are compared, an indication of ice/snow melt inception. The IR window channels are used to measure cloud distribution and to determine the temperature of the radiating surface (cloud or surface). Data from the two IR channels is incorporated into the computation of sea surface temperature. By using these two channels, it is possible to remove an ambiguity introduced when clouds fill a portion of the field-of-view. On later instruments in the series, a third IR channel was added for the capability of removing radiant contributions from water vapor when determining surface temperatures. Prior to inclusion of this third channel, corrections for water vapor contributions were based on statistical means using climatological estimates of water vapor content. Ground IFOV (end of scan) 59.5 km cross-track by 30.4 km along-track Distance between IFOV centers 42.0 km along-track Swath width (to far edge of outer FOV) 2,239.6 km 3.3 AVHRR Data Acquisition NOAA Polar Orbiting Environmental Satellites obtain global imagery daily. These data are transmitted to the Command and Data Acquisition (CDA) stations. The CDA stations relay the data to the National Environmental Satellite, Data and Information Service (NESDIS), located in Suitland, Maryland, for processing and distribution. As a result of the design of the AVHRR scanning system, the normal operating mode of the satellite calls for direct transmission to Earth (continuously in real-time) of AVHRR data. This direct transmission is called HRPT (High Resolution Picture Transmission). In addition to the HRPT mode, about 11 minutes of data may be selectively recorded on board the satellite for later playback. These recorded data are referred to as LAC (Local Area Coverage) data. LAC data may be recorded over any portion of the world, as selected by NOAA/NESDIS, and played back on the same orbit as recorded or during a subsequent orbit. LAC and HRPT have identical Level 1b formats. The full resolution data are also processed on board the satellite into GAC (Global Area Coverage) data which are recorded only for readout by NOAA's CDA stations. GAC data contain only one out of three original AVHRR lines. The data volume and resolution are further reduced by averaging every four adjacent samples and skipping the fifth sample along the scan line. POES satellites operate in relatively low orbits, ranging from 830 to 870 km above the earth. They circle the earth approximately 14 times per day (with orbital periods of about 102 minutes). The orbits are timed to allow complete global coverage twice per day, per satellite (normally a daytime and a nighttime view of the earth) in swaths of about 2,600 km in width. High resolution (1 kilometer) data are transmitted from the satellite continuously, and can be collected when the satellite is within range of a receiving station. Recorders on board the satellite are used to store data at a 4 kilometer resolution (processed by the on-board computers) continuously, and a limited amount of data at a 1 kilometer resolution on demand. The recorders are dumped when the satellite is within range of a NOAA receiving station. 3.4 AVHRR Data Description AVHRR Level 1b data are present as a collection of data sets. Each data set contains data of one type for a discrete time period. Thus, for AVHRR, there are separate HRPT, LAC, and GAC data sets. Time periods are arbitrary subsets of orbits, and may cross orbits (i.e., may contain data along a portion of an orbital track that includes the ascending node, the reference point for counting orbits). Generally, GAC data sets are available for corresponding time periods and usually have a three to five minute overlap between consecutive data sets. Level 1b (following FGGE terminology) is raw data in 10 bit precision that have been quality controlled, assembled into discrete data sets, and to which Earth location and calibration information has been appended, but not applied. Other parameters appended are: time codes, quality indicators, solar zenith angles, and telemetry. 3.5 AVHRR Spatial Coverage The AVHRR provides a global (pole-to-pole) on-board collection of data from all spectral channels. At an 833 km altitude, the 110.8 degree scan equates to a swath 27.2 degrees in width (at the Equator), or 2,600 km, centered on the subsatellite track. This swath width is greater than the 25.3 degree separation between successive orbital tracks, providing overlapping coverage (side-lap). For LAC and HRPT, the instantaneous field-of-view (IFOV) of each channel is approximately 1.4 milliradians (mr) leading to a resolution at the satellite subpoint of 1.1 km for a nominal altitude of 833 km. Since GAC data contain only one out of three original AVHRR lines and the data volume and resolution are further reduced by averaging every four adjacent samples and skipping the fifth sample along the scan line, the effective resolution is 1.1 x 4 km with a 3 km gap between pixels across the scan line. This is generally referred to as 4 km resolution. 3.6 AVHRR Comprehensive Information Additional information on the AVHRR sensor and data for satellites TIROS-N through NOAA-14 can be found at: http://www2.ncdc.noaa.gov/docs/podug/html/c3/sec3-0.htm Details of the AVHRR/3 sensor can be found at: http://www2.ncdc.noaa.gov/docs/klm/html/c3/sec3-1.htm while calibration information for AVHRR/3 is at: http://www2.ncdc.noaa.gov/docs/klm/html/c7/sec7-1.htm and data format details for AVHRR/3 LAC and HRPT are at: http://www2.ncdc.noaa.gov/docs/klm/html/c8/s831-3.htm and data format details for AVHRR/3 GAC are at: http://www2.ncdc.noaa.gov/docs/klm/html/c8/s831-4.htm 3.4 Format These data are in level1b format. For details on this format see: http://www2.ncdc.noaa.gov/docs/klm/html/c8/s8-0.htm 4.0 Quality Control Procedures UCAR/JOSS conducted no quality checks on these data. 5.0 File Naming Convention The file names are structured as follows: NSS.GHRR.NJ.D01188.S1418.E1611.B3360102.GC.L1309545 Where GHRR signifies GAC data NJ is the satellite: NJ = NOAA-14 NK = NOAA-15 NL = NOAA-16 D01188 signifies Year 2001 Julian Day 188 S1418 says the pass has a start time of 1418 UTC E1611 says the pass has an end time of 1611 UTC 6.0 References None