Click the question categories to see the questions. Click the questions to see the answers. If you have questions about using polar satellite data, please e-mail them to Patrick Dills (dills@ucar.edu) or Sherwood Wang (woody@ucar.edu). We will continue to expand this FAQ as the need arises.
FAQ Categories
• POES Sounder Characteristics
• Product Retrieval and Characteristics
• Product Availability and Operational Use -
The Polar Orbiting Satellite Sounding Evaluator (POSSE) and AWIPS
- When looking at multiple satellite composites, why are there gaps between orbits? Do these gaps remain fixed in position and do they change in size?
- When looking at the POES sounding coverage (using
POSSE
),
sounding locations across the swath of a given orbit appear to have
variable spacing. Why does this occur?
Product Retrieval and Characteristics
- POES systems produce temperature and moisture soundings in clear and cloudy atmospheres. How can this be done?
- Temperature and moisture profiles are computed for both GOES and POES observations. How are sounding products computed for each and are the approaches different?
- Most satellite derived product systems require first guess information to compute soundings. GOES soundings use a numerical weather prediction (NWP) forecast as the first guess. How important is the first guess, and how is it computed for POES?
- NESDIS produces moisture products in both ATOVS
and AMSU
-B based
sounding retrieval systems. What are the differences between these products?
- Can soundings be reliably retrieved over all terrain?
- POES soundings can be retrieved in cloudy areas, but not in areas of heavy precipitation. What is the threshold of precipitation at which a reliable retrieval becomes unlikely, and what if any is the effect of cloud liquid water (CLW)?
- RAOB and NWP forecast profiles represent a series of point measurements at specified horizontal and vertical locations. What are the horizontal and vertical characteristics of satellite derived sounding products?
- How do cloud liquid water (CLW) and total precipitable
water (TPW) products generated by the NESDIS MSPPS
compare with the same products available from ATOVS and AMSU-B systems?
- POES sounder measurements from AMSU-B used primarily for moisture retrieval are contaminated by significant radio frequency interference (RFI) from data transmitters onboard NOAA-15. What is the current status concerning this RFI and are the AMSU-B measurements and derived products useful?
- A known advantage of POES vs. GOES sounders is the ability to retrieve atmospheric temperature and moisture in cloudy regions. In clear regions where both systems provide soundings, how do the sensitivities of POES and GOES sounders compare? Are they equivalent?
Product Availability and Operational Use - POSSE and AWIPS
- NWS field offices currently have access to GOES sounder retrievals and some derived product imagery through AWIPS. Will the new generation of NOAA POES (NOAA-15, L, and M) provide the same or a different suite of products?
- How soon can I expect to have access to sounding retrievals
(with POSSE
) after a POES orbit?
- When looking at POES sounding products (using POSSE), are all displayed products valid for weather forecasting applications?
- The larger operational forecast centers around the world have begun switching from satellite derived soundings to satellite sounder measurements for assimilation into NWP models. What impact if any does this have when it comes to weather forecasting applications?
- What are the goals and attributes of the POSSE
system?
- Operational POES satellites are deployed with a complement of infrared and microwave
radiation sounders. How does an atmospheric sounder work?
Answer: An atmospheric sounder provides a vertical profile of atmospheric temperature and moisture. Sounder frequencies are selected based on their absorption characteristics for atmospheric gases such as carbon dioxide, nitrous oxide, and water vapor. The variation in absorption with frequency makes each channel uniquely sensitive to temperature and/or moisture within a discrete vertical atmospheric layer. The thickness of the layer depends on the variation of absorption with frequency as well as the width of the frequency (bandwidth) being sensed. Microwave radiometers typically have a broader vertical sensitivity than their infrared partners and reduced atmospheric sensitivity near the surface.
- When looking at multiple satellite composites, why are there gaps between orbits?
Do these gaps remain fixed in position and do they change in size?
Answer: The progression of orbits and orbital periods is not exactly the same for two satellites given their variation in height. Consequently, on some days their orbits can overlap considerably leaving large gaps (mainly toward the tropics). On other days the smaller overlap results in little or no gaps. Typically, no gaps occur poleward of 40 degrees latitude.
- When looking at the POES sounding coverage (using POSSE
), sounding locations across
the swath of a given orbit appear to have variable spacing. Why does this occur?
Answer: The variable spacing of POES data coverage is primarily the result of the POES sounding algorithm and the POSSE sequential display technique. The spacing of POES soundings can vary from as little as 17 km near nadir
, where sounder measurements are
most dense and highly resolved, to 150 km near the orbit edge, where sounder resolution
and measurement density are lowest.
Product Retrieval and Characteristics
- POES systems produce temperature and moisture soundings in clear and cloudy
atmospheres. How can this be done?
Answer: POES satellites are equipped with both microwave and infrared atmospheric sounders. Although clouds contaminate the infrared by absorbing upwelling radiation from the atmosphere and surface below, they are mostly transparent in the microwave. NESDIS sounding systems first determine the presence of clouds. Soundings for clear FOVs
are computed using both
infrared and microwave measurements. For cloudy FOVs only microwave measurements
are used. - Temperature and moisture profiles are computed for both GOES
and POES observations. How are sounding products computed for each and are
the approaches different?
Answer: GOES and POES sounding products are computed using a physical retrieval approach. This approach utilizes an atmospheric radiative transfer model and the observed sounder measurements to simultaneously solve for vertical profiles of temperature and moisture. The main difference in the scientific approach is how the necessary first guess is formed. GOES sounding retrieval depends on a 6-hour NWP forecast to provide the first guess information, whereas POES retrieval relies on a library search that determines first guess information based on 30- to 60-day global datasets of collocated RAOB and satellite measurements.
- Most satellite derived product systems require first guess
information to compute soundings. GOES soundings use a numerical weather prediction
(NWP) forecast as the first guess. How important is the first guess, and how
is it computed for POES?
Answer: The first guess provides initial temperature, moisture, and sounder information from which to compute soundings. Although accurate first guess information is desirable, it is most important that both the meteorological and sounder data represented in the guess be consistent. POES first guess information is independent of NWP forecasts, relying instead on carefully compiled datasets of collocated radiosonde and satellite observations. These datasets are global, updated daily, and directly accessed during orbital processing. Collocations for which the sounder and RAOB data agree best are identified during orbital processing. This matching is referred to as the library search approach. RAOB and sounder data for 10 selected collocations are averaged to compute the first guess temperature, moisture, and sounder radiances.
- NESDIS produces moisture products in both ATOVS
and
AMSU
-B based sounding retrieval
systems. What are the differences between these products?
Answer: At the time of publication, the NESDIS processing system for AMSU-B high-resolution moisture products is separate from the ATOVS system that produces AMSU-A and HIRS
temperature and moisture
soundings. This is unfortunate because the former lacks temperature information,
while the latter lacks moisture information in cloudy regions. It is therefore
recommended that users view ATOVS products for temperature profiles and
AMSU-B for coincident moisture products. NESDIS is developing a combined
AMSU-A, AMSU-B, and HIRS temperature and moisture product retrieval system,
tentatively scheduled for implementation before the end of 2000. - Can soundings be reliably retrieved over all terrain?
Answer: Yes, except for levels near the surface. The POES sounding products include terrain type, and product generation is segregated by terrain type. The terrain types available are sea and non-sea, with subcategories under non-sea including coast, snow, ice, and high terrain. Given the sensitivity of microwave channels to surface emissivity, those channels peaking in the lower troposphere are used less over land (particularly over snow and ice) than over sea. Soundings over high terrain use neither microwave channels peaking at low levels nor infrared data. Since coastal soundings can include microwave data affected by both land and sea, their use is not recommended.
- POES soundings can be retrieved in cloudy areas, but not
in areas of heavy precipitation. What is the threshold of precipitation at
which a reliable retrieval becomes unlikely, and what if any is the effect
of cloud liquid water (CLW)?
Answer: Profiles are retrieved in areas of clouds by relying on the microwave sensing capabilities of POES. Although insensitive to clouds, microwave measurements are affected by absorption and scattering due to CLW and contaminated by heavy precipitation. At the time of publication, a computation for CLW is being done over sea. If this value exceeds 2.5 mm, then heavy precipitation is assumed and the derived sounding should not be considered for weather applications. Over land, a separate precipitation index is computed. Correction of microwave channels for CLW up to 2.5 mm is currently not done but under investigation.
- RAOB and NWP forecast profiles represent a series of point
measurements at specified horizontal and vertical locations. What are the
horizontal and vertical characteristics of satellite derived sounding products?
Answer: Satellite sounder measurements represent a collage of data at various horizontal and vertical resolutions. The horizontal resolution of POES microwave and infrared data decreases from 50 and 17 km at nadir
, to 150 and 50 km
respectively at the orbit swath edges. The sounding path is vertical at
nadir viewing, but becomes increasingly slanted toward the swath edges.
When deriving POES soundings, the microwave data are interpolated to the
infrared FOVs and limb adjusted to achieve a vertical path through the atmosphere
at all swath locations. The spatial characteristics of a satellite sounding
can best be described as a vertical cylinder whose horizontal resolution
varies as a function of swath location, and whose vertical extent is the
integration of the vertical sensitivities for all the sounding channels.
The average POES sounding cylinder measures about 50 km wide and extends
from the surface to 0.1 mb. - How do cloud liquid water (CLW) and total precipitable water
(TPW) products generated by the NESDIS MSPPS
compare with the same products
available from ATOVS
and
AMSU
-B systems?
Answer: The algorithms for computing cloud liquid water (CLW) are similar for the MSPPS, ATOVS, and AMSU-B systems. The algorithms for total precipitable water (TPW), however, are quite different. TPW in the MSPPS system is based strictly on the radiometric data, namely the surface channels of the AMSU-A or AMSU-B, and is not provided over land. The TPW from the AMSU-A and AMSU-B atmospheric sounding systems is based on the integration of derived moisture profiles and is therefore available over sea and land. As a rule, TPW derived from moisture profiles is slightly lower than TPW retrieved directly from the sounder measurements.
- POES sounder measurements from AMSU
-B used primarily for moisture
retrieval are contaminated by significant radio frequency interference (RFI)
from data transmitters onboard NOAA-15. What is the current status concerning
this RFI and are the AMSU-B measurements and derived products useful?
Answer: Problems concerning RFI and AMSU-B were observed shortly after the launch of NOAA-15 in May 1998. The RFI degraded all of the AMSU-B measurements and initially appeared to render these data useless. After a period of intensive investigation and testing, however, NOAA, NASA, and United Kingdom Meteorological Office scientists successfully developed RFI correction schemes for removing a major portion of the RFI. Then, on 28 September 1999, the STX-1 and STX-3 data transmitters, the main sources of RFI, were permanently turned off. The result was a significant reduction in RFI, and the determination was made that RFI corrected AMSU-B measurements are suitable for operational use. NESDIS plans operational use of AMSU-B measurements during the spring of 2000, resulting in significant improvements to moisture retrieval.
- A known advantage of POES vs. GOES sounders is the ability
to retrieve atmospheric temperature and moisture in cloudy regions. In clear
regions where both systems provide soundings, how do the sensitivities of
POES and GOES sounders compare? Are they equivalent?
Answer: The POES ATOVS
deployed on NOAA-15 contains
a 20-channel AMSU
that provides continuous
vertical sounding of the atmosphere from the surface to 2 mb. The GOES infrared
sounder sounds from the surface to about 10 mb. AMSU also provides a much
improved sensitivity in the tropopause region between 300 and 100 mb where
the current GOES (and previous POES) sounders lack sensitivity. The GOES
sounder does, however, exhibit a higher horizontal resolution (8 km) than
the AMSU (48 km) and HIRS
(17 km).
Product Availability and Operational Use - POSSE
and AWIPS
- NWS field offices currently have access to GOES
sounder retrievals and some derived product imagery through AWIPS. Will
the new generation of NOAA POES (NOAA-15, L, and M) provide the same
or a different suite of products?
Answer: POES products were originally designed to provide a suite of global atmospheric sounding products. These consist primarily of derived temperature and moisture soundings, radiometric sounder measurements, and cloud products. However, the scope of the polar program has been revised with more emphasis on providing products that resemble those from GOES. POES products ported to AWIPS can be expected to include retrieved soundings and product imagery similar to what's available for GOES. The POSSE developed by NESDIS represents the first step in this process.
- How soon can I expect to have access to sounding retrievals
(with POSSE) after a POES orbit?
Answer: It takes 104 minutes for one POES to complete an orbit. Orbital processing at Suitland Maryland typically begins within 20 minutes of an orbit's completion, and takes about 20 minutes to process. Users can therefore expect to have POES data within 40 to 144 minutes of the observation time.
- When looking at POES sounding products (using POSSE),
are all displayed products valid for weather forecasting applications?
Answer: POES sounding products are computed from 1000 to 0.1 mb regardless of the terrain height. Since soundings below the ground are not screened in POSSE horizontal and vertical display modes, users must be careful not to consider such data in weather analysis. Users are advised to take precautions whenever using POES sounding products over high terrain. The POES sounding data record also contains an observational quality flag to help monitor product quality. For example, products contaminated by precipitation are flagged as "poor quality." Only those products exhibiting "good observational quality" should be considered for use in operational weather forecasting.
- The larger operational forecast centers around the world have begun
switching from satellite derived soundings to satellite sounder measurements
for assimilation into NWP models. What impact if any does this have
when it comes to weather forecasting applications?
Answer: The increasing tendency for larger forecast centers to assimilate sounder measurements reflects the notion that NWP forecast and observational background errors are more easily computed when satellite radiation measurements are assimilated rather than derived products (i.e., soundings). The scientific algorithms and derived satellite soundings produced by NESDIS are designed to satisfy the sounder radiometric measurements similar to any NWP forecast or application. Both exhibit solutions and performance (i.e., accuracy) constrained by the non-unique characteristics of atmospheric radiative transfer theory, the limiting factor in deriving an exact temperature and moisture solution given a vertical profile of radiation measurements. A thorough treatment of this subject can be found in Chapter 2 of the SATMET-3 CD-ROM module Using the GOES Sounder.
- What are the goals and attributes of the POSSE
system?
Answer: The POSSE system represents NESDIS's first attempt to provide POES sounding products in a format compatible to what most NWS users are familiar with for GOES. POSSE allows users to display horizontal imagery of sounder data (channels) and derived product (levels) from the latest NOAA-POES orbits. By selecting a particular location within a horizontal field, the user can view the complete derived sounding profiles (for temperature and moisture) and additional secondary derived parameters (i.e., stability). Another feature of POSSE is the display of NWP forecast data fields and profiles, when available. These typically consist of a 6-hour forecast valid within 3 hours of the POES observation, and the analysis from which the forecast was initialized. Occasionally no forecast data is shown, indicating that timely forecast data was not available at the time of the POES data capture.