Description of CAGEX Version 1 data
This page last updated on 1/2/99!!
On this page, we describe the data available in version 1 of the CAGEX data set. For information about how the dataset was created, especially the input data, click on DOCUMENTATION (some of this stuff is repeated in that section). Below is a list of the CAGEX parameters. Select an item to get you to that spot on this page, or scroll down to read the entire thing. If the parameter is not underlined and/or highlighted, there is no further description of that parameter in this document. The word "core" is used to represent the "default" state: NMC based soundings and fluxes computed with those soundings.
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- Core Longwave
- Upward flux profiles (W/m**2)
- Downward flux profiles (W/m**2)
- Top-of-atmosphere upward flux (OLR) (W/m**2)
- Surface upward flux (ULF) (W/m**2)
- Surface downward flux (DLF) (W/m**2)
- MODTRAN-3 TOA 10-12um TOA radiance (W/m**2/sr)
- TOA and SFC longwave clear-sky fluxes
- Other Longwave
- OLR using pyrgeometer-based skin temperature (W/m**2)
- ULF using pyrgeometer-based skin temperature (W/m**2)
- MODTRAN-3 TOA 10-12um TOA radiance using pyrgeometer-based skin temperature(W/m**2/sr)
- OLR using MAPS soundings, sounding-based skin temperatures (W/m**2)
- ULF using MAPS soundings, sounding-based skin temperatures (W/m**2)
- DLF using MAPS soundings, sounding-based skin temperatures (W/m**2)
- TOA and SFC longwave clear-sky fluxes (MAPS soundings)
- Core Shortwave
- Upward flux profiles
- Downward flux profiles
- Top-of-atmosphere albedo
- Surface upward flux (SFC reflected SW)
- Surface downward flux (SFC insolation)
- Surface downward direct flux
- Surface downward diffuse flux
- TOA and SFC clear-sky fluxes
- Other Shortwave
- TOA albedo using MAPS soundings (W/m**2)
- SFC reflected SW using MAPS soundings (W/m**2)
- SFC insolation using MAPS soundings (W/m**2)
- TOA and SFC clear-sky fluxes (MAPS soundings)
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- TOA Longwave
- Total-sky TOA broadband longwave flux (W/m**2)
- Clear-sky TOA broadband longwave flux (W/m**2)
- Total-sky TOA narrowband longwave flux (W/m**2)
- Clear-sky TOA narrowband longwave flux (W/m**2)
- GOES-7 TOA 10-12um radiance (W/m**2/sr)
- Surface Longwave
- Pyrgeometer ULF (W/m**2)
- Pyrgeometer DLF (W/m**2)
- Pyrgeometer ULF standard deviation (W/m**2)
- Pyrgeometer DLF standard deviation (W/m**2)
- TOA Shortwave
- Total-sky TOA broadband shortwave albedo
- Clear-sky TOA broadband shortwave albedo
- Total-sky TOA narrowband shortwave albedo
- Clear-sky TOA narrowband shortwave albedo
The core sounding data includes pressures, temperatures, water vapor mixing ratios, and ozone mixing ratios for the 3x3 CAGEX grid. Raw temperatures were obtained from NWS soundings (lower level) and TOVS (upper level). The humidities are based upon NWS soundings for lower levels and climatology for upper levels (we do not have confidence in humidity sounding data above ~-30 degrees C, around 100 mb). The ozone is an SBUV2 (Solar Backscattering UV measurements system flying on NOAA polar orbiters) product. See DOCUMENTATION for details on how the raw data is transformed into the CAGEX system. It should be noted here that the core soundings are based on 12-hourly sondes. The diurnal cycle, most evident near the surface, is not evident in these soundings.
The MAPS (Mesoscale Analysis and Prediction System) is an ARM product and consists of soundings for the central facility (center CAGEX gridbox) only. The surface temperatures for these soundings come from the ARM SMOS (Surface Meterological Observation System) data sets, since MAPS does not supply a surface temperature. Replacing the core soundings with those of MAPS results in a significant decrease in the number of flux calculations, since none are done for gridboxes other than the center one. The MAPS soundings are based upon 3-hourly output, describing the diurnal cycle of near-surface temperatures and humidities more faithfully. Again, TOVS and climatology are used for the upper levels in the soundings for temperature and humidity respectively, and the ozone source is SBUV2.
MAPS is a mesoscale model/data assimiliation system. It was developed by Forecast Systems Laboratory (FSL) in Boulder.
The cloud retrievals are a product of the Minnis et al.(1995) Layered Bi-spectral Thresholding Method applied to GOES-7 visible and infrared images. For each parameter, there are values for three cloud levels (low, middle, high), plus a total. A complete description of this method is described in Minnis et al. (1995) and can be found on the Langley Cloud Home Page under the "Reference" heading. See the Cloud Placement section of the Documentation page for some important notes regarding cloud placement.
Procedure for the calculation of solar zenith angles follows Iqbal
(1983). The effective cosines of the solar zenith angles were calculated by averaging the cosines of 31 solar zenith angles calculated for each
minute in the CAGEX 30-minute timestep. Temporally, the 31 zenith angles are centered
on the CAGEX time. For example, the zenith angle for CAGEX timestep 1
(1409 GMT) was calculated using the 31 zenith angles from 1354 GMT to
1424 GMT.
These zenith angles were inplemented in the radiative transfer code. For surface purposes (conversion of pyrheliometer direct normal flux to direct horizontal flux), a term was added to account for diffraction.
(corrected 9/23/97)
The solar constant used by CAGEX is 1365.0 W/m**2, adjusted for the day of the year. This relationship is given by:
solar constant = 0.9905 * 1365.0 / (r**2), where
r= 1 + a0 * sin ( ( 2 * pi/365.25 ) * ( nd - 365.25/4 ) ), where
a0 = 0.0167381 and
nd = The number of days since January 4, 1958.
This formula comes from the American Ephemeris and Nautical Almanac, United States Naval Observatory (1956). On January 4, 1958, the vector was at extreme displacement (.9832619); this is assumed to vary sinusoidally with date. The three-body interactions of Earth, the sun, and Jupiter are significant for the earth at periods of 100,000, 40,000, and 20,000 years (eccentricity of orbit, obliquity of ecliptic, precession of equinoxes). We're pretty confident with the present formula.
The .9905 represents the fraction of the 1365 W/m**2 which is present in the Fu-Liou shortwave spectrum (.2 - 4 microns) The remaining .0095 (13 W) exists between 4 and 10 microns.
The Fu & Liou radiation transfer code utilizes six
spectral bands in it's parameterization of the short
wave and near infra-red spectrum between 0.2 and 4.0
microns: 0.2-0.7, 0.7-1.3, 1.3-1.9, 1.9-2.5,
2.5-3.5, 3.5-4.0 microns. In order to determine a set
of spectral reflectivities to place in these bands it
is necessary to find spectral measurements from various
sources and try to match their results with these intervals.
Unfortunately there are very few if any data that can be
found that match these intervals one to one.
The method chosen was to use reflectances measured in
similar bands and either interpolate or extrapolate these
values to the Fu & Liou intervals. The weights used in
the interpolation are determined by the low resolution
radiation transfer model MODTRAN-3 calculated at the relatively
high resolution of 50 inverse centimeters. Essentially, the
incoming shortwave flux at the surface is integrated within the
spectral bands of the known reflectances. Then, if these bands
correspond to one of the Fu & Liou intervals, these values are
used as weights for distributing the known reflectences
across the Fu & Liou bands. For example, used in the CAGEX
are the short grass/meadow spectral reflectences found in
Briegleb et al. (1985) which are given for the spectral
regions 0.2-0.5, 0.5-0.7, 0.7-0.85, and 0.85-4.0 microns.
Hence, the first two of the Briegleb et al. intervals are
used to interpolate to the region 0.2-0.7 microns. The
second two numbers are used to interpolate a value for the
0.7-1.3 micron interval and the 0.85-4.0 reflectence is simply
inserted into the last four Fu & Liou intervals.
A weighting factor was calculated by dividing the Briegleb et al. spectral reflectance by these spectral reflectances integrated over the Fu-Liou spectral bands.
Broadband surface albedos were calculated from ARM upward and downward-looking pyranometer data. Where missing, the dataset average of these surface albedos (for each temporal period) was substituted. The CAGEX spectral reflectance was determined by multiplying this broadband albedo by the Briegleb weighting factor for each spectral band.
Included in the dataset are the weighting functions used to distribute the broadband surface albedo across each of the six Fu-Liou spectral bands, as well as the Briegleb et al. (1985) broadband albedo, found by integrating across the Fu-Liou shortwave spectrum.
Surface emissivities are set equal to 1 for each of the 12 Fu-Liou longwave bands. While there is some evidence to suggest we should decrease our emissivities somewhat, it is not done in release 1.
In the "core" dataset, the skin temperature is set equal to the sounding surface temperature. However, we believe a more accurate skin temperature can be found by utilizing tha ARM downward-looking pyrgeometer and applying the Stefan-Boltzmann law, assuming a surface emissivity of 1.0. Although this will result in a skin temperature which is too low for a non-black surface, this modification uses measured data which more accurately describes the change in temperature throughout the day. Since the core profiles are based on 12-hour soundings, there is not enough information to accurately describe the diurnal cycle. The CAGEX datasets includes some fluxes calculated using a pyrgeometer-based skin temperature.
Raw aerosol optical depths are an ARM product, and arrived on our doorstep courtesy of Dr. Joseph Michalsky (SUNY-Albany). They were measured in 5 spectral bands (.412, .498, .606, .663, and .856 microns) by a multi-filter rotating shadowband radiometer. Single scattering albedos and asymmetry parameters were extracted from D'Almeida et al.(1991). Spinhirne et al. (1993) provided a means for distribution of aerosols vertically. See the Aerosol section of the Documentation page for details of insertion of these values into the CAGEX dataset.
The core longwave products include upwelling and downwelling flux profiles, calculated at each CAGEX level. These profiles include a full complement of fluxes at the surface and top-of-atmosphere.
These sets of fluxes include OLR and ULF, calculated using a pyrgeometer-based skin temperature. Also included are OLR, DLF, and ULF calculated using the MAPS product as a sounding database. Clear-sky calculations were performed by setting all cloud amounts equal to zero.
The core shortwave includes the shortwave versions of the longwave fluxes, as well as TOA insolation and surface direct and diffuse downwelling radiation. Albedos are given at the TOA however. TOA insolation is indirectly present. It should be calculated by using the solar constant and the cosine of the solar zenith angle. Clear-sky calculations were performed by setting all cloud amounts equal to zero.
Shortwave was also calculated using the MAPS soundings. Clear-sky calculations were performed by setting all cloud amounts equal to zero.
TOA broadband longwave was estimated by the Minnis et al. LBTM (Layered Bispectral Thresholding Method) from narrowband GOES-7 measurements. The LBTM narrowband to broadband conversion is based on an earlier study that used GOES-6 TOA visible narrowband and ERBE (Earth Radiation Budget Experiment) TOA broadband. The rms error of this conversion should not exceed 5%. Flux values exist for the entire CAGEX spatial grid.
The surface longwave validation measurements are in the form of ARM "BSRN" pyrgeometers. We use these one-minute fluxes to calculate a 30 minute average flux, centered over the CAGEX timestep. The standard deviations of this data is included in the data set. These fluxes were measured in the central CAGEX gridbox only. The temporal standard deviations (of the 1-minute values within each 30-minute block) are included in the dataset.
TOA broadband shortwave was inferred in a similar manner as the longwave broadband. The rms error of this conversion should not exceed 15%. Flux values exist for the entire CAGEX spatial (3x3) grid.
(this text modified on 12/18/96)
The surface shortwave validation measurements are in the form of ARM "BSRN" radiometers. We use high-temporal resolution direct (pyrheliometer) and diffuse (shaded pyranometer) values to calculate a 30 minute average insolation, centered over the CAGEX timestep. The standard deviations of this data is included in the data set. These fluxes were measured in the central CAGEX gridbox only. The radiometric dataset was designed to provide the user with as much information as possible. Using different combinations of these parameters, more data can be extracted.
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The direct component of the surface downwelling shortwave is the normal component. To obtain a direct horizontal flux, this quantity must be multiplied by the cosine of the solar zenith angle. The cosine of the zenith angle given in CAGEX is that calculated by us, which is somewhat different than that given in the raw ARM radiometry datasets. To get direct horizontal flux, subtract the diffuse from the total, do not multiply the direct normal by the cosine of the CAGEX zenith. Please note that this data is what is averaged directly from the raw data files. Negative values are possible!
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The diffuse component is averaged directly from the raw data. Again, negative values are possible.
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The surface insolation (total SW down) is computed by multiplying the direct component by the cosine of the solar zenith, as given in the ARM dataset, and adding the result to the diffuse component. Another insolation can be determined by using the CAGEX cosine of the zenith. However, please remember that the CAGEX zenith angle is not corrected for atmospheric diffraction but the ARM zenith angle, not given in the dataset, is.
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The surface albedo is not part of the radiometry section of the dataset, but is included in the "extra" section. The surface albedo uses the downward-looking pyranometer for the upwelling radiation, and the combination of pyrheliometer and shaded pyranometer for the downwelling component of the albedo. 30 minute averaged albedos are determined by averaging the upward and the downward separately, then dividing them to obtain the average albedo..
Other validation of the CAGEX system include lidar cloud base heights, SMOS 2-meter temperatures and relative humidities, and 60-meter tower temperatures and relative humidities.
Once-per-minute lidar cloud base heights are averaged over 30 minutes, centered upon the CAGEX timesteps. Standard deviations are calculated, and a "temporal" cloud fraction is calculated by dividing the number of times in which the lidar detected clouds by the total number of timesteps in the 30-minute averages. Cloud base heights are measured in the central CAGEX gridbox only.
SMOS parameters include 2-meter temperatures and relative humidities, measured at the ARM central facility. These values are used as surface values when using the MAPS soundings.
Tower parameters include 60-meter temperatures and relative humidities, measured at the ARM central facility. I don't think we did anything with these measurements in version 1.