Atmospheric Transmission Model Research Articles

Global validation of the along‐track scanning radiometer against drifting buoys

The along‐track scanning radiometer (ATSR) was launched on the European Space Agency's first remote sensing satellite, ERS 1, on July 17, 1991. ATSR is designed to retrieve sea surface temperature (SST) to an accuracy of 0.25 K rms, which represents more than a factor of 2 improvement over any previously flown satellite radiometer. Early validation studies from limited regions suggest that ATSR is capable of measuring SST to near this design accuracy. We report a global validation study against quality‐controlled drifting buoys by examining 280 matchups worldwide with ATSR measurements at their full (1 km) resolution. We investigate optimizing the precision of ATSR using four different SST algorithms derived using a theoretical atmospheric transmission model, combined with various techniques to reduce remnant noise and other errors. We find that a “low‐noise” retrieval algorithm incorporating only the 3.7 and 11 μm nadir view channels gives the optimum precision, a global pixel precision of 0.26 K (or 0.25 K if 1/2° spatial averages are used). A standard deviation of 0.25 K against global drifting buoy data approaches the geophysical limit set by the inherent variability of the skin effect and by the buoy bulk temperature accuracy. Further progress will require comparison against quality in situ radiometer‐derived skin temperatures, although the problem of obtaining sufficiently large and diverse data sets will need to be addressed.

Comparison of modeled and empirical approaches for retrieving columnar water vapor from solar transmittance measurements in the 0.94‐μm region

Four atmospheric transmittance models, LOWTRAN 7, MODTRAN 3, FASCOD3P, and the Thomason model, are investigated to quantify the relationship between water vapor transmittance as function of water vapor amount, Tw(U), for an instrument specific band pass in the 0.94‐μm region. In a second step an empirical Tw(U) function is established using long term measurements with our high‐precision Sun photometer (SPM) in Bern, Switzerland along with 1300 simultaneous and collocated water vapor retrievals performed with a dual‐channel microwave radiometer (MWR). In order to avoid a possible bias in the empirical Tw(U) function, the MWR data set is prescreened by comparing retrievals coincident with radiosonde ascents. Over a 2½‐year period of common observations, radiosondes and SPM agreed to within 0.19 cm (13%) of columnar water vapor (CWV) using the empirical Tw(U) relationship. Completely independent comparisons with an additional MWR and two Fourier transform spectrometers yielded agreement within 13% and 9%, respectively. Comparing empirical and modeled results, we found that with respect to the experimental data, LOWTRAN 7, MODTRAN 3, and FASCOD3P reported higher water vapor transmittances over almost the entire range of realistic absorber amounts. By relating these differences to differences in retrieved CWV for the case of two standard atmospheres, we found that using Tw(U) predicted by LOWTRAN 7, MODTRAN 3, and FASCOD3P leads to an overestimate of CWV by about 18–30%, 7–20%, and 2–18%, respectively. The Thomason model yields good agreement with respect to the experimental data up to medium absorber amounts, whereas at slant path amounts larger than 10 cm, errors up to 60% in retrieved CWV occurred. We also show in this work that a misinterpretation of the LOWTRAN 7 water vapor output counterbalances incorrectly predicted Tw, leading to results that agree well with experimental ones.

Water vapor absorption in the atmospheric window at 239 GHz

Absolute absorption rates of pure water vapor and mixtures of water vapor and nitrogen have been measured in the atmospheric window at 239 GHz. The dependence on pressure as well as temperature has been obtained. The experimental data are compared with several theoretical or empirical models, and satisfactory agreement is obtained with the models involving a continuum; in the case of pure water vapor, the continuum contribution based upon recent theoretical developments gives good results. The temperature dependence is stronger than that proposed in a commonly used atmospheric transmission model.

Validation of the ATSR in Australian Waters

The Along Track Scanning Radiometer (ATSR) was launched on the ERS-1 satellite on 17 July 1991. During the following six months, a concentrated effort was made to validate the sea surface temperature (SST) derived from data supplied by this new generation radiometer. Ship and aircraft radiometers collected “ground truth” data that were coincident with ATSR measurements and thus allowed a comparison of the surface and space measurements. A large proportion of the early validation data was obtained during four research vessel cruises in Australian waters, and a detailed analysis of those results is presented here. Ancillary data were collected to support the shipborne radiometer measurements and to allow further analyses beyond the important validation task. These data included the standard surface meteorological data, bulk SST, and, in most cases, ship-launched radiosondes. Four different algorithms derived using a theoretical atmospheric transmission model were applied to the ATSR data to provide estimates of SST, and these estimates were compared to the surface-based measurements. All the algorithms gave reasonable agreement with each other as well as agreement with the surface data. The algorithm using all six infrared measurements gave the lowest standard deviation but showed a warm bias of 0.2 K when compared to the temperature of the skin layer of the ocean. The validation results show that the ATSR instrument can provide SST within the design accuracy of 0.3 K. The results of the validation presented here are in good agreement with those reported elsewhere using other datasets. Further improvements in SST accuracy, perhaps to 0.2 K, may be expected with a more rigorous analysis of the data.

Atmospheric transmittance models and an analytical method to predict the optimum slope of an absorber plate, variously oriented at any latitude

Two new atmospheric transmittance models are proposed to calculate the diurnal profile of solar radiation intensity. These two, together with six other models proposed by earlier researchers, are used to obtain analytical formulae for the optimum tilt angle for a plane absorber plate at any latitude in either hemisphere. In all cases two sky models, namely the isotropic model and the anisotropic model proposed by Hay and Davies, are used. This gives a total of 16 different analytical formulae for the optimum tilt angle. The optimum slopes calculated from these formulae for Kabul, Afghanistan ( φ = 34.5°) for certain days of the year, when γ = 0 and γ = ±45°, are compared with the experimentally measured optimum slopes for those days. A simple empirical formula which is a function of latitude φ and the day of a year n is also proposed to calculate the optimum slope when γ = 0. For any given day of the year, the empirical formula reduces to a linear correlation between the optimum tilt and the latitude. The linear correlations for the monthly optimum slopes (corresponding to the Julian days) and latitude are given. The analysis is further extended to predict the optimum tilt angle, and optimum surface azimuth angle when W p is not equal to zero, and it is concluded that when W p ≠ 0, the optimum surface azimuth angle is also not equal to zero. The mean year optimum slope and the mean heating season slope for Gaborone, Botswana ( φ = −24.5°) are calculated. A formula to calculate sunset and sunrise hour angles when β ≠ 0 and γ ≠ 0 is obtained. The various symbols used are given in the Nomenclature.

Analysis of the fascode model and its H_2O continuum based on long-path atmospheric transmission measurements in the 45–115-μm region

An experimental study performed to evaluate the atmospheric transmission model FASCODE and its water vapor continuum [Clough, Kneizys, and Davies (CKD) model, Atmos. Res. 23, 229-241 (1989)] in the 850-2250-cm-(1) spectral region is presented. The analysis is based on a comparison between model calculations and transmission measurements carried out at the Defence Research Establishment Valcartier over a 5.7-km horizontal path for a wide range of ambient temperature (from -8.6 to 29.4°C) and humidity (from 1.16 to 14.2 g/m(3)) conditions. The agreement between measurements and calculations is good on the average. However, there are three specific spectral intervals where the differences cannot be explained by experimental errors. For summer conditions, it is shown that FASCODE overestimates the transmittance by approximately 3-6% (absolute terms) in the 850-950-cm(-1) region. For winter conditions, measurements are higher than calculations by as much as a factor of 2 at the edges of the 6.3µm absorption band of water vapor, namely near 1250-1380 cm(-1) and 1800-2000 cm(-1). The continuous nature of these differences is interpreted as anomalies that are due to the broadening coefficients of the water vapor continuum (CKD model). A set of coefficients is derived from experimental spectra and compared with coefficients from the CKD model. The results suggest that first the self-broadening coefficients at high temperature, C¯(s)(ν, 296), need to be increased by 10-16% near 850-950 cm(-1) and second the foreign broadening coefficients, C¯(ν), need to be decreased by approximately a factor of 2 near 1250-1380 cm-(1) and 1800-2000 cm(-1) to recover a good model-measurement agreement in these three spectral intervals. A modified continuum (based on coefficients derived from transmittances) has been implemented in FASCODE and used to analyze emission spectra from the High-Resolution Interferometer Sounder instrument. The modified continuum reduces the discrepancy by almost a factor of 5 near 1350 cm(-1).

Water vapor correction method for advanced very high resolution radiometer data

Current sea surface temperature (SST) retrieval algorithms consist of linear equations involving brightness temperatures (of the infrared advanced very high resolution radiometer (AVHRR) channels) multiplied by fixed coefficients ai. These coefficients are derived from atmospheric transmission models and radiosonde profiles or from regression against ocean buoy measurements. Errors in SST retrieval occur when the actual atmospheric conditions differ from those assumed when the ai were originally computed; it is generally agreed that water vapor variations are the primary source of this error. We propose a novel radiance‐based SST algorithm which compensates for temporal fluctuations in the water vapor profile. The method does not use any form of regression; rather, we utilize a single radiosonde seed combined with full radiative transfer theory applied to an atmospheric model. As well as improving the accuracy of the remotely sensed SST, this approach allows us to estimate the total water vapor content for a given AVHRR pixel. We compare the performance of this technique with two standard SST models, using buoy data from the west coast of Tasmania as surface truth. Preliminary results from a sample of 33 NOAA 9 overpasses gathered over a 10‐month period in 1987 are most encouraging: the dynamic water vapor (DWV) algorithm outperforms the standard SST methods in terms of retrieval accuracy, underestimating the buoy temperature by an average of 0.22 K. This result demonstrates that the method has merit and suggests that the total water vapor content returned by the DWV algorithm is probably a reasonable first estimate.

Expected radiometric and spectral significance of MOMS-02 data for vegetation mapping: Calculations based on system parameters applied on spectral field measurements

The MOMS-02 instrument is designed as a “pushbroom” scanning device for spectral data acquisition in the visible (VIS) and near infrared (NIR) range (Band 1: 449–511 nm, Band 2: 532–577 nm, Band 3: 645–677 nm, Band 4: 772–815 nm, IFOV: 13.5 m × 13.5 m), combined with along-track, simultaneously acquired forward, backward Bands 6/7: 524–763 nm (IFOV: 13.5 m × 13.5 m) and nadir Band 5: 512–765 nm (IFOV: 4.5 × 4.5 m) looking stereo capability. The location of the MS bands were done with respect to the suitibility of these spectral regions to analyze vegetation type and status. The calculations of the spectral significance for thematic data evaluation is based on spectroscopic field measurements of crops, balanced with an atmospheric transmission model (Lowtran 7), and the system parameters. The simulation results lead to the conclusion that the spatial, spectral, and radiometric resolution, combined with the on-track stereo capability of the MOMS-02 sensor, will result in substantial improvements in vegetation classification compared to concurrent remote sensing instruments like Landsat TM and Spot HRV.

Comparison of radiances observed from satellite and aircraft with calculations by using two atmospheric transmittance models

An evaluation of two different atmospheric transmittance models is performed by using radiance data from the high-resolution infraRed Sounder (HIRS) instrument onboard the National Oceanic and Atmospheric Administration's NOAA-9 satellite and the airborne high-resolution interferometer sounder (HIS) instrument. Synthetic radiances have been derived from collocated radiosondes by using the television infrared observation satellite (TIROS) operational vertical sounder (TOVS) operational transmittance model and the fast atmospheric signature code (FASCOD2) line-by-line transmittance model for comparison with the two independent instrument observations. Radiance observations in various spectral channels from the HIRS and HIS instruments along with the synthetic radiances derived from the FASCOD2 and operational TOVS transmittance models are used for the performance evaluation. The results of the comparison reveal a significant discrepancy between 707 and 717 cm(-l) in the radiance calculation for both models. Exce llent agreement is observed between observation and calculation for the lower tropospheric long-wave temperature sounding channels. Serious problems are noted with the modeling of water vapor in the operational TOVS transmittance model. In addition, poor performance by FASCOD2 is revealed for the short-wavelength N(2)O-CO(2) HIRS spectral channels. In general the operational TOVS transmittance model is found to be only slightly inferior to the FASCOD2 model. Regarding the performance of the instruments, observations from the NOAA-9 HIRS and the aircraft HIS are comparable in terms of their agreement with theoretical computations.

An extension to the split-window technique giving improved atmospheric correction and total water vapour

Abstract Satellite remote sensing of ocean and lake surface temperature is of considerable importance for climate research, but the main problem with thermal infrared measurements is the determination of the atmospheric correction. We present a novel extension to the split-window method to retrieve the total atmospheric transmittance, and demonstrate the use of this information in improved algorithms for sea surface temperature (SST) and total column content of water vapour. Simulation studies, using radiosonde profiles and an atmospheric transmission model, show that the new SST algorithm has an intrinsic precision of ∼01 degKr.m.s. (representing an improvement by a factor of 2-4 over conventional algorithms) and that atmospheric water vapour content can be retrieved with an accuracy of ∼ 5 per cent independent of its absolute value. We also discuss potential sources of error and future applications of the technique.

High-latitude moisture structure determined from HIRS water vapour imagery

Abstract An atmospheric transmittance model has been applied to Antarctic radiosonde ascents in order to examine the high-latitude normalized weighting functions (NWF) of the water vapour channels on the High Resolution Infrared Radiation Sounder (HIRS). In the Antarctic coastal region the 6·7 μ radiances are shown to be largely atmospheric in origin and imagery created from these data can provide useful diagnostic information on tropospheric water vapour, even during the winter months. The 7·3μm data contain a large surface contribution and have less value as a diagnostic tool. An example is shown of HIRS water vapour imagery of the Antarctic at a time when a cold front was descending from the Antarctic Plateau to the coastal area. The imagery clearly showed the dry air behind the front and provided information that was not available with any other imagery channel. A second example of a North Atlantic polar low shows the contaminating effect of cloud when deep atmospheric systems are being examined.

Diode laser spectroscopy of the ν3 vibration of the HO 2 radical

The infrared spectrum of the ν 3 vibration of the hydroperoxyl radical has been examined using a tunable lead salt diode laser. Fifty-nine transitions were observed in the wavenumber interval between 1040 and 1120 cm −1. This work extends the earlier measurements of Johns, McKellar, and Riggin to significantly higher rotational states and results in an improved set of spectroscopic constants which should predict all atmospherically relevant HO 2 transition frequencies with a precision approaching a thousandth of a cm −1. The incorporation of these line positions in atmospheric transmission models using HITRAN or ATMOS data bases will be useful in remote sensing measurements of HO 2.

Transmission window near 2400 cm^−1: an experimental and modeling study

The absorption in the 2400-cm(-1) region is dominated by continuum absorptions from carbon dioxide and nitrogen, and it is important to be able to describe the temperature dependence of these two continua. A series of measurements of atmospheric transmission over a 5.7-km range were carried out during the summer and winter seasons of 1988. Following a brief description of the experiments a selected number of spectra, covering the temperature range from -21.4 to 30.3 degrees C, are presented. These measurements are compared with predictions by the atmospheric transmission model FASCOD2 and modified versions using models of the carbon dioxide and the nitrogen continua derived from experimental laboratory measurements. Finally, an improved temperature-dependent model for the nitrogen continuum is derived from atmospheric transmission measurements. The model covers the range of temperatures found in the troposphere and differs significantly from laboratory-based measurements.

An atmospheric transmittance model for the calculation of the clear sky beam, diffuse and global photosynthetically active radiation

A physical model is proposed for the estimation of the photosynthetically-active-radiation (PAR) components. Transmittances corresponding to ozone absorption, Rayleigh scattering and aerosol extinction are parameterized after numerical integration of spectral formulae. The parameters used in the model are station pressure, ozone amount, Angström turbidity coefficients, single-scattering albedo, ground albedo and solar elevation. Knowledge of these parameters is necessary if the beam and diffuse PAR are to be estimated accurately; while the global PAR is essentially a function of solar elevation. Model estimates compare well with predictions obtained with rigorous spectral codes (LOWTRAN and BRITE) and to measurements in Uccle, Belgium. It may be used in any area where the turbidity climatology is sufficiently known, to compute instantaneous, hourly or daily PAR clear sky values, even if simultaneous solar radiation measurements are not available.

Interpretation of Mauna Loa atmospheric transmission relative to aerosols, using photometric precipitable water amouts

Precipitable water measurements made coincident in time and space with direct broadband solar irradiance measurements are used in conjunction with an atmospheric transmission model to derive a parameter whose major dependence is on total aerosol extinction. Irradiance measurements are used to calculate an atmospheric transmission factor (ATF) that is independent of the instrument calibration and the extraterrestrial solar constant. The dependency of the ATF on precipitable water is determined using LOWTRAN5, an atmospheric transmission model with high spectral resolution. Precipitable water measurements are then used to adjust the measured ATF to correspond to an ATF value obtained for a constant precipitable water amount. The remaining variability in the adjusted ATF is due mostly to aerosol extinction. The technique is applied to a 6-year period (1978–1983) for clear-sky mornings at Mauna Loa, Hawaii (MLO). MLO ATF aerosol residuals are compared with independently measured monochromatic aerosol optical depth. Results show that the ATF aerosol residual is nearly equal to the 500 nm aerosol optical depth prior to the eruption of E1 Chichon, at which time a nonlinear time-dependent relationship between the two quantities is evident. ATF aerosol residuals reflect the spectrally integrated aerosol influence on transmission and, therefore, could indicate better than monochromatic optical depth the radiation balance perturbations due to aerosols. The 6-year precipitable water record for MLO, determined from a dual-channel sunphotometer, has a mean value of 0.3 cm. An annual cycle in precipitable water is evident, as is a 4-month 5-standard-deviation “drought” from December 1982 through March 1983.

Satellite multichannel infrared measurements of sea surface temperature of the N.E. Atlantic Ocean using AVHRR/2

AbstractThe accuracy with which sea surface temperature (s.s.t.) can be measured using satellite infrared radiometers is limited primarily by uncertainties in the correction for atmospheric effects upon the measured radiance. This paper reports an investigation of the accuracy with which this correction can be made over the north‐eastern and tropical Atlantic Ocean using data at 3.7, 11 and 12 μm wavelengths from the Advanced Very High Resolution Radiometer (AVHRR/2) on the NOAA‐7 satellite, and the results are used as a test of atmospheric transmittance models. Simulations of atmospheric transmittances based on line‐by‐line calculations, using published line listings and experimental data on the water vapour ‘continuum’ absorption, provide regression relationships which permit the s.s.t. to be calculated from the brightness temperatures measured in each channel. New algorithms for both the ‘split window’ (11 and 12 μm) and the ‘triple window’ (3.7, 11 and 12 μm) have been derived for a range of airmasses from 1 to 2 to enable the s.s.t. to be retrieved from a 2280 km wide swath centred on the sub‐satellite track. To test the validity of the simulations, the s.s.t. values derived from the satellite measurements were compared with near‐coincident in situ measurements from oceanographic research ships. The absence of significant bias (˜0.1 K) in the comparison constitutes an important experimental verification of the atmospheric transmittance model used in the simulations, and the r.m.s. difference between ship and satellite values indicates that daytime measurement of s.s.t. using the ‘split‐window’ can be made to an accuracy of about ±0.6 K, in the eastern North Atlantic. Some possible sources of this uncertainty are discussed.

High Resolution Atmospheric Spectroscopy Using A Diode Laser Heterodyne Spectrometer

Atmospheric'spectroscopy, employing the sun as a source, has been routinely used for a number of years as a means for identification and quantitative measurements of atmospheric species. Recent improvements in Pb-salt semiconductor laser technology when combined with infrared heterodyne technology provide the capability of obtaining solar spectra with sub-Doppler spectral resolution. In this paper we discuss ultrahigh resolution (0.007 cm-1) atmospheric solar absorption spectra that have been obtained from a tunable infrared heterodyne radiometer. The radiometer was developed for ground based observations in the 8 to 12µm region, and tunability is achieved through the use of Pb-salt semiconductor laser local oscillators (L0). Spectra have been obtained in a piece-wise fashion from 9.1 to 11.1 um using laser emission modes that exhibit characteristics suitable for LO operation. Spectra showing absorption features of HNO3, 03, CO2, and H2O are presented along with comparisons of experimental and synthetic spectra calculated using a line-by-line atmospheric transmission model.

Absolute measurements of the atmospheric transparency at short millimetre wavelengths

Spectral measurements of the atmospheric opacity in the 3–13 cm −1 region have been made using a phase-modulated Michelson interferometer. The data are calibrated using a black body source to give absolute measurements. An independent measurement of the total atmospheric water vapour concentration is obtained from a near-infrared hygrometer which is itself calibrated against simultaneous radio sonde data. A theoretical atmospheric transmission model is used to compare the two sets of data. We have found a large discrepancy between our theory and the experimental data amounting to more absorption than predicted. This anomalous absorption shows no spectral features and scales with water vapour concentration. There is no indication of any water vapour dimer lines or of any other minor species features except those due to ozone.

Atmospheric transmittance of an absorbing gas 3: A computationally fast and accurate transmittance model for absorbing gases with variable mixing ratios

Atmospheric transmittance models for absorbing gases with constant mixing ratios were described in the two preceding papers of this series. In this paper a method for calculating atmospheric transmittances for absorbing gases with variable mixing ratios is described. Because the model uses only arithmetic operations, it is computationally fast as well as accurate. Details of the computational algorithm are given, including the calculation of the expansion coefficients. In a test of eleven independent profiles, the resulting transmittances agreed with line-by-line calculations in an rms sense to within 0.0090 in the worst case and to within 0.0018 in all other cases. This paper also includes a discussion for computing transmittances when several gases absorb in the same spectral interval. These three papers provide a complete treatment for modeling transmittances in inhomogeneous atmospheres.

Infrared continuum absorption by atmospheric water vapor in the 8–12-μm window

We have carried out a detailed analysis of several long pathlength transmission measurements in the 8-12-microm atmospheric window in order to determine the extinction coefficient due to the water vapor continuum. Our results indicate that three modifications to the current LOWTRAN atmospheric transmission model are required. The first two corrections involve an improved fit to the pure water vapor continuum absorption together with the elimination of the atmospheric broadened continuum term. Finally, and most critically, a strong measured temperature dependence must be included in the water vapor continuum absorption coefficient. For pathlengths ranging from 10 km to 50 km, failure to incorporate these corrections can lead to errors in the computed transmission ranging from factors of 2 to more than 10,000.