TY - JOUR
T1 - Observational constraints on non-Lorentzian continuum effects in the near-infrared solar spectrum using ARM ARESE data
AU - Vogelmann, A. M.
AU - Ramanathan, V.
AU - Conant, W. C.
AU - Hunter, W. E.
N1 - Funding Information:
We thank the ARM SGP and ARM ARESE teams for access to and interpretation of the data. We thank J. Michalsky and J. Barnard for discussions about the MFRSR, J. Liljegren for discussions about the MWR, and W. Ridgway for providing the original versions of the line-by-line and correlated- k algorithms used. We are grateful to S. A. Clough and Eli Mlawer for their critical feedback on this work and discussions about the CKD continuum. This work was supported by the Department of Energy Atmospheric Radiation Measurement program grant DE-FG03-91ER61198. This is contribution #172 for the Center for Clouds, Chemistry and Climate.
PY - 1998/8
Y1 - 1998/8
N2 - Uncertainties exist in the magnitude of the water vapor continuum at solar wavelengths and many models do not include a solar continuum. We assess whether the neglect of the continuum in some models could explain a significant amount of the excess solar absorption found by several recent studies, in which the observed atmospheric solar absorption is significantly greater than that modeled. Towards this goal, we constrain the magnitude of the near-infrared water vapor continuum absorption using observations from the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE). Narrowband irradiances measured by two independent Multifilter Rotating Shadowband Radiometers (MFRSRs) are used to infer the clear-sky transmission by water vapor in the 0.94 μm band. Over 16000 such observations are compared to non-continuum (i.e., a pure Lorentzian model) and continuum calculations using a correlated-k distribution model, which shows excellent agreement with a line-by-line model and uses coincident measurements of the pressure, temperature and water vapor profiles. Continuum calculations use the CKD super-Lorentzian formulation. The data suggest the need for a far wing continuum in the 0.94 μm band with an absorption that falls between that computed for pure Lorentzian lines and the CKD continuum. A sensitivity analysis presents the effects of uncertainties in parameters affecting the calculations, including those in the line parameters, continuum magnitude and atmospheric state. Upper estimates for the absorption of broadband solar radiation by the continuum, beyond that computed for pure Lorentzian far wings, are only 1 to 2 W m-2 for a diurnal (24 h) average, and 4 to 6 W m-2 for local noon. Thus, uncertainties in or the neglect of the water vapor continuum at solar wavelengths are not likely explanations for excess absorption of the order of 15 to 30 W m-2 (diurnal average).
AB - Uncertainties exist in the magnitude of the water vapor continuum at solar wavelengths and many models do not include a solar continuum. We assess whether the neglect of the continuum in some models could explain a significant amount of the excess solar absorption found by several recent studies, in which the observed atmospheric solar absorption is significantly greater than that modeled. Towards this goal, we constrain the magnitude of the near-infrared water vapor continuum absorption using observations from the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE). Narrowband irradiances measured by two independent Multifilter Rotating Shadowband Radiometers (MFRSRs) are used to infer the clear-sky transmission by water vapor in the 0.94 μm band. Over 16000 such observations are compared to non-continuum (i.e., a pure Lorentzian model) and continuum calculations using a correlated-k distribution model, which shows excellent agreement with a line-by-line model and uses coincident measurements of the pressure, temperature and water vapor profiles. Continuum calculations use the CKD super-Lorentzian formulation. The data suggest the need for a far wing continuum in the 0.94 μm band with an absorption that falls between that computed for pure Lorentzian lines and the CKD continuum. A sensitivity analysis presents the effects of uncertainties in parameters affecting the calculations, including those in the line parameters, continuum magnitude and atmospheric state. Upper estimates for the absorption of broadband solar radiation by the continuum, beyond that computed for pure Lorentzian far wings, are only 1 to 2 W m-2 for a diurnal (24 h) average, and 4 to 6 W m-2 for local noon. Thus, uncertainties in or the neglect of the water vapor continuum at solar wavelengths are not likely explanations for excess absorption of the order of 15 to 30 W m-2 (diurnal average).
UR - http://www.scopus.com/inward/record.url?scp=0031713041&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0031713041&partnerID=8YFLogxK
U2 - 10.1016/S0022-4073(97)00151-9
DO - 10.1016/S0022-4073(97)00151-9
M3 - Article
AN - SCOPUS:0031713041
SN - 0022-4073
VL - 60
SP - 231
EP - 246
JO - Journal of Quantitative Spectroscopy and Radiative Transfer
JF - Journal of Quantitative Spectroscopy and Radiative Transfer
IS - 2
ER -