AER Inc., Report, January 2001

Analysis of the AERI-LBLRTM infrared radiative flux differences at SGP during 1997-1998 of the ARM program: The role of aerosols

C. Andronache

AER Inc., Lexington, Massachusetts, USA

Abstract. Data of infrared (IR) radiance measured with the Atmospheric Emitted Radiance Interferometer (AERI) during clear-sky conditions at the Southern Great Plains (SGP) site for the observation period 1997-1998 (Julian days 250-450), were compared with LBLRTM calculations. The overall agreement between AERI measurements and LBLRTM results is excellent, with errors generally less than 1% in the radiative flux residuals (the residual is defined as the difference between measured AERI flux and the calculated LBLRTM flux). However, these residuals are not randomly distributed around zero and generally they show systematic positive values. Possible causes of the discrepancies between measurements and model calculations are:

  1. errors in measurements and instrument problems;
  2. contamination of data by unresolved clouds, and
  3. the sampled atmosphere has significant aerosol that is not represented in the LBLRTM model.

The purpose of this work is to detect possible aerosol effects on the flux residuals. The presence of aerosols in the real atmosphere, particularly absorbing aerosols such as carbon based aerosols generated by combustion and biomass burning can enhance the IR radiation toward Earth's surface. In this study we estimate the magnitude of these effects for the observation period 1997-1998 at SGP. Based on available measurements at SGP during 1997-1998, and on model calculations, we found that:

  1. AERI downward radiative flux in the spectral interval 500-3000 cm-1 is generally larger than the LBLRTM by about 2.4 Wm-2 on average for the observation period 1997-1998.
  2. The correlations between flux residuals, dF, and pwv, as well as RH, indicate that possible problems with water measurements and water radiative properties in the model remain to be addressed. A MODTRAN calculation for a SGP case also shows a high sensitivity of dF to ambient dew point. (dF=F(AERI)-F(LBLRTM) is the difference in the downward IR radiative flux).
  3. The correlations between the residuals (dF) and the aerosol absorption coefficient indicated that part of the residual can be explained by the presence of absorbing atmospheric particles in the atmosphere. The magnitude of dF caused by these particles is generally less than 1 Wm-2.
  4. Calculations with MODTRAN indicate that for the visibility at SGP during the observation period 1997-1998, and for rural aerosol model characteristics, about 0.34 to 1.17 Wm-2 can be attributed to aerosols (the cases correspond to extreme conditions in terms of diagnosed visibility). Under the assumption of urban type of aerosol, the low visibility case shows a 1.46 Wm-2 effect caused by aerosols. Most of the cases have aerosol contributions below 1 Wm-2.

While the aerosol data from SGP does not justify all the bias in the residuals, dF, between AERI measurements and LBLRTM calculations, these results show that aerosols can explain a significant part of dF, and that future inclusion of aerosols in radiative models is important especially for clear-sky polluted regions.

Related links:
Atmospheric Radiation Measurement (ARM) Program
Southern Great Plains (SGP) Site
Atmospheric Emitted Radiance Interferometer (AERI)