A Method for Continuous Estimation of Clear-Sky Downwelling Longwave Radiative Flux Developed Using ARM Surface Measurements.
We present a methodology for the continuous estimation of downwelling clear-sky longwave (LW) radiative flux based on analysis of surface irradiance, air temperature, and humidity measurements. While there is no exact means of determining the uncertainty associated with the clear-sky LW estimations, our analyses and comparison with detailed radiative transfer model calculations suggest that our estimations on average are no worse than model calculations that require temporally and spatially averaged input information. Our technique exhibits a high degree of repeatability for the downwelling LW cloud effect, with agreement at about the 3 Wm-2 level even including the results for a measurement system with a known calibration offset problem. Applying our technique, as well as that of Long and Ackerman (2000) for the shortwave (SW) radiative flux, to 15 years of data from the Atmospheric Radiation Measurement Program Southern Great Plains site shows that the maximum downwelling LW lags the SW maximum by about a month on yearly time scales, and by about 3 hours diurnally. The maximum all-sky and clear-sky SW and LW occurs during the summer months, with the greatest year-to-year clear-sky SW variability occurring in the fall, but the yearto-year clear- and all-sky LW variability is about the same in all seasons. The downwelling LW cloud effect is fairly constant in the aggregate across the seasons, but the greatest SW cloud effect occurs in the spring. Overall, the downwelling net cloud effect is dominated by the SW, with the greatest decrease in radiative energy reaching the surface due to clouds occurring in the spring (-64 Wm-2), and the smallest magnitude net cloud effect occurring during winter (-21 Wm-2).
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 946640
- Report Number(s):
- PNNL-SA-58933; KP1205010; TRN: US200903%%441
- Journal Information:
- Journal of Geophysical Research. D. (Atmospheres), 113:D18206, Vol. 113
- Country of Publication:
- United States
- Language:
- English
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