the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Mar 2025
The Component Summation Technique for Measuring Upwelling Longwave Irradiance in the Presence of an Obstruction
Abstract. The CERES Ocean Validation Experiment (COVE) was an instrument suite located at the Chesapeake Light Station approximately 25 kilometers east of Virginia Beach, Virginia (36.9° N, 75.7° W). COVE provided surface verification for the Clouds and the Earth’s Radiant Energy System (CERES) satellite measurements for 16 years. However, the large light station occupied approximately 15 % of the field of view of the upwelling longwave flux measurement (LW↑), so radiation from the structure artificially perturbed the measurements. Hence, we use data from multiple instruments that are not influenced by the structure to accurately obtain LW↑; we call this the longwave component summation technique. The instruments required for the component summation are an infrared radiation thermometer to measure sea surface temperature, a pyrgeometer to measure downwelling longwave irradiance, and an air temperature probe. We find a strong negative bias between the obstructed upwelling pyrgeometer measurements and the component summation LW↑ in the colder months, less so in the warmer months. The bias ranged from -6 % to +5 % over COVE from 2004–2013. These range of biases are larger than the Baseline Surface Radiation Network (BSRN) targeted uncertainties of 2 % or 3 W-m−2 (whichever is greatest), indicating that the component summation technique provides a significant correction to standard BSRN protocols when an obstruction is present. This work documents how we determine the component summation LW↑ irradiances and presents guidelines for how this method could be used at other locations.
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RC1: 'Comment on egusphere-2025-872', Anonymous Referee #1, 11 Apr 2025
Overall, the paper discusses a highly important alternative to determining longwave irradiance if an obstruction is present. This change will help improve data collection at environmental monitoring stations, both current and that may be in use in the future. I recommend this article for publication, following some minor revisions outlined below.
Line 28: Please remove the acronym definition of NASA from Line 32 to Line 28. This is the first place it is mentioned.
Equation 2: Is this a derived equation? Or created by the authors as a model for collected data? A reader would benefit from having some background on the established relationships and source of all equations, not just Equation 2.
Line 260—262, related to Figures 6 and 7: is the expectation that the LWcs is to be pretty consistent and not impacted by air temperatures? Mentioning that the adjusted data matches expectations (and why) could help emphasize the validity of the new measurement technique.
Line 280: How is FOV found/calculated? This explanation may help readers trying to apply this to their own situation.
Figure 8: The x-axis should have a lower-case F to match the variable in Equation 11.
Citation: https://doi.org/10.5194/egusphere-2025-872-RC1 -
CC1: 'Reply on RC1', Bryan E Fabbri, 21 Apr 2025
Hello, thank you for taking the time to review this paper, recommending the article for publication and for your suggestions. To respond to your suggestions:
- I will make the subtle change moving the NASA acronym from Line 32 to 28 and make the change to the Fig. 8 plot (having a lower case f to be consistent with Eq. 11).
- Eq. 2 is a hybrid. That is, the equation was taken from an existing equation for deriving upwelling LW radiation and then other variables were added to account for the emissivity of air and the air temperature. The existing equation part should have been included in the paper and the source in the bibliography, and this will be added. I will also review the other equations and determine any background information that may need to be added.
- Yes. The expectation for Fig. 6 and 7 (Line 260-262) is that LWcs is pretty consistent and not impacted by air temperature. New words can be added to emphasize this point.
- FOV (line 280) was initially determined using a grid system and later refined (or confirmed) using a software package called "ImageJ" to separate the water from the structure obstruction. This can be added to the bibliography and to the body of the article.
Citation: https://doi.org/10.5194/egusphere-2025-872-CC1
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CC1: 'Reply on RC1', Bryan E Fabbri, 21 Apr 2025
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RC2: 'Comment on egusphere-2025-872', Anonymous Referee #2, 30 Jun 2025
Review of “The Component Summation Technique for Measuring Upwelling Longwave Irradiance in the Presence of an Obstruction” by Fabbri et al.
The paper introduces a long-wave component-summation technique to obtain accurate upwelling long-wave (LW_up) irradiance when a support structure partially blocks a pyrgeometer’s field-of-view, a problem encountered at NASA’s Chesapeake Light Station (COVE) surface validation site. The method reconstructs unobstructed LW_up by combining (i) sea-surface emission from an infrared radiation thermometer, (ii) reflected downwelling LW from a standard pyrgeometer, and (iii) emission from the thin air layer between the water and the sensor. Applying the approach to ten years of COVE data (2004–2013) shows that conventional pyrgeometer readings suffer a systematic bias, largely outside the Baseline Surface Radiation Network (BSRN) target of ±2 %. The component-summed fluxes remove these seasonal biases and recover a long, homogeneous LW_up record suitable for Clouds and the Earth’s Radiant Energy System (CERES) satellite validation. Sensitivity tests indicate that if an obstruction occupies less than ~5 % of the sensor’s hemispherical view, residual errors remain below the BSRN threshold 99 % of the time, offering a clear siting guideline.
Overall, this manuscript is well organized and easy to follow. However, the topic is quite narrow and highly specialized; the manuscript reads more like a technical support note than a broadly scoped research article. Several technical details—particularly the justification of key assumptions and the sensitivity of the component terms—should be clarified and documented in greater depth. In light of these considerations, I recommend major revision. More specific questions and comments are provided below.
- Why is the pyrgeometer measurement even needed? Throughout the manuscript the IRT-derived upwelling long-wave irradiance (LW_IRT) is treated as the reference against which both the obstructed pyrgeometer and the component-summation reconstruction are evaluated. If the IRT alone already delivers an accurate, obstruction-free LW_up measurement, what additional scientific or operational value does the pyrgeometer provide? Please clarify in the Introduction or Discussion why a long-wave pyrgeometer record remains necessary (e.g., calibration traceability, stability for climate time series, complementary performance in cloudy or high-humidity conditions, logistical redundancy, cost, or broader site comparability). Addressing this point will better motivate the need for the component-summation technique.
- Clarification of the physical basis for the component-summation method (Eq. 2). Equation (2) is the pivotal relation in the manuscript, yet its right-hand side is presented in compact form and the physical meaning of each term is only briefly touched upon. I recommend rewriting Eq. (2) so that the three component terms are shown explicitly—ordered from largest to smallest contribution—and providing a short paragraph that explains the origin, units, and expected magnitude of each term (sea-surface emission, reflected downwelling LW, near-surface air-layer emission). Doing so will help readers grasp both the physical logic and the relative importance of the terms.
- Assumptions and limitations of Eq. (1). Eq. (1) is written as if it were generally valid, yet for a hemispheric pyrgeometer the effect of an obstruction depends not only on the fractional blockage f but also on its position within the field-of-view (FOV). For the same f, an obstruction near zenith (nadir-viewing for LW ↑) will bias the measurement far more than one near the horizon. Please (i) state the physical basis of Eq. (1), (ii) discuss whether the derivation assumes uniform angular radiance, and (iii) clarify how, or whether, blockage geometry is accounted for in the 10-year COVE application.
- Treatment of angular and spectral variation. Both Eq. (1) and Eq. (2) ignore the angular and spectral dependence of the LW radiation. But in reality, upwelling LW radiance varies with zenith angle and wavelength. While a gray, isotropic assumption may be defensible for this specific problem, the manuscript should (a) acknowledge the simplification, (b) justify its adequacy (e.g., by order-of-magnitude estimates or citing prior studies), and (c) discuss how angular or spectral departures from gray-Lambertian conditions might influence the stated ±2 % accuracy target
- Line 384: Plank’s constant should be Planck’s constant
Citation: https://doi.org/10.5194/egusphere-2025-872-RC2
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