Inhomogeneity effects on cloud shortwave absorption measurements: Two‐aircraft simulations

Abstract

Abstract. We set up an Observation System Simulation in order to quantify the idiosyncratic uncertainties caused by cloud inhomogeneity in two-aircraft measurements. An independently validated fractal model is used to simulate the horizontal variability of optical depth for boundary layer stratus, with and without gaps. A spectral band between 0.9 and 1.0 gm, with a combination of strong and weak water vapor absorption and negligible weak liquid water absorption, is selected for a detailed study of column absorption. Measurements of upwelling and downwelling radiative fluxes are simulated by the Monte Carlo method at two altitudes in a realistic cloudy atmosphere. We consider three methods of estimating column absorption: cloud forcing ratio, reflectance (R) versus transmittance (T) regression, and Ackerman-Cox correction. Emphasis is on the second method, R versus T, which is shown to be biased in the direction of enhanced absorption as long as the R and T data points are affected by horizontal fluxes. In other words, to reduce the bias to an acceptable level, (R, T) measurements must be at a scale well within the regime where the independent pixel approximation is accurate. Under completely cloudy skies, radiative smoothing processes dominate the bias in the absorption estimate at small scales; hence spatial averages over at least 30 times the radiative smoothing scale, which is commensurate with cloud thickness, are required. If there are substantial gaps in the cloud cover, geometrical shadowing and cloud side-illumination effects overwhelm the radiative smoothing. As a result, the bias in R versus T methodology is worsened, and aircraft positioning becomes more critical. We also examine the effect of Sun angle and instrumental parameters such as vertical separation and horizontal offset between aircraft. The Ackerman-Cox correction offers an alternative to spatial averaging; a modification of this intrinsically small-scale approach that can virtually eliminate horizontal flux contributions in the data is presented.

On the Solar Radiation Budget and the Cloud Absorption Anomaly Debate

“…Use of monthly-mean fluxes averaged over the GCM grids may be sufficient in this regard . Another closely related debate is concerned with analysis of measurements made by two aircraft flying in stack below and above a cloud (Marshak et al 1997). Horizontal linkage of photons has also been blamed for apparent CAA in studies using aircraft data.…”

Investigation of the effect of surface heterogeneity and topography on the radiation environment of Palmer Station, Antarctica, with a hybrid 3‐D radiative transfer model

“…That is, the horizontal position of the photon remains fixed immediately before and after the cloud interaction. For optically thick clouds the horizontal displacement a photon undergoes in passing through the cloud layer is roughly approximated by the vertical thickness of the cloud [Marshak et al, 1997]. Hence disabling horizontal transport is equivalent to assuming that the cloud layer opacity is compressed to a scattering layer of zero thickness.…”

Atmospheric Radiation Measurement Program Science Plan. Current Status and Future Directions of the ARM Science Program

“…In response to these studies, ARM supported a special IOP -the ARM Enhanced Shortwave Experiment (ARESE-I) -in 1995, in the SGP vicinity, to deal exclusively with this problem. Valero et al (1997) and Zender et al (1997) found enhanced absorption in ARESE-I, but these results and those of the previous studies were questioned by many different investigators citing the small number of observations for extensive cloud cover, the methodology used to analyze the measurements (Stephens 1996;Barker and Li 1997;Cess et al 1997;Zhang et al 1998), the effects of 3-D radiative transfer and sampling (Marshak et al 1997;Marshak et al 1999), the potential problems of aerosols (Li and Trishchenko, 2001), and potential errors in the observation (Trishchenko and Li 1998;Rapp 2001). …”