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  • Author or Editor: Bernhard Mayer x
  • Journal of Applied Meteorology and Climatology x
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Kathrin Wapler
and
Bernhard Mayer

Abstract

Cloud-resolving models—in particular, large-eddy simulation (LES) models—are important tools to improve the understanding of cloud–radiation interactions. A method is presented for accurate, yet fast, three-dimensional calculation of surface shortwave irradiance within an LES model using the tilted independent column approximation with smoothing of the diffuse irradiance. The algorithm calculates a tilted optical thickness for each surface pixel that is then used as input to a one-dimensional radiative transfer code. In a sensitivity analysis, it is shown that this calculation can even be replaced by a simple precalculated lookup table that tabulates surface irradiance as a function of only solar zenith angle and cloud optical thickness. Because the vertical variability of the cloud is of little relevance for the surface irradiance, this approximation introduces little extra uncertainty. In a final step, surface irradiance is smoothed to account for horizontal photon transport between individual columns. The algorithm has been optimized for parallelization, which enhances its applicability in LES models. In this implementation, the total computational time of the LES model increased by only 3% relative to the reference run without radiation. Comparisons between the fast approximation and detailed three-dimensional radiative transfer calculations showed very good agreement for different cloud conditions and several solar zenith and azimuth angles, with a root-mean-square difference of 6%.

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Ulrike Wissmeier
,
Robert Buras
, and
Bernhard Mayer

Abstract

The resolution of numerical weather prediction models is constantly increasing, making it necessary to consider three-dimensional radiative transfer effects such as cloud shadows cast into neighboring grid cells and thus affecting radiative heating. For that purpose, fast approximations are needed since three-dimensional radiative transfer solvers are computationally far too expensive. For the solar spectral range, different approaches of how to consider three-dimensional effects were presented in the past—in particular, the tilted independent column approximation (TICA), which aims at improving the calculation of the direct radiation, and the nonlocal tilted independent column approximation (NTICA), which is used to additionally correct the diffuse radiation. Here a new version of NTICA is presented that—in contrast to earlier approaches—is applicable for a variety of cloud scenes and grid resolutions and for arbitrary solar zenith angles. This new parameterization for the diffuse irradiance is then applied to the two different TICA approaches and the results are compared with a full 3D Monte Carlo calculation. It is shown that both approaches strongly improve the calculation of radiation fluxes if the new parameterization for the diffuse irradiance—what the authors call “parameterized NTICA (paNTICA)”—is applied. It is found that the method in which TICA is only applied to direct radiation yields the better results. The studies show that consideration of three-dimensional effects is inevitable if higher model resolutions are used in the future. This paper proposes ways to consider these effects and, thus, to substantially reduce the errors made with one-dimensional radiative transfer solvers.

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Sebastian W. Hoch
,
C. David Whiteman
, and
Bernhard Mayer

Abstract

The Monte Carlo code for the physically correct tracing of photons in cloudy atmospheres (MYSTIC) three-dimensional radiative transfer model was used in a parametric study to determine the strength of longwave radiative heating and cooling in atmospheres enclosed in idealized valleys and basins. The parameters investigated included valley or basin shape, width, and near-surface temperature contrasts. These parameters were varied for three different representative atmospheric temperature profiles for different times of day. As a result of counterradiation from surrounding terrain, nighttime longwave radiative cooling in topographic depressions was generally weaker than over flat terrain. In the center of basins or valleys with widths exceeding 2 km, cooling rates quickly approached those over flat terrain, whereas the cooling averaged over the entire depression volume was still greatly reduced. Valley or basin shape had less influence on cooling rates than did valley width. Strong temperature gradients near the surface associated with nighttime inversion and daytime superadiabatic layers over the slopes significantly increased longwave radiative cooling and heating rates. Local rates of longwave radiative heating ranged between −30 (i.e., cooling) and 90 K day−1. The effects of the near-surface temperature gradients extended tens of meters into the overlying atmospheres. In small basins, the strong influence of nocturnal near-surface temperature inversions could lead to cooling rates exceeding those over flat plains. To investigate the relative role of longwave radiative cooling on total nighttime cooling in a basin, simulations were conducted for Arizona’s Meteor Crater using observed atmospheric profiles and realistic topography. Longwave radiative cooling accounted for nearly 30% of the total nighttime cooling observed in the Meteor Crater during a calm October night.

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