Single interval longwave radiation scheme based on the net exchanged rate decomposition with bracketing

Abstract

The main obstacle to efficient calculation of longwave radiative transfer is the existence of multiple radiative sources, each with its own emission spectrum. The work presented here overcomes this problem by combining the full spectrum broadband approach with the net exchanged rate decomposition. The idea is worked out to suit the needs of numerical weather prediction, where the most costly contribution representing the sum of internal exchanges is interpolated between cheap minimum and maximum estimates, while exchange with the surface and dominant cooling to space contributions are calculated accurately. The broad‐band approach must address the additional problems related to spectral integration and many ideas developed previously for the solar spectrum are reused. Specific issues appear, the dependence of broadband gaseous transmissions on the temperature of the emitting body being the most important one. The thermal spectrum also brings some simplifications—aerosols, clouds and the Earth's surface can safely be treated as grey bodies. The optical saturation of gaseous absorption remains the main complication and non‐random spectral overlaps between gases become much more significant than in the solar spectrum. The broadband character of the proposed scheme enables the use of an unreduced spatial resolution with an intermittent update of gaseous transmissions and interpolation weights, thus ensuring a full response of longwave radiation to rapidly varying cloudiness and temperature fields. This is in contrast to the mainstream strategy, where very accurate and expensive radiative transfer calculations are performed infrequently, often with reduced spatial resolution. The approach proposed here provides a much better balance between errors coming from the radiation scheme itself and from the intermittency strategy. The key achievement, ensuring a good scalability of the scheme, is a computational cost essentially linear in the number of layers, with straightforward inclusion of scattering as an additional bonus.

The HARMONIE–AROME Model Configuration in the ALADIN–HIRLAM NWP System

“…The ACRANEB2 radiation scheme from the ALARO model Geleyn et al 2017) and the HLRADIA radiation scheme from the HIRLAM model (Savijärvi 1990; see also Nielsen et al 2014) are now available for testing in HARMONIE-AROME; we will investigate whether the ability to have fast interactions between clouds and radiation and the surface and radiation are of greater importance for model performance than accounting for the spectral details of clear-sky radiation. Also, it will be investigated whether these radiation schemes can be used in individual ensemble members for HARMONIE-AROME ensemble simulations.…”

“…Because of computational constraints the full radiation calculations are currently performed every 15 min. The more affordable single-band radiation schemes from ALARO physics (ACRANEB2; Masȇk et al 2016;Geleyn et al 2017) and HIRLAM (HLRADIA ;Savijärvi 1990;Wyser et al 1999), which can be run at each time step, have also been implemented in HARMONIE-AROME for experimentation purposes. Hereafter, unless stated otherwise, when we refer to the SW and LW radiation schemes the default parameterizations are implied.…”

The ALADIN System and its canonical model configurations AROME CY41T1 and ALARO CY40T1

“…The second version, called ACRANEB2 (Mašek et al, 2016;Geleyn et al, 2017), was developed with the goal to increase the accuracy of gaseous transmissions, cloud optical properties and the NER technique, while still keeping the full cloud-radiation interaction. Several spectrally unresolved effects had to be parameterized.…”

“…The performance of such an inclusion has been tested by Hamdi et al (2014) in cycle CY36 of the ALADIN System. Additionally the interfaces to the radiation scheme have been cleaned and the ACRANEB2 radiation scheme (Mašek et al, 2016;Geleyn et al, 2017) of the ALARO configurations can be called from the AROME physics package, relying on the common physics-dynamics interface INTFLEX.…”

27 Years of Regional Cooperation for Limited Area Modelling in Central Europe

“…To safely afford the long time steps allowed by the ALADIN dynamics, Geleyn et al (2008) implemented a statistical approach to the sedimentation of precipitation.…”