A Flexible and Efficient Radiation Scheme for the ECMWF Model

Abstract

This paper describes a new radiation scheme ecRad for use both in the model of the European Centre for Medium-Range Weather Forecasts (ECMWF), and off-line for noncommercial research. Its modular structure allows the spectral resolution, the description of cloud and aerosol optical properties, and the solver, to be changed independently. The available solvers include the Monte Carlo Independent Column Approximation (McICA), Tripleclouds, and the Speedy Algorithm for Radiative Transfer through Cloud Sides (SPARTACUS), the latter which makes ECMWF the first global model capable of representing the 3-D radiative effects of clouds. The new implementation of the operational McICA solver produces less noise in atmospheric heating rates, and is 41% faster, which can yield indirect forecast skill improvements via calling the radiation scheme more frequently. We demonstrate how longwave scattering may be implemented for clouds but not aerosols, which is only 4% more computationally costly overall than neglecting longwave scattering and yields further modest forecast improvements. It is also shown how a sequence of radiation changes in the last few years has led to a substantial reduction in stratospheric temperature biases. Plain Language Summary Solar and thermal infrared radiation provide the energy that drivesweather systems and ultimately controls the Earth's climate. Accurately simulating these energy flows is therefore a crucial part of the computer models used for weather and climate prediction. This paper describes a flexible and efficient new software package, ecRad, for computing radiation exchange. It became operational in the forecast model of the European Centre for Medium-Range Weather Forecasts (ECMWF) in July 2017, and is 41% computationally faster than the previous package. This offers the possibility to update the radiation fields in the model simulations more frequently for the same overall computational cost, which we show in turn can improve the skill of weather forecasts. A unique feature for a radiation package of this kind is the ability to simulate radiation flows through the sides of clouds, not just through the base and top, making it well suited as a tool for research into atmospheric radiation exchange.

Description and evaluation of the tropospheric aerosol scheme in the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS-AER, cycle 45R1)

“…In this section, a short evaluation of the simulated AOD against observations from the Aerosol Robotic Network (AERONET; Holben et al, 1998) is shown, as well as of PM 2.5 and PM 10 against observations from the AirNow and AirBase networks in the United States and Europe, respectively. The simulations evaluated here consist of 24 h cycling forecasts with meteorological initial conditions provided by an analysis and aerosol and chemical initial conditions provided by the previous forecast.…”

“…Section 7 presents simulation results and budgets; Sect. 8 is dedicated to a preliminary evaluation of simulations against aerosol optical depth (AOD) observations from the AERONET network (Holben et al, 1998) and against European and North American PM observations.…”

Balancing Accuracy, Efficiency, and Flexibility in Radiation Calculations for Dynamical Models

“…RTE and RRTMGP operate on multiple columns at a time to exploit this parallelism. The column dimension is innermost; despite good reasons for having the spectral dimension vary fastest (Hogan & Bozzo, 2018), this choice allows user control over vector length and can be easily adapted to different architectures. RTE and RRTMGP are agnostic to the ordering of the vertical axis.…”

Overview of the Meso-NH model version 5.4 and its applications

“…The latest radiation code of ECMWF, ecRad (Hogan and Bozzo, 2016), was implemented in Meso-NH in 2017. This code is highly modular, which allows the user to conveniently choose between multiple options.…”