Aerosol feedback mechanisms

Offline way of integration with reading ACTM output files by HIRLAM can also improve the NWP if aerosol feedback mechanisms are incorporated into HIRLAM. However, this way could be too expensive if high-resolution ACT forecast is available. For example, some tests can be done using the CAC, DERMA, EMEP, MATCH or SILAM models (considering feedbacks with Enviro-HIRLAM version). The OAS1S4 coupler software (Redler et ah, this volume) could be tested for this work. 

Possibility exists to consider feedback mechanisms, e.g. aerosol forcing... 

Feedback Mechanisms, Aerosol Forcing in Meso-meteorological Models... 

In this concern one of the important tasks is to develop a modelling instrument of coupled Atmospheric chemistry/Aerosol and Atmospheric Dynamics/Climate models for integrated studies, which is able to consider the feedback mechanisms, e.g. aerosol forcing (direct and indirect) on the meteorological processes and climate change (see Fig. 1.2). 

Improvement of NWP itself by implementation of ACTMs and aerosol/gases forcing/feedback mechanisms into HIRLAM (mostly by online integration, like in Enviro-HIRLAM) as well as by further development of the radiation and cloud microphysics modules to introduce feedback mechanisms. 

The Enviro-HIRLAM (see its description and validation in a separate paper by Korsholm et al. in this volume) is a fully online NWP-ACT integrated system. The following steps towards Enviro-HIRLAM are being incorporated or have already been achieved (1) nesting of models for high resolutions, (2) improved resolution of boundary and surface layer characteristics and structures, (3) urbanisation of the model, (4) improvement of advection schemes, (5) implementation of chemical mechanisms, (6) implementation of aerosol dynamics, (7) realisation of feedback mechanisms, (8) assimilation of monitoring data (ongoing). 

Implementation of Chemistry, Aerosols and Feedback Mechanisms into Online Coupled ACTM-HIRLAM/ HARMONIE... 

The path towards a coupled meso-scale HARMONIE-chemistry model is not clear yet. There is a possibility to use chemical components that have recently been included in AROME, but they have proved to be quite difficult to use. The Enviro-HIRLAM community is considering two different options for the development of a mesoscale coupled system either the present HARMONIE chemistry code is used and extended with the chemistry and aerosol modules and feedback mechanisms that are already included in Enviro-HIRLAM, or Enviro-HIRLAM can be extended with an updated non-hydrostatic core NH-HIRLAM from Estonia (Room et al. 2007). 

Implementation of feedback mechanisms, direct and indirect effects of aerosols... 

Feedback mechanisms are considered to be important for a coupling strategy. This is especially relevant for aerosols. Aerosol forcing mechanisms influence radiative and optical properties as well as cloud processes, leading for instance to changes of precipitation and circulation. 

Implementation of Feedback Mechanisms, Direct and Indirect Effects of Aerosols... 

It is necessary to highlight the effects of aerosols and other chemical species on meteorological parameters, which have many different pathways (direct, semi-direct, and indirect effects, etc.) and they have to be prioritised and considered in on-line coupled modelling systems. Sensitivity studies are needed to understand the relative importance of different feedback mechanisms for different species and conditions relevant to air quality and climate interactions. A concerted action to mobilise and coordinate research in this area is needed. 

As stated earlier the major reason for developing on-line systems is to take account of feedback mechanisms for accurate modelling of NWP/MM-ACTM and quantifying direct and indirect effects of aerosols. Several questions can be identified in this regard ... 

Unfortunately, the incorporation of feedback mechanisms into climate models is not easily done. Thus, we are sure that changes of the radiation balance (due e.g. to the rise of C02 level) produce modifications in the atmospheric circulation, but we do not know the nature of these modifications. We have also no doubt that variations in cloud cover (due, e.g., to anthropogenic alteration of the aerosol burden) can substantially influence the radiation balance, but we are not able to predict thedirection of this effect in a reliable way sinceclouds control the transfer of incoming shortwave radiation as well as of outgoing infrared radiation. 

Schematic representation of global iron connections linked through the state of the land surface, aerosol loading, marine productivity, and climate. Positive feedbacks are represented by solid arrowheads and negative feedbacks by open circles. Feedbacks of uncertain sign are represented by open arrowheads. Linking mechanisms are in italics. Taps represent mechanisms that modulate primary processes. Source-. From Jickells, T. D., et al. (2005). Science, 308, 67-71. (See companion website for color version.)...

For simulation of the aerosol (especially anthropogenic) effects more detailed microphysics in HIRLAM is needed, in the former version it is very difficult to consider all the aerosol indirect effects. The current STRACO (Soft TRAnsition condensation) scheme in HIRLAM (Sass 2002) needs modifications and gives a possibility of developing simpler indirect mechanisms. One of such feedback semi-empirical model was developed in Enviro-HIRLAM by Korsholm et al. (2008b). The new AROMA/HARMONIE cloud scheme (Pinty and Jabouille 1998 Caniaux et al. 1994) is more suitable for implementation of aerosol dynamics and indirect effects of aerosols (CCN) models, but will be more expensive computationally. So, the main focus of our collaboration in this field should be in the improvement of the cloud/microphysics and radiation schemes in HIRLAM. 

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