UKCA Chemistry and Aerosol Tutorial 12

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What you will learn in this Tutorial

In this tutorial you will learn about how to quantify the radiative effects of aerosol simulated by GLOMAP-mode in UM-UKCA.

In the first task you will update your copy of the UKCA tutorial job to request radiative fluxes allowing the radiative flux perturbation (or effective radiative forcing) to be diagnosed based on difference in the fluxes between a pair of UM-UKCA jobs with some difference (e.g. pre-industrial and present-day emissions jobs).

The second task involves configuring a copy of the UKCA tutorial job to run in double-call configuration whereby the aerosol radiative effects can be diagnosed at each radiation timestep.

Task 12.1: Update your copy of the UKCA tutorial job to diagnose Top Of the Atmosphere (TOA) radiative fluxes

In this task you will add STASH requests for SW and LW outgoing radiative fluxes at the top of the atmosphere to enable the radiative forcing from a particular change to be diagnosed.

The user should note however that to illustrate the task we are adding these requests to the UKCA tutorial job which is just a 1-day simulation.

One would need to average the flux-difference between the pair of simulations over an appropriate timescale (e.g. multi-annual monthly-means) or run with nudging to re-analysed meteorology, in order to diagnose an effective radiative forcing appropriately.

Noting the above caveat, proceed and add daily-mean STASH requests for section 1 item 208 (all-sky outgoing short wave flux at the top-of-the-atmosphere) and section 2 item 205 (all-sky outgoing long wave flux at the top-of-the-atmosphere) to your copy of the UKCA tutorial job (xkvxe).

The radiative fluxes are 2-dimensional diagnostics (longitude by latitude) so you should use the DIAG domain profile in this case. For daily-means use the TDAYM time profile. Again, since we require the daily-mean fluxes to be output to the .pa file you should request the diagnostics with the UPA usage profile.

Task 12.2 Configure the UKCA tutorial job to run as a double-call job diagnosing aerosol radiative effects

In this task you will copy your copy of the standard tutorial job (xkvxe) and configure it to run with double-call to the radiation scheme to diagnose the radiative effects of the aerosol simulated by GLOMAP in UM-UKCA.

The UM has been coded to allow the user to diagnose radiative effects of a particular forcing agent by calling the radiation scheme twice with one of the calls setting the agent's concentration to zero. Special forcing STASH items are included within the UM which store the difference in the radiative fluxes between the two radiation calls.

In the UMUI go to Atmosphere --> Scientific Parameters and Sections --> Section by section choices and then choose Section 1: SW radiation.

In the SW Radiation UMUI panel that opens, you see that at the top there is an "Options for multiple calls to radiation" button-selector.

The UKCA tutorial job is set to "Timestepping scheme" which is the recommded way of running the model. In this configuration the UM has a single call to the radiation scheme every radiation timestep (here 3 hours) with a 2nd reduced-call being applied on other timesteps (for more details see Manners et al., QJRMS 2009). The single-call option is the same as the Timestepping scheme but does not apply the reduced radiation call on interim timesteps.

The other option supported here is to select "Diagnose radiative forcings" which activates the double-call approach where the radiation scheme is called twice on each radiation timestep with and without the forcing agent.

By default, if one selects the Diagnose radiative forcings option, then the model diagnoses the radiative forcing based on the advancing call including the forcing agent as usual, and the species is set to zero in the 2nd diagnostic call to the radiation.

This operation is applied via the SLWForc panel which is available after selecting the Gen2 follow-on window. See that it is possible to individually select each of the CLASSIC aerosol types to diagnose their radiative effects whereas for GLOMAP-mode it only makes sense to diagnose the effects over all the types considered since the different types become internally mixed within a given mode. Note that the user needs to be very careful to specify exactly how the effects should be applied in the second radiation call and this is specified in the Call2 follow-on window.

We strongly recommend only making changes to the default settings after discussing with relevant experts at within NCAS or the Met Office.

Although the default UM setting for the double-call is to set the species mixing ratio to zero in the diagnostic call, it is often very useful to be able to suppress the fast feedbacks from the forcing agent in question by reversing the operation of the double-call including the aerosol radiative effects only in the diagnostic call and setting the species mixing ratio to zero in the advancing call.

With this double-call radiative forcing configuration, the difference in radiative fluxes between the two calls provides the aerosol radiative perturbation with respect to an atmosphere containing no aerosols. One can diagnose the present-day to pre-industrial aerosol radiative forcing by taking the difference between two parallel double-call nudged simulations with aerosol and precursor emissions set to 1850 and 2000. All other forcing agents, such as greenhouse gases or land-use change, remain fixed at a reference time period.

This approach has been used extensively in aerosol forcing intercomparisons (e.g. the AeroCom direct forcing experiments, Myhre et al., 2013, ACP) with the radiative forcings diagnosed from each model with fast feedbacks disabled. Often nudging to meteorological re-analysis fields to applied in tandem with the double-call simulations in which case the composition-climate model is being run in a similar way to an offline chemistry-transport model.

To run UM-UKCA with this double-call radiative forcing configuration, you will need to add an extra FCM branch to the job and also add an extra hand-edit in the UMUI to configure the double-call.

So first, in the FCM panel, add an entry to point to revision 17632 of the following FCM branch:

  fcm:um-br/dev/gmann/vn8.4_RADAERupdates_for_dblcalaerforc/src

Then, in the hand-edits panel in the UMUI add the hand-edit to configure the double-call:

  ~gmann/umui_jobs/hand_edits/vn8.4/c2c_dustADE_glomapADEandAIE1_v84.ed

Written by Graham Mann 2014