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) 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 each size class. 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 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 simulations with aerSLWForc panelosol 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 configuration 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

When the double-call forcing configuration is selected in the SW (or LW) radiation panels it synchonises to the setting in the LW (or SW) panel and a submitted job adds a series of C2C switches to the RADFCDIA namelist in the file CNTLATM according to the settings specified in the SLWForc panel.

In our double-call forcing job however, we will keep the SLWForc" UMUI panel unchanged with all C2C switched set to be false.

Instead we are using the above hand-edit to set the values of the C2C switches in CNTLATM.

If you view the c2c_dustADE_glomapADEandAIE1_v84.ed hand-edit you see it changes the C2C_DUST_D, C2C_UKCA_D and C2C_UKCA_I switches to be true so that the model is configured to diagnose the flux-difference (forcing) based on including the direct radiative effects from the total of the CLASSIC simulated dust combined with the direct and (1st) indirect effects from the GLOMAP simulated aerosol properties.

Configuring the radiation scheme for the double-call requires more than just selecting the Diagnose radiative forcings option.

You will need to update your job making also the following changes:

First, in the SW radiation window change the Number of times a day ot calculate increments (Diagnostic) from 24 to 8. This reverts the diagnostic call to only be carried out on radiation timesteps (3 hourly) rather than every hour as the reduced-radiation-call was applied in the timestepping configuration. Make the same change in the Section 2: LW radiation window.

Second, in the "Gen2" follow-on window for the SW radiation, change the Diagnostic File to be the same shortwave spectral file as for the Prognostic File. I.e. change spec_sw_cloud3_0 to spec_sw_ga3_0. Do the same for the Gen2 follow-on window under the LW radiation panel changing the Diagnostic File from spec_lw_cloud3_0 to spec_lw_ga3_0.

Finally, in the Call2 follow-on window (from the SW or LW Radiation panels) the radiation settings for the double-call need to be set to match those used in the main model. In order to achieve this, in the make the following changes:

• change the method for representing horizontal water content variability from Homogeneous to McICA.
• change the option for overlapping clouds from Maximum-random to Exponential-random.
• switch on the button to include SW absorption by O2
• switch on the buttons to include LW absorption by CFC113, CFC114, CFC11, CFC12, HCFC22, HCFC125, HFC134A, CH4 and N2O.

All that remains then is to add in the extra STASH requests for the double-call forcing diagnostics and to note a change to the operation of the Aerosol Optical Depth diagnostics.

The approach taken to index the STASH numbers for the radiative forcing items (the flux-difference between the two radiation calls) is to apply an offset of +200 to the item number to the corresponding item for the conventional radiative fluxes.

In task 12.1 we added STASH requests for the all-sky TOA outgoing SW and LW radiative fluxes which are references in STASH as section 1 item 208 and section 2 item 205.

To request the all-sky TOA outgoing SW and LW radiative forcings (between the two radiation calls) the corresponding item numbers are section 1 item 408 and section 2 item 405.

Go to the STASH Specification of Diagnostic requirements window and add daily-mean (TDAYM) requests for both of these forcing diagnostics. As in Task 10.1, the domain profile should be set as DIAG and the usage profile as UPA.

Since we have now configured the model to run with GLOMAP aerosol set to zero in the advancing call, the conventional AOD diagnostics introduced in tutorial 10 (section 2 items 300 to 305) will now contain zero values when the model is run.

The UM therefore has a second set of AOD diagnostics giving the aerosol optical depth as calculated in the diagnostic call.

The approach for the double-call AOD diagnostics is the same as for the forcing diags, i.e. to apply an offset of +200 to the item number to find the corresponding AOD item in the diagnostic call.

The double-call GLOMAP AOD diagnostics are therefore found in section 2 items 500 to 505.

At v8.4 it is necessary to add an extra user-STASHmaster file to enable these STASH items to be requested in the job. In the User-STASHmaster files. Diags, Progs & Ancils window off the STASH panel you need to add in the following file

 ~gmann/stashfiles/dblecall_aods_only.stash

If you view that file you see that as well as providing the STASH settings for the GLOMAP double-call AOD diagnostics for each mode, it also provides the information to the UMUI to allow double-call AOD diagnostics to be requested for each of the CLASSIC aerosol types.

Once you have have added the dblecall_aods_only.stash user-STASHmaster file you should proceed to the STASH Specification of Diagnostic requirements window and add daily-mean (TDAYM) requests for the double-call GLOMAP AODs (section 2 items 500 to 505) with usage profile UPA and domain profile DIAG_AOT.

The simulation will then output daily-mean forcing and double-call AOD diagnostics to the .pa file for your UM-UKCA job.

Written by Graham Mann 2014

Written by Graham Mann 2014