Radiation balance

[MartinDix]

Latest CM3 runs (e.g. cm3-run-16-09-2024) have a net TOA energy flux around 2.7 W/m^2 initially.

Met Office GC5 AMIP runs were tuned to give the desired radiation balance at N216, not N96. The tuning is described at https://code.metoffice.gov.uk/trac/gmed/wiki/ticket/573/TicketDetails describes the radiation tuning of the N216 version.

This is an extract of the table near the end of https://code.metoffice.gov.uk/trac/gmed/wiki/ticket/577/TicketSensitivityTests

	rsut	rlut	net
Obs CERES EBAF 4.1	99.0	240.3	+1.0
N96 RC3	98.56	239.53	+2.31
N216 RC3	99.07	240.59	+0.74

RC3 is almost the same as the final version, except for an update to the boundary layer buoyancy https://code.metoffice.gov.uk/trac/jules/ticket/1242.

Alejandro Bodas's presentation at the 2023 Momentum Workshop says they retuned the GC5-N96ORCA1 PI control aiming at rsut=99, rlut=240.5, close to CERES EBAF4 values (because UKESM suggested PI and current were pretty close). Used two_d_fsd_factor = 1.70, cape_timescale =2700 s in place of standard two_d_fsd_factor=1.65 cape_timescale=1800.0, though not clear if this is the final value.

[MartinDix]

The Met Office n96orca1 PI control, e.g. u-ct598, instead tunes some cloud ice detrainment parameters. Compared to the AMIP configuration we've used until now changed parameters are

Parameter	AMIP	PI control
cff_spread	1.0e-5	1.0e-10
tice	263.15	253.15
starticeTKelvin	263.15	265.15
alliceTdeg	-20	-32
two_d_fsd_factor	1.65	1.67

From the UM metadata

• cff_spread_rate: Ice cloud fraction spreading rate
  When the effects of shear on falling ice cloud are either ignored or derived from model winds, this controls the rate at which ice cloud fraction spreads.

• starticetkelvin: Start detraining condensate as ice at (Kelvin)
  When the method for coupling PC2 to convection scheme is 3 or later, this sets the temperature at which detraining condensate as ice begins.

• allicetdegc: All condensate is detrained as ice by (deg C)
  When the method for coupling PC2 to convection scheme is 3 or later, this sets the temperature at which all condensate has detrained to ice.

• tice: Specify phase change temperature /K
  Use a phase change other than 0 degrees - choose this if running with PC2. The value commonly used with PC2 is 263.15.

• two_d_fsd_factor: Factor to convert 1D FSD for straight line to 2D value
  Current parametrizations of FSD (fractional standard deviation, a measure of subgrid variability) of cloud water content used in radiation and microphysics schemes are based on observations taken in a 1D line. For the UM we require the FSD for a 2D gridbox, which is generally larger. We account for this difference by multiplying the 1D fsd by this value.

[kieranricardo]

Run cm3-run-29-10-2024-radiation-balance with two_d_fsd_factor = 1.70, cape_timescale =2700 didn't show any improvement.

Run cm3-run-06-11-2024-radiation-balance-ice-detrainment with ice detrainment started.

[kieranricardo]

TOA flux not looking promising over 5 years. I've extended the run, and it's currently running for 20 years.

[MartinDix]

Changes in the right direction, but only a small improvement from these parameters in an AMIP run.

	Std	New options	CERES
rsut	98.41	98.84	98.93
rlut	239.62	239.82	240.35
net	2.37	1.74