Raising the Roof -Diagnosing the Instability

Matthew GriffithM.J.Griffith@bath.ac.ukUniversity of BathSupervisors:Prof C. J. Budd OBE, Prof N. J. Mitchell,Prof D. Jackson, Prof J. Thuburn

Introduction

Project Aims•Develop a stable numerical model of the MLT(Mesosphere & Lower Thermosphere) which alsorepresents high-frequency and other atmosphericwaves.

•Explore a number of important meteorologicalphenomena in the upper atmosphere which arebeyond the reach of current forecasts.

•Use Radar observations obtained by N. Mitchellcombined with satellite observations to validatethe model’s results.

Model Setup

This work will develop the Met Office’s UM (UnifiedModel). The model and specific setup used here havethe following features:•The model is a GCM (General Circulation Model)of the full non-hydrostatic, fully-compressible deepatmosphere equations of motion.

•The dynamical core uses a SISL (Semi-ImplicitSemi-Lagrangian) formulation, with the benefit ofbeing able to take long timesteps in a stablemanner; not limited by the CFL condition.

•A terrain following hybrid height coordinate isused as the vertical variable.

•The model lid is extended from 85km to 100kmusing a total of 88 levels (L88), adding 3additional ≈5km levels at the top of the model.

•The horizontal resolution is 1.25◦N x 1.875◦E.•The UKCA (United Kingdom Chemistry &Aerosols) Scheme is switched off (it is currently indevelopment above 85km) and is replaced bybackground ozone and aerosol ancillaries.

Current Problems•Current model runs with an increased lid of100km are unstable, and crash after variousperiods of time ranging from just under a monthto just under a year.

•An instability develops at the top of the model inthe timesteps leading up to the crash, whichcauses anomalies in many of the modelled fieldsincluding wind, temperature and pressure.

•These anomalies are large in magnitude, and socause issues in the advection scheme.

Plotting the Instability

Top Level Plots

In order to study the instability, we produce the following plots of a 100km lid model run with88 vertical levels. We plot some important model variables at the model crash in mid-December.

−180 −150 −120 −90 −60 −30 0 30 60 90 120 150 180Longitude / degrees

−90

−60

−30

0

30

60

90

Latitude / degrees

-48 -42 -36 -30 -24 -18 -12 -6 0LW Heating / K d-1

−180 −150 −120 −90 −60 −30 0 30 60 90 120 150 180Longitude / degrees

−90

−60

−30

0

30

60

90

Latitude / degrees

0 6 11 16 22 28 33 38 44 50SW Heating / K d-1

↙

−180 −150 −120 −90 −60 −30 0 30 60 90 120 150 180Longitude / degrees

−90

−60

−30

0

30

60

90

Latitude / degrees

8.0e-05 1.2e-04 1.6e-04 2.0e-04 2.4e-04 2.8e-04Air Pressure / hPa

−180 −150 −120 −90 −60 −30 0 30 60 90 120 150 180Longitude / degrees

−90

−60

−30

0

30

60

90

Latitude / degrees

-200 -100 0 100 200 300Westerly Wind / m s-1

↙

−180 −150 −120 −90 −60 −30 0 30 60 90 120 150 180Longitude / degrees

−90

−60

−30

0

30

60

90

Latitude / degrees

134 140 146 153 159 166 172 178 185 191Air Temperature / K

−180 −150 −120 −90 −60 −30 0 30 60 90 120 150 180Longitude / degrees

−90

−60

−30

0

30

60

90

Latitude / degrees

-120 -60 0 60 120 180Southerly Wind / m s-1

↙90 120 150

Longitude / degrees

−90

Latitude / degrees

-200 -100 0 100 200 300Westerly Wind / m s-1

−90Longitude / degrees

−90

Latitude / degrees

-120 -60 0 60 120 180Southerly Wind / m s-1

From left to right, top to bottom we have L88 plots (≈98km) of LW (Long Wave) Heating,Pressure, Temperature, SW (Short Wave) Heating, Westerly Wind, Southerly Wind and zoomedplots of the latter two showing problematic areas around the South Pole.

South Polar Slice & Zonal Mean Plots

Here, we examine the anomalies in several of the variables further; these highlight theinstabilities encountered.

−50 0 50Latitude / degrees

0

20000

40000

60000

80000

100000

Height / m

0 6 11 16 22 28 33 38 44 50SW Heating / K d-1

−50 0 50Latitude / degrees

50

60

70

80

Model level number

Mass fraction of ozone in air

0.000002 0.000004 0.000006 0.000008 0.000010 0.000012 0.000014 0.000016

50 100 150 200 250 300 350Longitude / degrees

75000

80000

85000

90000

95000

Height / m

-58 -50 -43 -36 -29 -22 -14 -7 0LW Heating / K d-1

0 50 100 150 200 250 300 350Longitude / degrees

75000

80000

85000

90000

Height / m

-600 -400 -200 0 200 400 600Westerly Wind / m s-1

50 100 150 200 250 300 350Longitude / degrees

75000

80000

85000

90000

Height / m

110 120 130 140 150 160 170 180 190 200Air Temperature / K

50 100 150 200 250 300 350Longitude / degrees

75000

80000

85000

90000

Height / m

-240 -160 -80 0 80 160 240Southerly Wind / m s-1

From left to right, top to bottom we have lat/long vs. height plots of SW Heating (Zonal Mean),LW Heating, Temperature, Ozone (Zonal Mean), Westerly Wind and Southerly Wind.

Future Plans• Investigate the possibility of linear solver issues.•Fine tune the vertical resolution at the top of the model and the timestep used in the model run.• Include non-LTE (Local Thermodynamic Equilibrium) effects, which could help with current overestimates of heating.•Relax to climatological temperature structure in the MLT region. Currently the ≈100K change in the MLT is not taken into account in the model.•Strip down to just a "dynamics + radiation" model to eliminate problems between interacting processes.