NERC Centre for Global Atmospheric Modelling Department of Meteorology, University of Reading
Scale Interactions on Diurnal to Seasonal Timescales: Their Relevance to Seasonal
Model Systematic Error
Julia Slingo, Peter Inness, Richard Neale,
Steve Woolnough and Gui-Ying Yang
CGAM Tropical Group
TOWARDS TROPICAL CLIMATE PREDICTION
SST Variability e.g. El Nino
SST Variability e.g. El Nino
Diabatic Heating Response
Diabatic Heating Response
Global Circulation Anomalies
Global Circulation Anomalies
Lagged Ocean/Land
Response Lagged Ocean/Land
Response
Applications e.g. Crop Models
Applications e.g. Crop Models
Final Impact on Statistics of
Local Weather Final Impact on Statistics of
Local Weather Scale
Interactions e.g. MJO
Scale Interactions
e.g. MJO
Primary route
Translation Teleconnection
Atmospheric Bridge
‘THE TALE OF TWO ERRORS!’
– The Maritime Continent
– The Madden-Julian Oscillation
TOOLS
• Integrations of the Met Office Unified Model -HadAM3 AMIP II (observed SST, 1979-95) -Aquaplanet version of HadAM3
-HadCM3 Control
• CMAP Precipitation data
• High resolution (0.5
0, 3 hourly) window brightness
temperature data from the EU Cloud Archive User Service
(CLAUS)
Typical window brightness (K) image from the CLAUS dataset
12z 1 January 1992
Annual Mean Precipitation (CMAP) and AMIP II Mean Model Errors
JMA ECMWF
Climatology NCAR
NCEP
Met Office
Topography of the Maritime Continent
Resolving the Maritime Continent in GCMs
Annual Mean Precipitation Errors in HadAM3:
Sensitivity to Horizontal Resolution
HadAM3 Sensitivity Experiments: Impact of removing the islands of the Maritime Continent
(Neale and Slingo, 2001: Submitted to J. Clim.)
•Land grid-points removed and replaced by ocean grid- points.
•Increased moisture availability from the sea surface leads to enhanced convection and partial correction of the model dry bias.
•Note also corrections to model’s wet bias in adjacent areas.
Global Impacts of Improved Maritime Continent Heat Source
DJF: 500hPa height (m) and Surface Temperature (K)
•Potential improvements in the
Maritime Continent heat source can have significant remote effects.
•Related to the generation of Rossby waves by the enhanced divergent outflow from the Maritime Continent heat source.
•Substantially reduces model systematic error over the extra- tropics of the winter hemisphere.
•Emphasizes the importance of
considering the global context
of model systematic error in
which biases in the tropics may
be a key factor.
The Diurnal Cycle in the Tropics
(Yang and Slingo, 2001: MWR, 129, 784-801)
Amplitude (K) of the diurnal harmonic
DJF
JJA
Phase of diurnal harmonic: Local time of maximum brightness temperature
DJF
JJA
Phase of diurnal cycle showing systematic propagation of convective signal away from the coast
Bay of Bengal, JJA:
Implied propagation speed ~15-20 ms
-1?Deep gravity wave
Mexico, JJA:
Implied propagation speed ~ 10 ms
-1?Shallower gravity wave
associated with land/sea breeze
Rapidly propagating squall lines down Bay of Bengal as
observed in JASMINE (Webster et al. 2001)
Maritime Continent, DJF: Evidence of complex land/sea breezes which organize convection for several 100 km
Are sub-gridscale land/sea breezes a crucial component of
the energy and hydrological budgets of the Maritime Continent?
Schematic of a sea breeze
Sea breeze has two major impacts:
•Convergence along sea breeze front provides additional convective mass flux
•Winds associated with land and sea breezes enhance surface
fluxes leading to increased moisture supply
Investigating Maritime Continent sea breezes using a Investigating Maritime Continent sea breezes using a
mesoscale model (MM5) mesoscale model (MM5)
Embedded MM5 simulations with Kain-Fritsch convection scheme.
Morning versus evening precipitation differences show signal over ocean, indicative of land-sea breezes.
System of propagating land-sea breezes evident in model. Precipitation is generated over the ocean during the early morning by the convergence initiated by the land breeze.
Evidence that orographic effects enhance the
CONCLUSION I
• Maritime Continent heat source is a key component of the global climate. Improvements in its simulation may have significant impacts on remote systematic errors.
• Specifically, land/sea breezes may be a crucial part of the energy and hydrological budgets of coastal regions and especially around large island complexes.
• In general, horizontally propagating gravity waves, generated by convection, may be important for
organizing convection on larger scales?
Typical window brightness (K) images showing scales of convective organization
Note tendency for cloud clusters to congregate together to form super-clusters
Temporal behaviour of convection around the equator from window brightness temperature for Jan.-Feb. 1992
Note evidence of coherent propagation.
Kelvin
Rossby
Inertio-gravity Inertio-gravity
Mixed Rossby-gravity
Space-time spectra showing the organization of convection in association with theoretical equatorial waves.
Anti-symmetric Symmetric
MJO
Space-time spectra from R30 version of GFDL model
Note lack of organization, an error common to many GCMs.
Lack of self-organization mechanism?
Spectra of the zonal and meridional wind in the upper troposphere.
Data points show actual observations from commercial aircraft flights.
Solid curve is for the N270L40 SKYHI model along the 45°N latitude circle and at 211hPa, monthly averaged for a single July. For clarity the results for the meridional wind have been shifted one decade to the right.
July mean spectra as a function of total horizontal wave-number of (a) the total KE
spectrum, (b) the rotational part of the total KE spectrum, (c) the divergent part of the total KE spectrum.
Presence of strong divergent component at meso-scales consistent with
presence of resolved gravity waves?
GOES Visible Images for 30 September 2001
20:00 UTC
21:30 UTC
22:45 UTC
23:45 UTC
Florida sea breezes and mesoscale organised convection
Super-parametrizations
Results from T21 simulation with an embedded 2-D CRM (1km resolution) in place of convective parametrization.
Note that the cloud-resolving models from neighbouring
columns interact only through the large-scale dynamics. Therefore limits the propagation of gravity waves from one GCM column to another but does allow gravity waves to organise convection within the GCM column.
Note dramatic improvement in MJO (upper panels) and synoptic waves (lower panels).
Standard AGCM AGCM + CRM
Why the MJO is important
• Intimately related to active/break cycles of the Australian and Asian Monsoons
• Offers potential to provide extended predictability up to 15-20 days in tropics
• Affects weather over the western US and possibly elsewhere
• Associated westerly wind events generate ocean Kelvin waves which may significantly modify the evolution and amplitude of El Nino (e.g. 1997)
• Large interannual variability in the activity of the MJO has implications for the predictability of the coupled ocean-
atmosphere system
Sensitivity of the MJO to AGCM vertical resolution
( Inness et al., 2001: Clim. Dyn., 17, 777-793.)
Model Levels: L19 vs. L30
Note additional levels in free troposphere
Exploring sensitivity of convective organization to vertical resolution
• A water-covered or “aqua-planet” version of the UM is used to investigate the behaviour of tropical convection when the vertical resolution is doubled.
• Aqua-planet version of the UM chosen because:
- Homogeneity of the model allows us to obtain a large sample of convective events over warm SSTs - Removal of the land areas excludes circulations forced by land-sea contrasts. Convective events in
different geographical locations are subject to the same large scale forcings, giving a cleaner comparison.
- Aqua-planet provides a more realistic test of the convection scheme than using a single-column model with idealized boundary and large scale forcing functions.
• Aqua-planet setup:
- Zonally symmetric SST distribution, typical of equatorial Indian Ocean/West Pacific warm pool values.
- Incoming solar radiation fixed at zonally symmetric, equinoctial (March) values.
- Aqua-planet model integrated for 15 months with both 19 and 30 levels in the vertical. First 3 months of each integration were discarded.
Time-height evolution of convective cloud over 3x3 model grid boxes (7.5
0lat. x11.25
0long.) centred on the equator
L30
L19
Time-height sections of specific humidity increment (g/kg/day) by the convection scheme
Note periods of moistening in the L30 case – convection is generally a moisture sink.
L30
L19
TOGA-COARE IFA apparent heat source (Q
1) and moisture sink (Q
2) for suppressed (A) and active (B) periods
(Lin and Johnson, 1996: J. Atmos. Sci., 53, 3367-3383)
A: Suppressed B: Active
Q
1Q
2Time-height sections of potential temperature lapse rate
Note presence of stronger stable layer between 600 and 400hPa in L30, and similarity with observations.
L30
L19
Observations from TOGA-COARE
(Johnson et al. 1999, J. Clim.)
Inferences from aqua-planet model results
• When the vertical resolution is increased, the spectrum of tropical cloud types changes from a bimodal to a tri-modal distribution with a third peak in mid-troposphere near the melting level. Associated with periods when these mid-level congestus clouds are
dominant, the detrainment from these clouds significantly moistens the mid-troposphere.
• The appearance of these congestus clouds is shown to be partly due to improved resolution of the freezing level and the convective processes occurring at this level.
• The resulting cloud distribution more closely resembles observations, particularly during the suppressed phase of the MJO when cumulus congestus is the dominant cloud type.
• The moistening of the free troposphere by cumulus congestus clouds acts to
precondition the atmosphere for deep convection. This preconditioning may set the timescale for the next active phase of the MJO and thus influence the intraseasonal organization of convection.
Trimodal distribution of convection and cumulus congestus
Many conceptual models of tropical
convection are based on a BIMODALcloud distribution, emphasizing shallow “trade- wind” or boundary layer cumuli and deep cumulonimbi.
TOGA COARE results emphasize the
dominance of cumulus
congestus and point to
a TRIMODAL cloud
distribution in which the
freezing level inversion
is the key
CONCLUSION II
• Vertical resolution in the free troposphere must be adequate to resolve the formation of the freezing level inversion and the cooling associated with melting precipitation
• Convective parametrizations need to represent a
TRIMODAL rather than bimodal cloud distribution.
Coupling with the upper ocean:
Bringing together the diurnal cycle and the MJO
TOGA-COARE buoy data showed pronounced diurnal variations in skin
temperature in excess of 1K are evident, as well as slower variations related to the MJO. Note that the
diurnal variations occur only during break (B)
periods. Active (A) periods are preceded by a warming on sub-seasonal
timescales.
(From Anderson et al. 1996, J. Clim. )
Nov.
B A B
TOGA-COARE observations also suggest that cumulus congestus clouds are most prevalent during light wind
conditions in the presence of a strong diurnal cycle in SST.
Further, these cloud occur most frequently in the late afternoon suggesting that they are
triggered by the diurnal cycle in SST.
Coupling with the upper ocean is important on diurnal
timescales
Cumulus congestus and the diurnal cycle
The MJO and coupling with the ocean: Observations
(Woolnough et al., 2000: J. Clim., 13, 2086-2104)
Observations show a coherent relationship
between
convection and SST. Warm SSTs precede convection by 5- 10 days and are the result of
weaker winds,
reduced LH flux
and increased
SW flux during
suppressed
phases of the
MJO.
The MJO and coupling with the ocean: Modelling
(Inness, personal communication)
CGCM has a propagating convective signal compared with standing
oscillation in AGCM. Coherent variations in SST in CGCM
Coupling with the
upper ocean is
important for the MJO
BUT intraseasonal SST variations in CGCMs are too small and the MJO signal is still weak:
Is the representation of the upper ocean adequate?
Large freshwater flux sets up a salt stratified barrier layer so that a
shallow mixed layer forms which can respond rapidly to flux variations, such as the diurnal cycle in solar
radiation. The presence of this barrier layer can potentially provide much stronger local coupling in the warm pool region than is currently found in coupled models which do not resolve the detailed structure of the warm pool upper ocean.
Schematic showing formation of salt barrier layer
Temperature
cross-section from the TOGA-COARE
WHOI mooring
Note complex temperature
structure in top 40 meters
during periods of light winds,
associated with suppressed
phase of the MJO and a
strong diurnal cycle.
CONCLUSION III
• Good evidence that MJO and diurnal cycle of cumulus congestus involve coupling with the upper ocean
• To simulate diurnal and intraseasonal variations in SST requires detailed representation of salinity and temperature gradients in the mixed layer
• Need to consider an upper ocean/atmosphere system in which the structure of the upper ocean is
adequately resolved
Probability distribution functions (PDF) of monthly mean SST and precipitation over the tropical Pacific:
DJF (upper panels), MAM (lower panels)
CMAP HadAM3 HadAM3-CMAP
Note tendency for HadAM3 to overestimate precipitation over warm SSTs. PDF is also too tight, following closely the exponential relationship implied by the Clausius-Clapeyron equation for saturated vapour