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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.
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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
e.g. El Nino
on Statistics of
e.g. Crop Models
-HadAM3 AMIP II (observed SST, 1979-95)
-Aquaplanet version of HadAM3
12z 1 January 1992
Sensitivity to Horizontal Resolution
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.
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.
Amplitude (K) of the diurnal harmonic
signal away from the coast
Bay of Bengal, JJA:
Implied propagation speed ~15-20 ms-1
?Deep gravity wave
Implied propagation speed ~ 10 ms-1
?Shallower gravity wave
associated with land/sea breeze
Are sub-gridscale land/sea breezes a crucial component of
the energy and hydrological budgets of the Maritime Continent?
Sea breeze has two major impacts:
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 land breeze.
showing scales of convective organization
Note tendency for cloud clusters to congregate together to form super-clusters
with multi-day life cycles e.g. Madden Julian Oscillation
from window brightness temperature for Jan.-Feb. 1992
Note evidence of coherent propagation.
in association with theoretical equatorial waves.
From Wheeler and Kiladis 1999: J. Atmos. Sci.
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.
From Koshyk and Hamilton 2001: J. Atmos. Sci.
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?
From Koshyk and Hamilton 2001: J. Atmos. Sci.
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).
Courtesy: David Randall, CSU
AGCM + CRM
Note additional levels in free troposphere
Note periods of moistening in the L30 case – convection is generally a moisture sink.
TOGA-COARE IFA apparent heat source (Q1) and moisture sink (Q2) for suppressed (A) and active (B) periods(Lin and Johnson, 1996: J. Atmos. Sci., 53, 3367-3383)
Note periods of moistening (negative Q2) during suppressed period
Observations from TOGA-COARE
(Johnson et al. 1999, J. Clim.)
Note presence of stronger stable layer between 600 and 400hPa in L30,
and similarity with observations.
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
Many conceptual models of tropical convection are based on a BIMODALcloud distribution, emphasizing shallow “trade-wind” or boundary layer cumuli and deep cumulonimbi.
From Johnson et al. 1999, J. Clim.
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. )
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
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.
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?
Schematic showing formation of salt barrier layer
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.
(From Anderson et al., 1996: J. Clim)
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.
SST and precipitation over the tropical Pacific:
DJF (upper panels), MAM (lower panels)
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 pressure.