A cloud radiation interaction parameterization package has recently been incorporated into a global spectral General Circulation Model (GCM) used for extended range prediction studies at GFDL. The elements of this package are summarized. Analysis of the time mean radiative and dynamical responses suggests that cloud radiation interaction has a favorable impact, overall, on systematic errors. The possible relevance of this sensitivity study to FIRE is mentioned

Gordon, C. T.

English

NASA Technical Reports Server

N90-28274
_.o
Comparison of 30 Day Integrations with and without Interactive Clouds
C.T. Gordon
Geophysical Fluid Dynamics Laboratory/NOAA
Princeton University, P.O. Box 308
Princeton, NJ 08542
A cloud-radiation interaction parameterization package has recently been
incorporated into a global spectral GCM used for extended range prediction
studies at GFDL. The elements of this package are summarized. Analysis of the
time mean radiative and dynamical responses suggests that cloud-radiation
interaction has a favorable impact, overall, on systematic errors. The
possible relevance Of this sensitivity study to FIRE is mentioned.
Fractional cloud amount is predicted empirically in a manner similar to
J. Slingo (1987), although the humidity threshold is reduced to 70%, the
relationship between cloud fraction and relative humidity is linear, and a
shallow convective cloud type is predicted for radiative purposes. The optical
depth of sub-freezing, cold clouds varies quadratically with temperature
following Platt and Harshvardhan (1988). Otherwise, distinct constant values
are specified for high, middle and low clouds. The long- and shortwave cloud
optical properties are linked to the cloud optical depth, employing an
algorithm of V. Ramaswamy (1987, personal communication).
The GCM's sensitivity to interactive clouds is investigated for the
extended forecast range, by performing two sets of 30 day integrations for
3 winter and 3 summer cases (referred to by the respective dates of the initial
conditions): (i) CLDRADI employs the above package of parameterizations;
(ii) LONDON utilizes the GCM's standard specification of zonal mean cloud
amount, absorptivity, reflectivity and blackbody emissivity. The CLDRADI and
LONDON GCM's are identical in all other respects. The model resolution is
R21L18, denoting rhomboidal spectral truncation at wave number 21 and 18 sigma
levels in the vertical. The usual physical parameterizations are retained -
Mellor-Yamada turbulence closure, Monin-Obukhov surface boundary layer, water
bucket hydrology, Fels-Schwarzkopf radiation, moist convective adjustment and
orography; and linear mountain gravity wave drag is incorporated. A few
auxilliary integrations are performed for a single case to help clarify the
results. For example, parameterized shallow convection in the spirit of Tiedke
et al. (1988) is added in KSHLCNV; and the Platt-Harshvardhan temperature
dependence of cloud optical depths is suppressed in WARM T.
The 30 day mean CLDRADI total cloud amount for one winter and one summer
case are shown in Fig. 1, and the CLDRADI, LONDON and observed OLR (winter case
only) in Fig. 2. The GCM-predicted cloud amount fields and corresponding OLR
fields are plausible in many respects, especially in the tropics, where they
exhibit ITCZ-like and SPCZ-like features. In contrast, the OLR fields from the
control integrations do not. The CLDRADI OLR fields agree better, overall,
than LONDON with NIMBUS 7 earth radiation budget data. The tropical OLR minima
are nonetheless too weak, especially over the Amazon. Some stratus may be
noted off the west coasts of South America and Africa in January. Moving on to
longwave radiative heating/cooling rates (not shown), the CLDRADI cirrus level
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• TOTAL CLOUD 05 JAN 79
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Fig. 1 Day 0-30 30 day mean CLDRADI total cloud amount for 05 Jan 1979 (top)
and 11 June 1979 (bottom) cases. B]ank= 0-20_; proqressively darker
shades of stippling for 20-100% in intervals of 20%.
heating is moderately stronger in amplitude, is much more zonally asymmetric,
and occurs ~75 hPa higher in the atmosphere, compared to LONDON. Meanwhile, at
low cloud top level (e.g., at ~700 hPa in the tropics), the longwave cooling is
dramatically reduced in CLDRADI, accompanied by weaker latent heating and
adiabatic cooling. The response is considerably enhanced, because the longwave
cooling for clouds more than I layer thick is not smoothed. LONDON ]ow clouds
are particularly sensitive to this adjustment, being always ? layers thick and
zonally symmetric.
The zonal mean OLR tropical bias is positive and surprisingly similar
(~7 or 8 Wm-2) for both CLDRADI and LONDON. The higher tops of LONDON vs.
CLDRADI low clouds apparently tend to compensate for the higher tops of CLDRADI
high clouds. But the CLDRADI tropical bias can he almost eliminated, by
suppressing the temperature-dependent Platt-Harshvardhan parameterization of
cloud optical depth (not shown). Conversely, we were able to increase the OLR
5 to 6 Wm-2 by incorporating shallow convection and another 2 to 4 Wm-2 by
confining the shallow convective cIoucl tops beneath 800 hPa instead of 750 hPa.
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Fig. 2 Day 0-30 30 day mean outgoing 1ongwave radiation. CLDRADI (top) and
LONDON (middle) For case of 05 Jan 1979; NIMBUS 7 NFOV Obs (bottom).
Contour int. = 25 wm-2.
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Fig. 4 30 day, 8 case ensemble mean, zonal mean temperature error in the
latitude-pressure plane for N48L9 GCM with moist cony. adj. (top)
and Arakawa-Schubert (bottom) parameterizations. Contour int. = 1°k.
After Sirutis and Miyakoda.
Since contributions to OLR come from various vertical levels, it would be
useful to have observations of cloud top and cloud base pressure in the
subtropics and tropics as well as the observed vertical profiles of radiative
flux and water vapor.
To assess the GCM's dynamical response to cloud-radiation interaction, we
shall focus on systematic errors (with respect to NMC observation) and
CLDRADI-LONDON differences of the zonal mean temperature and wind fields,
because the CLDRADI-LONDON differences (not shown) tend to be statistically
significant. The 3-case winter ensemble mean, zonal mean zonal wind and
temperature errors are illustrated in Figs. 3a and 3b. The following features
of the CLDRADI distributions compare more favorably with observation: a weaker
cold bias, i.e., ~2k warmer, centered near the tropical tropopause, and weaker
easterlies in the tropical lower stratosphere; weaker SH mid-latitude westerly
jet and slightly stronger westerlies poleward of the jet during SH summer; a
weaker cold bias in the tropical middle troposphere, and tropical and
mid-latitude lower troposphere. In particular, the cold bias near the 700 hPa
level in the 30°N-30°S latitude belt is _4k. Near the South Pole, the lower
troposphere is considerably warmer during SH winter and colder during SH
summer, in closer agreement with observation, when cloud-radiation interaction
is incorporated. The latter responses are essentially radiatively driven by a
relative reduction in cloud cover. During NH winter, the CLDRAD[ zonal mean
wind in the tropical upper troposphere is westerly, like observation, but too
305
CLDRAD__'_ " _ SUMMER
Fig. 5 Day I0-30 20 day ensemble _an 500 hPa
geopotential height, for $H summer,
strong, while the corresponding LONDON
winds are easterly. Fig. 3b indicates
that these westerly and easterly wind
errors are of comparable magnitude.
The sensitivity of the systematic
temperature error of a 30 day mean
forecast by a 2° resolution 9 level GCM
to the parameterization of cumulus para-
meterization is illustrated in Fig. 4.
The results are based on an 8 case study
by Sirutis and Miyakoda (1989). Note the
dipole error structure associated with
the Arakawa-Schubert (AS) parameteriza-
tion in the tropical upper troposphere
and the sign reversal near tropopause
level when moist convective adjustment is
replaced by AS. While the result may not
surprise anyone, it illustrates that the
systematic temperature error may not be a
good indicator of the performance of a
cloud-radiation parameterization.
Clearly, some pairs of cumulus + cloud-
radiation parameterizations are more
effective than others at minimizing the
systematic temperature error in the upper
troposphere. Our present cloud
prediction scheme tends to compensate for
the apparent cold bias of the moist
convective adjustment scheme. Similarly,
the anvil cirrus parameterization of
Randall et al. (1988), which promotes
radiative cooling at tropopause level and
warming at ~400 hPa, would tend to oppose
the dipole structure of AS in Fig. 4.
Conversely, the combination of Randall et
al. + moist convective adjustment would
probably increase the cold bias in our
GCM. Under these circumstances, observed
vertical profiles of radiation fluxes in
the tropics or subtropics would be a
useful verification tool.
The phase and amplitude prediction
of geopotential height troughs and ridges
at 500 hPa in SH summer is discernably
improved when cloud-radiation interaction
is incorporated into the model (Fig. 5).
Also note the amplification of wavenumber
3. It may be primarily forced by
stronger (radiative) diabatic heating in
the SH tropics near the tropopause in the
CLDRADI(top).LONDON(middle)and NMC presence of more favorable zonal winds.
observation (bottom}, ContourInt. - 60 m. In particular, the CLDRADI upper
306
tropospheric zonal mean westerlies may favor wave propagation to the SH
extratropics, whereas the LONDON easterlies may inhibit it. In addition,
radiatively induced warming in the tropics, and hence, a strengthening of the
equator-to-pole temperature gradient could play a complimentary role, in
analogy to Meehl and Albrecht (1988). Their GCM predicted a stronger
circumpolar trough in SH summer, in response to enhanced tropical diabatic
heating associated with a new parameterization of cumulus convection.
Meanwhile, in the NH, where there is more background asymmetric forcing, the
500 hPa ridges tend to be stronger and troughs weaker; and there is no visible
evidence of improved phase prediction, at least at R21 resolution.
The relevance of a global GCM sensitivity study to FIRE is not immediately
obvious. But perhaps, the favorable impact of cloud-radiation interaction on
systematic forecast error in our GCM is at least reassuring. More importantly,
awareness gained of the limitations of OLR and temperature data for validating
cloud prediction schemes, from the perspective of a GCMer poses a challenge to
FIRE II or ASTEX. Namely, can their future observational programs measure
vertical profiles of radiative cooling rates, cloud-related variables, water
vapor and temperature over the entire troposphere for a region the size of a
GCM grid box 250 km square? Horizontal means, computed from these measurements
for a range of horizontal scales, could prove valuable for the development and
validation of meso-scale and GCM scale cloud parameterizations.
References
Meehl, G.A., and B.A. Albrecht, 1988: Tropospheric temperatures and southern
hemisphere correlation. Mon. Wea. Rev., 116, 953-960.
Platt, C.M.R., and Harshvardhan, 1988: Temperature dependence of cirrus
extinction: Implications for climate feedback. J. Geoph_s. Res., 93,
No. D9, 11051-11058.
Randall, D.A., Harshvardhan, D.A. Dazlich, and T.G. Corsetti, 1988:
Interactions among radiation, convection, and large-scale dynamics in a
general circulation model. Submitted to J. Atmos. Sci.
Sirutis, J., and K. Miyakoda, 1989: Subgrid-scale physics in one-month
forecasts. Part I: Experiment with four parameterization packages.
Manuscript now in process of internal review at GFDL.
Slingo, J., 1987: The development and verification of a cloud prediction
scheme for the ECMWF model. Q.J.R.M.S., 113, 899-927.
Tiedke, M., W.A. Heckley, and J. Slingo, 1988: Tropical Forecasting at ECMWF:
The influence of physical parameterization on the mean structure of
forecasts and analyses. O.J.R.M.S., 14__44,639-664.
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Comparison of 30 day integrations with and without interactive clouds

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