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%\footline={\hss \folio \hss}
%\pageno=0

%---------------Reply begins here------------


\null
\vskip.75truein

\centerline{\bf A REPLY TO}
\centerline {\bf ``A COMMENT ON: ``ON LARGE-SCALE CIRCULATIONS IN}
\centerline{\bf CONVECTING ATMOSPHERES"}
\centerline {\bf BY EMANUEL, NEELIN AND BRETHERTON''}
\vskip.5truein

\centerline {Kerry A.\ Emanuel}
\medskip
{\baselineskip=12pt
\centerline {Center for Meteorology and Physical Oceanography}
\centerline {Massachusetts Institute of Technology}
\centerline {Cambridge, MA 02139}

}
\bigskip

\centerline {J.\ David Neelin}
\medskip
{\baselineskip=12pt
\centerline {Department of Atmospheric Science}
\centerline {University of California, Los Angeles}
\centerline {Los Angeles, CA 90024}

}
\bigskip

\centerline {Christopher S.\ Bretherton}
\medskip

{\baselineskip=12pt
\centerline {Department of Atmospheric Science}
\centerline {University of Washington,  Box 351640}
\centerline {Seattle, WA 98195}

}
\vfill\eject


\beginsection 1.\ Introduction

We very much welcome the comments on our paper (Emanuel, Neelin, and
Bretherton, 1994; hereafter ENB) by Bjorn Stevens, David Randall, Xin
Lin, and Michael Montgomery (hereafter SRLM).  We had hoped to 
provoke discussion and are very much gratified to learn of the semester-long
seminar at Colorado State University, leading to the thoughtful
response by SRLM.  Herewith our response to their comments.

\footline={\hss\folio\hss}\pageno=2


\beginsection 2.\ CISK

In contrast to SRLM, we feel that the concept of CISK, as originally
defined by Charney and Eliassen (1964) and also discussed by Ooyama
(1964) is conceptually clear.  Simply stated, it is an instability of
a large-scale circulation owing {\it specifically} to its interaction
with an ensemble of convective clouds.  We find no evidence in either
of the above-cited papers that their authors intended CISK to
encompass a feedback between surface winds and evaporation; such a
feedback is not present in the simple models presented in those
papers, nor is it entailed in the term ``moisture convergence.''
Moreover, in all the rather prodigious literature on CISK that has
appeared since 1964 there is not, to our knowledge, a single
formulation that holds surface fluxes, rather than convection, to be
the ``rate-limiting'' process for tropical cyclones, to use the
terminology of Craig and Gray (1996).

SLRM maintain that since its original 1964 definition, the term
``CISK'' has evolved to encompass a broader range of processes,
including WISHE.
%(This strikes us akin to claiming that there exist dragons, finally
%going into the jungle and emerging with an elephant, and then
%broadening the definition  of ``dragon'' to include elephants.)
Their reasoning seems to be as follows:
\medskip

\item {1.} Ooyama was an early proponent of CISK
\item {2.} Ooyama's 1969 model very clearly depends on surface fluxes
to produce hurricanes
\item {3.} Therefore, the importance of surface fluxes is entailed in
the definition of CISK
\medskip

\noindent We strongly concur that Ooyama's 1969 model captures the
essence of hurricane physics, but we do not agree with SLRM that
Ooyama's concept of CISK, as articulated in that or subsequent papers,
leads to a more favorable view of the theory. Indeed, Ooyama (1982)
continued to argue that tropical cyclone intensification is due to ``a
cooperative process between the organized moist convection and the
cyclone-scale vortex", and his own ``broadening" of the definition of CISK
was meant strictly to account for the nonlinear contraction of the local
deformation radius and does not account for feedbacks involving surface
fluxes. In his discussion of intensification and CISK, Ooyama (1982)
mentions surface fluxes only briefly and only in connection with
steady-state maintenance.
We cannot discern any sense in which WISHE can possibly be viewed as a
refinement of Ooyama's (or anyone else's) articulation of CISK. 

One confusing aspect of Ooyama (1969, hereafter O69) is that his
initial condition contained a high degree of conditional instability,
allowing his linear analysis of the model to exhibit instability.
Thus, Ooyama's initial condition was linearly unstable to CISK while
it is equally clear that all but a trivial initial amplification of
the model vortex is owing to WISHE.  (Subsequent experiments with
Ooyama's model demonstrate that it exhibits finite amplitude
instabilities to WISHE but no linear CISK instability when the high
degree of conditional instability is removed from the initial
condition.)  We believe that the presence of both CISK and WISHE in
Ooyama's simulation, coupled with subsequent attempts to ``broaden''
the definition of CISK have had the unfortunate effect of muddling
what should be clear and distinct physical concepts.  We interpret
SLRM's three alternative definitions of CISK as being so broad as to
encompass anything anyone has ever believed to be true about the
physical process of tropical cyclogenesis, thus rendering the concept
useless.  We defer to Charney and Eliassen (1964), the authors of the
acronym ``CISK,'' for its definition and that definition is clear.
We stand by our original opinion that CISK has been an influential
and lengthy dead-end road in atmospheric science.


\noindent{\bf 3.\ Latent heating and kinetic energy production}
\smallskip

\noindent In ENB we attempted to point out that heating by
convection cannot be
regarded as necessarily being positively correlated with temperature,
and gave some simple examples of models in which the reverse is true.
 We argued that the interaction between convection and large-scale
circulations, {\it by itself,} would lead to a negative correlation
between convective heating and temperature, implying a sink of
kinetic energy.

SRLM give some examples of instances in which observed tropical
circulations demonstrate positive correlations between heating and
temperature.  These observations neither refute nor confirm ENB's
argument, because they do not and cannot treat the interaction
between large-scale circulations and convective clouds in isolation
from other processes, such as WISHE, or feedbacks involving radiative
transfer.  A hurricane is an excellent example of a storm in which
potential energy is converted to kinetic energy; this is manifest by
a positive correlation between convective heating and temperature.
Hurricanes are driven by WISHE, however, as envisioned by Riehl
(1950) and Kleinschmidt (1951) and demonstrated by the numerical
simulation of Ooyama (1969).  Without elevation of subcloud-layer
entropy by surface fluxes, an initial warm-core vortex decays owing
to its interaction with convection (Emanuel, 1989).  But we do not
expect to observe circulations with negative growth rates in the real
world.

   While we do not regard the observation of positive correlations between
heating and temperature as a test of any particular theory about the origin
of
tropical circulations, particular applications of WISHE can be tested
against
observations.  For instance, predictions of Emanuel's (1987) model of the
intraseasonal oscillation as an equatorially trapped WISHE mode with Kelvin
wave structure have been compared with data from TOGA-COARE (Lin and
Johnson
1996) and large-scale surface analyses (Zhang 1996) and upper air/satellite
analyses (e. g. Hendon and Salby 1994).  These studies do not support the
model, showing that the ISO is prominent in a region of weak mean surface
westerlies rather than easterlies, and that the strongest surface enthalpy
fluxes are west rather than east of the maximum convection.  However, this
does not rule out the possibility of other wave-like WISHE modes (e.g.
those described by Emanuel, 1993) being relevant
in other contexts, or that WISHE may combine with mechanisms such as surface
drag variations to create the observed waves.  In addition, the zonal
structure of the ISO is more complex than a Kelvin wave, so it may be
misleading to base conclusions on wind and flux measurements only near the
equator, rather than over a broader region. 

From the modeling perspective, WISHE does appear to rationalize the
behavior
of some aquaplanet GCM simulations of the ISO in which there are mean
easterlies around the equator (Neelin 1988). Xie et al (1993) found
ISO-like
WISHE modes even in a model with no mean wind.  On the other hand, linear
studies using the Betts-Miller scheme
(Neelin and Yu, 1994) and the Emanuel scheme (Brown and Bretherton,
1995) fail to produce CISK.



\beginsection 4.\ Quasi-equilibrium

We are well aware of the distinction between quasi-balance
approximations and exact balance, and ENB introduced the term
``strict quasi-equilibrium (SQE)'' to distinguish it from
quasi-equilibrium.  Our discussion of SQE was intended as a
conceptual aid in understanding tropical dynamics, not as an
appropriate closure for a convective scheme, except perhaps
as a leading order balance.  Indeed, one of ENB's central points was
that the departure of the atmosphere from SQE has important dynamical
consequences, including moist convective damping (MCD).  Even the
simplest models of convection with a very small time lag
are consistent with substantial fractional
fluctuations of CAPE.  For example, the linear model of Emanuel (1993) with a 
small but nonzero convective time lag produces fluctuations of CAPE of about 
200 $J\ Kg^{-1}$ for moist wavenumber 1 Kelvin-type waves, given a zonal 
wind perturbation amplitude of $4\  ms^{-1}$. This is consistent with the 
amplitude of CAPE fluctuations shown in Figure 4 of SLRM. Even so, we urge 
caution in evaluating CAPE from actual soundings. Figure 1 of this paper 
shows reversibly defined CAPE calculated using over 344 soundings from one 
of the same sounding stations used by SLRM to construct their Figure 1. (All 
sounding parcel origin levels associated with negative values of CAPE were 
excluded from the averaging.) Clearly, the magnitude of CAPE is very 
sensitive to the assumed origin level of lifted air, with almost no CAPE for 
air lifted reversibly from above the surface layer. (We note, however,
that serious flaws in the calibration and launch procedures of
rawinsondes used in TOGA/COARE may strongly affect CAPE calculations
using the first 10 or 20 mb of data.)  Figure 1 also shows that the magnitude 
of fluctuations in CAPE is sensitive to the assumed parcel origin level. The 
magnitude of CAPE fluctuations for air originating above the surface layer 
does not exceed that which arises solely from measurement noise, adding to 
problems of interpretation. A better understanding of the thermodynamics of 
convection as well as its connection to the boundary and surface layers is 
clearly desirable.   


\beginsection 5.\  Summary

ENB introduced ``statistical equilibrium thinking'' as an alternative to
the extant view of the interaction between convection and large-scale
flows, a view that might be called ``CISK thinking.'' We do not agree with SLRM
that one aspect of statistical equilibrium thinking, WISHE, can in any
sensible way be viewed as a refinement of CISK theory, which has always
entailed a cooperative interaction between large-scale flows and
convection, with surface fluxes playing, at most, a background role,
never entering as an important feedback mechanism. 
We do agree with SRLM
that there is a large range of what might constitute CISK thinking,
but we believe that it ranges between wrong, in the worst cases, to merely
awkward in others. The awkwardness arises, we believe, from a failure to
take advantage of the fact that the buoyancy adjustment time scale of
convection is small compared to that of many larger-scale circulations. We
hold statements such as ``hurricanes
are driven by latent heat release in cumulus clouds'' to be analogous
to the statement that automobiles are driven by the torque
transmitted through the drive train.  Neither statement is wrong;
both are misleading.  Both attribute causation to processes with
relatively short time scales. But hurricanes would not exist without
convection any more than automobiles would operate without drive
trains.

The advantage of statistical equilibrium thinking is that it places
the emphasis on processes with comparatively long time scales and
regards convection as a fast, if complex, connector. In this view of the
Hadley and Walker circulations, for instance,
convection acts quickly to distribute the effect of the underlying sea
surface temperature gradient over a deep layer,
resulting in baroclinic pressure gradients that maintain the circulation
(Held and Hou, 1980). We find this more appealing than making a conceptual
division between
a dry circulation first being driven by latent heat release and then
feeding back on it.
Similarly, the long-time-scale process that drives a hurricane is the
feedback between the storm's circulation and evaporation from the
ocean; no nonlinear model (beginning with that of Ooyama, 1969) has
contradicted this.  While our view discounts convection as the {\it cause}
of large-scale circulations, we by no means wish to understate its
fundamental importance to their dynamics.  The relative magnitude of
convective updrafts and downdrafts, which is governed by large-scale
dynamical cloud dynamics and microphysical processes, strongly
influences the development of tropical cyclones, for example, and no
proper description of large-scale tropical circulations can be made
without accounting for convective downdrafts.  But we believe that
the fundamental premise of statistical equilibrium thinking, that
convection acts as a fast equilibrator of thermodynamic profiles, offers a
simple and physically consistent view of tropical circulations that
is in agreement with some, but not all, available observations. 
We hope that its
predictions, for example that the interaction between large-scale
waves and convection per se damps the former, will be subjected to
rigorous tests using observational data and ensemble convection
models.
\vfill\eject



{\frenchspacing

\def\ref{\leftskip 20pt\parindent -20pt\parskip 12pt plus 1pt}
\def\rbar{\leavevmode\kern .06em\vbox{\hrule width .35truein}}
\def\vol #1, {{\bf #1,} }
\def\jas{{\it J.\ Atmos.\ Sci.,} }
\def\qjrms{{\it Quart.\ J.\ Roy.\ Meteor.\ Soc.,} }
\def\mwr{{\it Mon.\ Wea.\ Rev.,} }

\centerline {\bf References}
\bigskip
{\baselineskip=20pt
\ref
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Craig, G.\ C., and S.\ L. Gray, 1996: CISK or WISHE as the mechanism for
tropical cyclone intensification. \jas, {\it in press}. 

Emanuel, K.\ A., 1989: The finite-amplitude nature of tropical
cyclogenesis.  \jas \vol 46, 3431--3456.

Emanuel, K.\ A., 1993: The effect of convective response time on
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Emanuel, K.\ A, J.\ D.\ Neelin, and C.\ S.\ Bretherton: On
large-scale circulations in convecting atmospheres. \qjrms \vol 120,
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Held, I.\ M., and A.\ Y.\ Hou, 1980: Nonlinear axially symmetric
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Hendon, I.\ M., and M.\ L. Salby, 1994: The life cycle of the Madden-Julian
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Kleinschmidt, E., Jr., 1951: Gundlagen einer Theorie des tropischen
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Lin, X., and R.\ H. Johnson, 1996: Kinematic and thermodynamic
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Neelin, J.\ D., 1988:  Reply to ``Comments on an air-sea interaction
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Neelin, J.\ D., and J.\ Yu, 1994: Modes of tropical variability under
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Xie, S.-P., A. Kubokawa and K. Hanawa, 1993: Evaporation-wind feedback and
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}
}
\vfill\eject

\centerline {\bf Figure captions}
\bigskip

{\parskip=10pt \parindent=0pt
{\bf Fig.\ 1} The mean (solid) and standard deviation (dashed) of 
reversibly defined convective available potential energy (CAPE) 
as a function of the parcel origin level, calculated using over 344 soundings
from Kapangamarangi during TOGA/COARE.  Sounding
origin levels showing no positive buoyancy have been excluded from the 
averaging, and no freezing occurs in the lifted parcel. 



\bye



