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Workshop on Large-Scale Circulations in Moist Convecting Atmospheres
October 16 - 17, 2009
Abstracts |
Larissa Back, University of Wisconsin-Madison |
Title: A toy model for understanding the observed relationship between column-integrated water vapor and tropical precipitation
Author(s): Larissa Back
Abstract: Several observational studies have shown a tight relationship between
tropical precipitation and column-integrated water vapor. While this
relationship is an integral part of theories for many tropical
phenomenon (e.g. MJO, convectively-coupled waves, hurricanes), we do
not at present have a full understanding of the mechanisms underlying
the observed humidity-rainfall relationship. It has previously been
argued that features of this relationship could be explained by
analogy with continuous phase transitions and self-organized
criticality.
We show that the observed relationship in the tropics between
column-integrated water vapor, precipitation, and its variance can be
qualitatively reproduced by a simple and physically motivated
two-layer model. Our model assumes that the onset of precipitation is
governed by a stability threshold involving boundary-layer water
vapor. When the column exceeds this stability threshold, rainfall is
modulated column-integrated water vapor path. This allows us to
explain the precipitation-humidity relationship over a broad range of
water vapor values, and may explain the observed temperature
dependence of the relationship. Similarities and differences to other
work on this humidity-precipitation relationships will be discussed.
Full Presentation |
William Boos, Harvard University |
Title: Mechanisms of poleward propagating, intraseasonal convective anomalies in a cloud-system resolving model
Author(s): William R. Boos and Zhiming Kuang
Abstract: The phenomenon accounting for the largest fraction of
tropical intraseasonal variance during boreal summer consists of an
envelope of convection that propagates both poleward and eastward in
Asian longitudes. Over the past thirty years, a number of mechanisms
for the poleward propagating component of this phenomenon have been
proposed based on results from zonally symmetric (latitude-height)
models with diverse and highly idealized parameterizations of moist
convection. Here we test some of these hypothesized mechanisms using
a nonhydrostatic model that explicitly represents moist convection.
Robust poleward propagating convective anomalies with a structure
similar to that observed are obtained only when this cloud-system
resolving model is integrated in a domain wide enough in its
longitudinal extent to resolve large-scale (O(1000 km)) eddies. We
present diagnostics of the moist static energy budget to show how
these eddies cause the convection to migrate poleward. By turning off
various feedback processes in the model, we also show that these
poleward propagating anomalies can result from multiple instability
mechanisms, although a moisture-radiation feedback produces the
structure that most closely resembles observations. These results
illustrate the need to validate theoretical models having simple
convective closures against cloud-system resolving models, and
suggest a novel mechanism for the poleward propagation of
intraseasonal convective anomalies.
Full Presentation |
Chris Bretherton, University of Washington |
Title: The interaction of SST, low-level convergence and rainfall over the tropical oceans
Author(s): Chris Bretherton and Larissa Back
Abstract: A two-mode model calibrated to climatological observations illustrates how both SST and SST gradients affect tropical oceanic rainfall. One bottom-heavy mode of vertical motion is tied to near-surface horizontal wind convergence. It is well predicted by a mixed-layer model of the atmospheric boundary layer, and is driven primarily by gradients both in SST and secondarily by free-tropospheric temperature variations. A second top-heavy mode of vertical motion is correlated with SST itself, conditional on there being surface convergence. Together, these modes explain approximately 90% of observed space-time rainfall variability on monthly and longer timescales. Combining this atmospheric model with a simple interactive ocean along the lines of Peters and Bretherton would be an illuminating thesis problem or research topic.
References:
Back, L. E., and C. S. Bretherton, 2009: On the relationship between SST gradients, boundary layer winds and convergence over the tropical oceans. J. Climate, 22, 4182-4196 http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F2009JCLI2392.1
Back, L. E, and C. S. Bretherton, 2009: A simple model of climatological precipitation and vertical motion patterns over the tropical oceans. J. Climate, in press.
ftp://eos.atmos.washington.edu/pub/breth/papers/in-review/precipmodel.html
Peters, M. E., and C. S. Bretherton, 2005: A simplified model of the Walker circulation with an interactive ocean mixed layer and cloud-radiative feedbacks. J. Climate, 18, 4216-4234.
http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175/JCLI3534.1
Full Presentation |
Steve Derbyshire, Met Office |
Title: From "dynamical thinking" to parametrization progress?
Author(s): Steve Derbyshire
Abstract: There are many motivations to study tropical "convection and dynamics"
but how far can we exploit theoretical progress to speed up
parametrization progress?
I will review a few simple examples of how we've used (or possibly
underused) dynamical or dynamical-feedback thinking, and discuss
potential benefit from further advances. Amongst other goals, we need to
understand mechanistically where any large-scale benefits of
structurally new techniques really originate, and conversely what is/is
not achievable through enhancement of current schemes.
Full Presentation |
Kerry Emanuel, MIT |
Title: Aggregated convection and the regulation of tropical climate
Author(s): Kerry A. Emanuel and Marat Khairoutdinov
Abstract: Simulations with cloud-permitting models run under horizontally homogeneous forcing in doubly periodic domains show that under certain circumstances, the near random spatial distribution of convective clouds collapses into one or a few superclusters. This appears to require, among other things, a strong enough feedback between convectively induced surface wind gusts and surface enthalpy flux; this in turn requires a high enough specified value of the sea surface temperature.
These simulations show that the phase transition to the self-aggregated state is accompanied by a dramatic drying of the atmosphere above the boundary layer. This implies that were the sea surface calculated rather than specified, self-aggregation could lead to a decrease of the SST owing to the dramatic reduction of water vapor, an important greenhouse gas. At the same time, smaller SST is not conducive to aggregation, so it is possible that the convection would disaggregate as the SST falls. This suggests that the actual model state might be attracted to the phase transition to aggregation, an example of self-organized criticality.
We test this idea first with a toy model, with very simple convective and radiative physics. This shows self-aggregation, provided there is sufficient feedback between convection and surface fluxes, and the system is indeed attracted to the phase transition and thus to a particular surface temperature. We will also report on preliminary efforts to demonstrate self-organized criticality in a cloud-permitting, full-physics model in which the surface temperature is calculated from the surface energy budget. We will discuss possible implications of these findings for tropical climate sensitivity.
Full Presentation |
Dragan Frierson, University of Washington |
Title: CCKWs and the MJO in a hierarchy of GCMs
Author(s): Dargan M. W. Frierson
Abstract: We present some recent results on convectively coupled Kelvin waves (CCKWs) in simplified and comprehensive GCMs, and the MJO in a series of fixed SST aquaplanet GCM simulations. A key unifying property of the CCKW work is the usefulness of the gross moist stability to explain the changes in phase speed across simulations. The gross moist stability can be changed by inhibiting the parameterized convection, and this affects the CCKW properties in these simulations even if some 2nd baroclinic mode structure exists in the waves.
We also present some work-in-progress results from a series of aquaplanet simulations with the GFDL AM2 model, using variations on the AMIP fixed SST boundary conditions suggested by Neale and Hoskins. A MJO-like wave can be generated in this model even with zonally symmetric SSTs, or with a Walker cell in which surface westerlies exist in part of the equatorial atmosphere.
Full Presentation |
Željka Fuchs, University of Split |
Title: Convectively coupled gravitywaves and moisture
modes
Author(s): Željka Fuchs, David J. Raymond and Sharon L. Sessions
Abstract: A simplified model for convectively coupled tropical disturbances in a nonrotating environment is presented. The rainfall rate produced by the model
is a function of tropospheric precipitable water and convective inhibition. The
sensitivity of precipitation rate to convective inhibition is represented by two
terms, one that represents the surface flux variations and the other representing
the variations in wave-induced buoyancy anomalies just above the planetary
boundary layer. Two types of unstable modes are predicted by this model, a
slowly moving “moisture mode” in which the primary control of precipitation is
the precipitable water, and a more rapidly propagating “gravity mode” in which
precipitation is controlled primarily by changes in convective inhibition.
The model is vertically resolved with an upper radiation boundary condition.
The heating profile has the form of the first baroclinic mode. This represents a
big simplification over the models of Mapes (2000), Majda and Shefter (2001b),
Khouider et al. (2006), etc., in which the two-mode vertical structure of the
equatorial Kelvin wave is linked to a similar two-mode structure in the vertical
heating profile. Our model shows that a simple first baroclinic mode heating
profile is sufficient by itself to produce the observed two-mode structure of
the Kelvin wave. As the gravity waves map onto equatorial Kelvin waves in
the earth’s atmosphere, we conclude that the modeled propagation speeds for
the gravity waves ( 18 19ms1) are close to the observed phase speeds of
convectively coupled equatorial Kelvin waves. The growth rate peaks at zonal
wavenumbers (l 5 7) at which Kelvin waves exhibit the greatest spectral
energy. Furthermore, the computed vertical structure, in particular the temperature
structure, matches that of observed Kelvin waves.
In a series of experiments using a cloud resolving model in two-dimensional
domain (7000 km in the horizontal, horizontal grid spacing 1 km and 2 km)
large-scale waves with phase speed of 15 m=s developed. All simulations were
run for 15 days. The analysis of the produced wave shows that it matches
a convectively coupled gravity mode, but an interesting point is that among
many parameters (humidity, CAPE, CIN) it showed the strongest correlation
with convective inhibition. A crucial hypothesis for convectively coupled gravity
wave in the analytical model is that the convection is largely controlled by the
wave-related adiabatic lifting of the capping layer just above the PBL. The
numerical simulations support this hypothesis.
The moisture mode produced by the model is very similar to that seen in
models which omit the dependence of precipitation rate on convective inhibition,
such as Fuchs and Raymond (2002, 2005, 2007), but concentrate on precipitation
rate being a strong function of precipitable water. Deep convection that
produces the precipitation must act to further moisten the atmosphere which
generally occurs when gross moist stability (GMS) is negative. The instability
of the moisture mode in our model depends on the existence of negative gross
moist stability (GMS). Cloud-radiation interactions and wind-induced heat exchange
(WISHE) are able to destabilize the moisture modes as well, but only
by virtue of supplying moisture feedback similar to those supplied by GMS.
The modeled moisture mode propagates to the east as a result of the WISHE
mechanism. This is reflected in the eastward shift of the heating maximum relative
to the buoyancy maximum. The buoyancy anomaly does not exhibit the
tilted structure seen in the gravity mode in the troposphere. However, in the
stratosphere an eastward tilt is seen, as is to be expected of an eastward-moving
disturbance. One difference between our results and those of Sobel et al. (2001)
is that negative gross moist stability (or at least a negative equivalent gross
moist stability including the effects of cloud-radiation interactions) is needed to
destabilize the moisture mode in our case. However, Sobel et al. (2001) show
that moisture modes can become unstable in a three-dimensional, rotating environment
exhibiting meridional moisture gradients even when the gross moist
stability is positive.
There are two big advantages to the presented model:
1. It is able to produce both the convectively coupled gravity mode and the
moisture mode.
2. The model reproduces the observed structure of the gravity mode without
imposing it a priori via complicated heating profiles associated with shallow
convection and stratiform rain.
The first element arises from the observationally driven realization that both
convective inhibition and tropospheric precipitable water act to control precipitation.
Our results suggest that different types of atmospheric phenomena over
tropical oceans act to produce precipitation primarily through one or the other.
The second element gets to a fundamental point of convectively coupled Kelvin
wave dynamics: Does the observed two-mode vertical structure result from the
cloud physics of stratiform rain areas or does it come from the intrinsic dynamics
of the Kelvin wave itself? Our results point to the latter and further suggest
that the evolution of cloud behavior through the life cycle of a Kelvin wave
passage is governed primarily by wave dynamics rather than cloud physics.
Identification of the moisture mode with actual tropical disturbances is still
in question, but it might be a candidate mechanism for easterly waves, monsoon
depressions and the Madden-Julian oscillation (MJO).
Full Presentation |
Dennis Hartmann, University of Washington |
Title: Tropical clouds and cloud feedback
Author(s): Dennis L. Hartmann
Abstract: Hartmann and Larson (2002) argued on the basis of physical reasoning and a radiative-convective equilibrium calculation with parameterized convection that tropical cloud anvil top temperatures should be insensitive to sea surface temperature because tropical convection is maintained in a near radiative-convective equilibrium state and the radiative cooling rate by water vapor, the principle emitting gas, is constrained by the abundance of water vapor, which at saturation is controlled only by temperature. This is called the Fixed Anvil Temperature (FAT) Hypothesis. This hypothesis was later confirmed in a cloud-resolving model calculations by Kuang and Hartmann (2007), and strongly supported in an observational study by Kubar, et al. (2007). Recently we have further explored the nature of the constraint on cloud top temperature using a cloud-resolving model in which the water vapor seen by the radiation code is modified. The cloud top temperatures cool when the upper tropospheric water vapor emission is increased and warm when it is decreased, as expected. Experiments in which the latent heating is altered have also been conducted to investigate the relative roles of radiative cooling and latent heating in determining the cloud top temperature at statistical equilibrium in the model. In addition, we have looked at the long wave cloud feedback in AR4 global warming simulations. The models produce uniformly positive cloud longwave feedback. We show that this positive cloud longwave feedback is consistent with the fact that the cloud temperatures in the AR4 models remain approximately constant as the surface warms, in basic agreement with the FAT hypothesis, although the models show a wide range of basic cloud distributions. In contrast to the longwave cloud feedback, the shortwave cloud feedback is highly variable from one AR4 simulation to the next. In a separate study, we show that cloud-resolving model calculations can be compared directly with MODIS satellite cloud data (Lopez et al., 2009). This reveals that in one particular cloud-resolving model the simulation of tropical anvil clouds is poor, as evidenced by too little cold cloud of intermediate optical depth, which we associate with the extension of anvil clouds in the Tropics. The comparison of the model clouds with the data can be improved by altering the simple bulk microphysics scheme to reduce the amount of cloud liquid water and increase the amount of ice cloud.
References:
Hartmann, D. L. and K. Larson, 2002: An important constraint on tropical cloud - climate feedback. Geophys. Res Lett., 29(20), doi:10.1029/2002GL015835.
Kuang, Z. M. and D. L. Hartmann, 2007: Testing the fixed anvil temperature hypothesis in a cloud-resolving model. J. Climate, 20, 2051-2057.
Kubar, T. L., D. L. Hartmann, and R. Wood, 2007: Radiative and Convective Driving of Tropical High Clouds. J. Climate, 20, 5510-5526.
Lopez, M., D. L. Hartmann, P. N. Blossey, R. Wood, C. S. Bretherton, and T. L. Kubar, 2009: A Test of the Simulation of Tropical Convective Cloudiness by a Cloud-Resolving Model. J. Climate, 22, 2834-2849.
Full Presentation |
Isaac Held, NOAA |
Title: Energy balance constraints on the tropical climate
Author(s): Isaac Held
Abstract: In theories for the tropical climate there is a tension between frameworks in which SSTs play
a central role and theories in which the surface energy fluxes are more fundamental. Some
new results are presented on this issue bases on the Ph.D. thesis of Sarah Kang (2009).
Sarah’s published results as they pertain to this issue will be summarized. In addition,
perturbing a model’s boundary layer physics will illustrate how the response to thermal
forcing that results in a displacement of the ITCZ is robust when posed as a response to an
energy flux perturbation, but is model dependent when viewed as a response to SST
anomalies.
Full Presentation
|
George Kiladis, NOAA |
Title: Forcing of convectively coupled Kelvin waves by extratropical wave activity
Author(s): George N. Kiladis
Abstract: Forcing of deep tropical convection by extratropical Rossby wave
activity is a well-known feature of regions of upper level westerly
flow. These interactions are commonly observed within South
Pacific and South Atlantic Convergence Zones (SPCZ and SACZ),
and in the eastern Pacific ITCZ during winter, where westerlies and
equatorward wave guiding by the basic state occur at low enough
latitudes to interact with tropical and subtropical moisture sources.
In these regions convection is commonly initiated by Rossby wave
activity ahead of upper level troughs, characteristic of forcing by
quasi-geostrophic dynamics. However, recent observational
evidence indicates that extratropical wave activity is also associated
with equatorial convection even in regions where there is a "critical
line" to Rossby wave propagation at upper levels, that is, where the
flow goes from westerly to easterly towards the equator. A common
manifestation of this type of interaction involves the initiation of
convectively coupled Kelvin waves. Kelvin waves are responsible
for a large portion of the convective variability within the ITCZ over
the Indian, Pacific, and Atlantic sectors, as well as within the
Amazon Basin of South America. The waves originating within the
western Pacific ITCZ are often triggered by Rossby wave activity
propagating into the Australasian region from the South Indian
Ocean extratropics. The resulting Kelvin waves frequently propagate
across the entire Pacific, and continue uninhibited across the Andes
into South America and even as far as Africa. At other times, Kelvin
waves are seen to originate along the eastern slope of the Andes. In
the latter case the initial forcing is sometimes linked to a low-level"pressure surge," initiated by wave activity propagating equatorward
from the South Pacific storm track. In yet other cases, such as over
Africa, the forcing appears to be related to wave activity in the
extratropics, which is not necessarily propagating into low latitudes,
but appears to "project" onto the Kelvin structure, in line with past
theoretical and modeling studies. Observational evidence for such
interactions will be presented, along with a review of some recent
theoretical work aimed at explaining their dynamical causes.
Full Presentation
|
Zhiming Kuang |
Title: Large-scale organizations of transient variabilities in tropical deep convection
Author(s): Zhiming Kuang
Abstract: Idealized aquaplanet simulations are performed using the superparameterized Community Atmosphere Model (SPCAM)1 and the Weather Research Forecast (WRF) model using the Diabatic Acceleration and Rescaling (DARE) approach, to study large-scale variations in tropical deep convection. No cumulus parameterizations are used in these two models, and both models, when run in realistic settings, produce realistic spectra of tropical deep convection, including convectively coupled waves and the Madden-Julian Oscillation (MJO). The roles of sea surface temperature distributions, radiative and surface heat flux feedbacks, horizontal advection of moisture are explored in a set of aquaplanet simulations, mostly with zonally symmetric SST distributions. The results indicate that radiative feedback is key to enhancing the low wavenumber-frequency variance, thus reddening the spectrum, while horizontal advection of moisture is responsible for the propagation of the low frequency, "MJO-like", disturbances in the simulations.
1 Courtesy of Marat Khairoutdinov.
Full Presentation
|
Eric Maloney, Colorado State University |
Title: Aquaplanet simulations of the MJO: The importance of wind-evaporation feedback and horizontal moisture
advection
Author(s): Eric D. Maloney, Adam H. Sobel and Walter M. Hannah
Abstract: The Madden‐Julian oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics, characterized
by eastward‐propagating coherent wind and precipitation fluctuations at 30‐60 day timescales. The MJO strongly
modulates tropical cyclones and monsoon variability, and occasionally initiates ENSO events. While the existence of
the MJO has been known since the early 1970s and is well-characterized in observations, an understanding of the
dynamics of the MJO has remained elusive. We still do not have a fundamental understanding of the spatial and
temporal scale selection of the MJO, its slow 5 m s‐1 eastward propagation, or its seasonality. General circulation
modeling evidence is presented that suggests the MJO is destabilized by wind‐evaporation feedbacks and
propagated eastward by horizontal moisture advection, and resembles an intraseasonal moisture mode.
An aquaplanet atmospheric general circulation model simulation with a robust intraseasonal oscillation is analyzed.
The SST boundary condition resembles the observed December-April average, although with the meridional SST
gradient reduced to be one-quarter of that observed poleward of 10o latitude. Slow, regular eastward propagation at 5
m s-1 in winds and precipitation with amplitude greater than that in the observed MJO is clearly identified in
unfiltered fields. The local relationship between precipitation rate and column precipitable water is strongly
increasing and nonlinear, as in observations. The model intraseasonal oscillation resembles a moisture mode that is
destabilized by wind-evaporation feedback, and that propagates eastward through advection of anomalous humidity
by the sum of perturbation winds and mean westerly flow.
Moistening of the troposphere occurs to the east of (in quadrature with) enhanced precipitation, and is dominated by
a column-integrated horizontal advection moistening rate of greater than 2 mm day-1. Zonal and meridional
moisture advection are of approximately equal amplitude, although meridional advection tends to damp tropospheric
moisture anomalies, while zonal advection propagates them eastward. At the time of peak moistening in the model,
the total zonal wind near 850 hPa is 5 m s-1. This value is approximately the same as the phase speed of the
intraseasonal disturbances; together with other diagnostics, this suggests that horizontal advection of moisture is the
dominant propagation mechanism. Latent heat flux is the second largest term in the intraseasonal moisture budget
and has a positive covariance with precipitation anomalies. A mechanism denial experiment in which intraseasonal
latent heat flux variability is removed largely eliminates intraseasonal wind and precipitation variability, although
weak, small spatial scale convective features that move slowly eastward, presumably through advection, are still
present in the simulation. Reducing the 850 hPa westerly flow in the model by reducing the zonal SST gradient
slows eastward propagation, supporting the importance of horizontal advection by the low-level wind for eastward
propagation.
Aquaplanet simulations with different basic states are also conducted and discussed. An SST boundary condition
that resembles the December-April average produces realistic amplitude intraseasonal wind variability, although
precipitation contains more variance at low frequencies than in observations. A zonally symmetric SST basic state
produces weak and unrealistic intraseasonal variability between 30 and 90 day timescales, indicating the importance
of mean low-level westerly winds and hence a realistic phase relationship between precipitation and surface flux
anomalies for producing realistic tropical intraseasonal variability.
Full Presentation |
Brian Mapes, University of Miami |
Title: Capturing convection sensitivities in a plume-based algorithm suitable for GCM parameterization
Author(s): Brian Mapes
Full Presentation |
David Neelin, UCLA |
Title: Deep convection--transition and tails
Author(s): David Neelin
Abstract: After briefly reviewing recent observational results that provide ways to collapse the
statistics of the onset of strong tropical deep convection, this talk will focus on some
aspects related to strong events. The probability distribution for column integrated water
vapor (CWV) under precipitating conditions has a Gaussian core but an approximately
exponential tail. The exponential tail is associated with much more frequent excursions
into the high precipitation regime than if Gaussian statistics applied. Prototypes from
passive tracer forced-advection-diffusion problems exhibit such tails. Evidence that these
prototypes are widespread for three-dimensional tropospheric transports with complex
sources is provided using other tracers and CWV in non-precipitating regimes.
Full Presentation |
David Nolan, University of Miami |
Title: Idealized Simulations of the ITCZ
and its Multi-level, Cross-Equatorial Circulations
Author(s): David S. Nolan, Scott W. Powell and Chidong Zhang
Abstract: A mesoscale numerical model with an idealized, tropical channel environment is used to study the dynamics of intertropical convergence zones (ITCZs). Prior work with a similar model illustrated the dynamics and variability of the meridional circulations associated with the ITCZ, including the recently discovered shallow meridional circulation and the dry, mid-level inflow. This work is extended to model domains that span across the equator, allowing for more realistic cross-equatorial circulations for both equatorially symmetric and asymmetric sea surface temperature (SST) distributions.
In agreement with recent findings based on several reanalysis products, the simulations show that both the shallow meridional circulation and the mid-level inflow are robust features of the ITCZ when the meridional temperature gradient is strong. When the SST maximum is off the equator, the meridional circulations become highly asymmetric with strong cross-equatorial components. For broader SST profiles, a double ITCZ appears with very weak meridional circulations. The prior result that the shallow meridional circulation is a dynamic, sea-breeze-like response to surface temperature gradients is further supported, while the mid-level inflow is found to be closely related to upper-level maxima in diabatic heating and vertical motion.
Since the shallow return flow and mid-level inflow are typically not well resolved in lower-resolution climate simulations, their contributions to the regional budgets of water and moist static energy (MSE) are considered. When present, the shallow return flow transports substantial amounts of water and MSE out of the ITCZ region. The water transport of the mid-level inflow is minimal, but the MSE transport is significant, accounting for 20-40%percent of the overall budget. The possible implications on the larger-scale circulations by the neglect of the shallow and mid-level circulations are discussed.
Full Presentation |
Olivier Pauluis, New York University |
Title: Water vapor and mechanical work in the atmosphere: comparison between Carnot and steam cycles
Author(s): Olivier Pauluis
Abstract: The impact of water vapor on the production of kinetic energy in the atmosphere are discussed here by comparing two idealized thermodynamic cycles: the Carnot cycle and the steam cycle. A steam cycle transports water from a warm, moist source to a colder, dryer sink. It can be viewed as a heat engine in which the energy source is the latent heat of evaporation. It is shown here that a steam cycle always produces less work than the corresponding Carnot cycle. The degree of saturation in the cycle has a direct impact on its mechanical output. It is argued that such reduction of the work
produced in a steam cycle is directly tied to the irreversible entropy production associated with the diffusion of water vapor.
The Carnot and steam cycles can be combined into a mixed cycle that is heated by a combination of sensible and latent heating at the warm source. It is argued that such cycle can provide some practical insight on the work generated by atmospheric circulation. In particular, it is shown that the
expression for the buoyancy flux in stratocumulus convection is directly related to the work performed by a mixed cycle.
This analysis of idealized thermodynamic cycles furthermore indicates that the work performed
by atmospheric circulations should depend on four key parameters: the total energy transport, the temperature difference between the energy source and sink, the Bowen ratio, which measures the partitioning between the sensible and latent heat transports, and the relative humidity of the atmosphere.
Full Presentation |
David Raymond, New Mexico Tech |
Title: On moist convective adjustment
Author(s): David J. Raymond
Abstract: Moist convective adjustment has been largely superseded by mass flux schemes in the
parameterization of convection in global atmospheric models. However, it still constitutes a
useful conceptual tool and a basis for simplified convective parameterizations in large-scale
models of intermediate complexity, such as those of Neelin and Raymond.
The Neelin scheme (which is a simplified form of the Betts-Miller parameterization) in
its simplest manifestation relaxes both the temperature and mixing ratio profiles to predetermined forms with a very short time scale. However, imposed on top of this relaxation
is an additive correction to the temperature profile which enforces conservation of vertically
integrated moist static energy. This is needed because the vertical rearrangement and mixing
of parcels which occurs in convective adjustment, even with condensation, evaporation, and
precipitation, does not change this integral.
Raymond's scheme takes a different approach by assuming that the adjustment process
tends to homogenize the vertical profiles of the quasi-conserved variables moist entropy and
water vapor mixing ratio. In the presence of non-conservative processes, the total tendencies
of these variables are the sums of conservative mixing and non-conservative tendencies. Due
to the rapid development of precipitation under saturated conditions, the mixing ratio profile
is subject to much more rapid non-conservative modification than the moist entropy profile.
Thus, the vertically averaged water vapor changes much more quickly than the vertically
averaged moist entropy. As a consequence of the short-term conservation of entropy, vertically
averaged temperature fluctuations are anticorrelated with moisture fluctuations on short time
scales, as in the Neelin model.
Observations indicate that deep convection covers a small fractional area, with convection
suppressed elsewhere as a result of convective inhibition by a stable layer just above the
boundary layer. This is true even over tropical oceans. A typical global model grid cell
contains both suppressed and convectively active regions. Raymond's method handles this
by applying convective adjustment separately to the planetary boundary layer and the full
depth of the unstable troposphere, computing the resulting net tendencies as a weighted
average of the two. The weighting factors are determined by the value of the convective
inhibition.
Plausible estimates of throttling, convective mixing rates, and non-conservative processes
such as precipitation, surface fluxes, and radiation yield interesting transient and equilibrium
calculations of temperature and moisture profiles. The advantage of this scheme over the
Betts-Miller and Neelin schemes is that such profiles are predicted a priori and are not
imposed externally. Furthermore, the variation of these profiles with changes in various
physical processes can be explored. Finally, comparison with the results of cloud-resolving
models run under similar conditions provides insight into such models' behavior.
Full Presentation |
David Romps, Harvard University |
Title: A direct measurement of entrainment
Author(s): David M. Romps
Abstract: Evidence is mounting that convectively coupled waves,
including the MJO, are made possible by the sensitivity of convection
to mid-tropospheric humidity. In turn, the sensitivity of convection
to mid-tropospheric humidity is dictated by the convective entrainment
rate. But, just how entraining is convection?
To date, we have relied on estimates of entrainment diagnosed using
the bulk-plume equations. But, convection is nothing like a bulk
plume: in-cloud heterogeneity is quite large and is an essential
feature of convection. By using the bulk-plume equations to diagnose
entrainment, I argue that we have been underestimating entrainment
rates and underestimating the sensitivity of convection to
environmental humidity.
I will introduce a technique for directly measuring entrainment and
detrainment at the grid-cell level. Comparing the direct measurement
technique to the bulk-plume method in large-eddy simulations of
shallow and deep convection, it is found that the bulk-plume equations
underestimate entrainment by a factor of two. When those bulk-plume
entrainment rates are used in convective parameterizations, they will
produce convection that is too insensitive to humidity, which will
likely result in spuriously weak convectively coupled waves.
With the new estimates in hand, it is possible to evaluate existing
proposals for the dependence of the entrainment rate on height and
buoyancy. Also, since direct measurement produces spatially and
temporally resolved entrainment and detrainment rates, it is possible
to visualize the entrainment and detrainment process and, in future
work, to correlate entrainment and detrainment with the local
properties of the cloud and the environment.
Full Presentation |
Sharon Sessions, New Mexico Tech |
Title: The relevance of gross moist stabilities in multiple equilibria
Author(s): Sharon L. Sessions, Satomi Sugaya, David J. Raymond and Adam Sobel
Abstract: We have recently performed a series of numberical experiments to investigate the conditions which support multiple equilibria in a cloud resolving model. The equilibrium states correspond to either a dry, non-precipitating state or one with persistent deep convection. Several factors control the existence of multiple equilibria in our model, including: 1) surface fluxes, 2) the size of the modeled domain, and 3) the time for the local vertical profile of potential temperature to relax to the prescribed reference profile. The time scale is used in the implementation of the weak temperature gradient approximation. In regimes where both equilibrium states exist, the initial moisture dictates which state the model converges to.
We define the gross moist stability (GMS) to be the ratio of moist entropy export to moisture stages of convection. In the precipitating state, the GMS is larger than 0.3. In the dry state, all equilibrium values of GMS are smaller than 0.3, and can even become negative. Negative values of GMS in this case correspond to a domain which is exporting both moisture and moist entropy.
Several experiments which were initiated with a completely dry troposhere eventually developed convection and converged to the precipitating equilibrum. These cases provided an opportunity to analyze the transient stages of convection, and the corresponding evolution of GMS. An initially dry domain typically exports moisture and imports moist entropy (GMS > 0). If the boundary conditions are unable to support a dry equilibrium, the free troposphere will begin to moisten, resulting in a net moisture import. As convection develops, moist entropy will evolve from net import to export. The sign change for moisture and moist entropy convergence need not occur simultanenously, which results in a short time where the system is importing both quantities. In this time period the GMS is negative. The mature stages of convection export moist entropy and import moisture, which results in a postive GMS in the precipitating equilibrium.
Full Presentation |
Adam Sobel, Lamond-Doherty Earth Observatory |
Title: The axisymmetric QTCM2 prototype--a survey of recent results
Author(s): Adam Sobel
Abstract: Sobel and Neelin (2006) developed a prototype version of what we have thought of
as “QTCM2” – a model based on the original QTCM1 but with the addition of a
prognostic atmospheric boundary layer. We constructed an axisymmetric
(longitude‐independent) version of this model, with the intent of eventually
developing a full “3D” version. The latter is still in development, but in the
meantime we (mostly Gilles Bellon, now at CNRS, Meteo‐France, Toulouse) have
performed a number of studies using the axisymmetric version, of which I will
present an overview. The phenomena addressed include:
1. ITCZ dynamics – we find that horizontal diffusion of moisture is important to
obtaining a reasonable steady solution, something that has found some
support in observational studies (Back and Bretherton, Peters et al.)
2. Northward propagating Indian monsoon rainbands – these are (perhaps
surprisingly) easy to simulate well in this model, and the construction of the
model allows the key mechanisms to be unambiguously identified.
3. Multiple equilibria – for some SST profiles, the Hadley circulation and ITCZ
have multiple stable equilibrium states. These map onto multiple equilibria
of a single‐column version of the model under the WTG approximation, when
the latter is solved at a particular point of interest (the one which either does
or does not contain a second ITCZ).
4. Comparison to observations – we have run the model over SST
representative of the eastern Pacific and compared the resulting atmospheric
solution to observations. The results are not spectacular but have some
interesting features.
Links to papers on this work are below. The first is on topic 1, the next three are on
topic 2, and the last (still in review) is on topic 3; nothing has been published (or
submitted) on topic 4.
http://www.appmath.columbia.edu/users/sobel/Papers/sobel_neelin_tcfd_06.pdf
http://www.appmath.columbia.edu/users/sobel/Papers/bellon_sobel_2008a.pdf
http://www.appmath.columbia.edu/users/sobel/Papers/bellon_et_al_08.pdf
http://www.appmath.columbia.edu/users/sobel/Papers/bellon_sobel_08b_JGR.pdf
http://www.appmath.columbia.edu/users/sobel/Papers/ME_Hadley.pdf
Full Presentation |
Stefan Tulich, NOAA |
Title: On the pattern of biases in a regional climate simulation of the tropics
Author(s): Stefan Tulich
Abstract: This talk will discuss the performance of the WRF model in simulating the global tropics at 36-km horizontal grid spacing with convection parameterized using the Kain-Fritsch scheme. Results show that several key synoptic modes of variability are well captured, including convectively coupled Kelvin and easterly waves. However, significant biases in wave activity are seen, with easterly (Kelvin) waves being generally more (less) active than observed. Evidence is presented to suggest that these biases in wave activity (which are also correlated with biases in time-mean rainfall, as well as biases in the frequency of tropical cyclogenesis) stem from convection in the model coupling to strongly to rotational circulation anomalies. Possible explanations for this overly tight coupling between convection and rotation will be discussed.
Full Presentation |
Chidong Zhang, University of Miami |
Title: Tropical diabatic heating: Its bi-modal ubiquity and its MJO signals
Author(s): Chidong Zhang
Full Presentation |
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