Workshop on Large-Scale Circulations in Moist Convecting Atmospheres

October 16 - 17, 2009


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.


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

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.

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.

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

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 􀀀 19ms􀀀1) 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.


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.

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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