Linear response functions of cumulus ensembles to temperature and moisture perturbations
To study the large-scale organization of cumulus ensembles, it is useful to separate the system into a large- scale dry component and a moist component that consists of the cumulus ensemble and consider the interaction of two. Because the response of the large-scale dry circulation to convective heating is well described by current large-scale models, the key of this problem lies in how cumulus ensembles respond to changes in the large-scale conditions. In this study, a cloud-system-resolving model (CSRM) is used to assess such responses. A set of small perturbations in temperature and moisture are introduced to the CSRM to obtain its linear response functions to such perturbations. The convective adjustment process is then studied in terms of an eigenvalue problem and implications to convectively coupled waves will be discussed.
An Observational Case Study of a Convectively Coupled Tropical Wave
The dynamic and thermodynamic structure of a convectively coupled tropical wave is constructed by using measurements from a network of balloon sounding stations and satellite as well as operational analysis during the NASA Kwajalein field experiment. The sources and sinks of lower tropospheric moisture associated with the wave are analyzed. The gradual deepening of a moist layer of air is shown to arise from the succession of boundary turbulence, shallow convection, and cumulus congestus. The subsequent development of deep convection and its phase relationship with the wave are presented. I will finally discuss the role of the wave in preconditioning the deep convection, and the impact of deep convection on the dynamic behavior of the wave.
Energy Sources for Madden-Julian Oscillations
The energy sources for the enhanced perturbed kinetic energy during Madden-Julian Oscillations (MJOs) are calculated with the NCEP/NCAR reanalysis. According to the energy equation, there are three main sources for the perturbed kinetic energy: perturbed potential energy, mean kinetic energy, and the energy source due to sea surface heat flux. Fifty-four MJO events are determined based on the MJO index from 1979 to 2006. Averaging over all 54 MJO events, about 82% of the total energy sources for the enhanced perturbed kinetic energy come from the perturbed potential energy, about 20% of the total energy sources come from the surface heat flux, while about -2% go to the mean kinetic energy, which is actually a weak sink for the perturbed kinetic energy. In most MJO events, the energy is derived from the perturbed potential energy and the ocean can feedback to the atmosphere, prolonging the period of MJOs, which can explain why the simulated MJO lifetime becomes longer and thus more consistent with the observations in some ocean-atmosphere coupled model experiments. Some MJO events are driven by a reinforced surface heat flux, which is caused by warm sea surface temperature anomalies induced by the internal oceanic variabilities and occurring about 5 days before the deep convection. During the transition period between monsoon seasons, some MJO events can be triggered by the energy transferred from the mean kinetic energy due to the barotropic instability. Although all MJO events are characterized as intraseasonal oscillations in the atmosphere, they have various energy sources which indicate that the observed MJOs are probably a consequence of multiple-mechanisms. The relative importance of various energy sources can shed light on how to apply appropriate theories and how to resolve appropriate processes to explain and better simulate different MJO events.
A one-dimensional cloud model with trimodal convective outflow
We describe a new buoyancy based parcel model that may be useful as a convective parameterization. It produces distinct deep and congestus outflow modes. The relative strength and height of these modes are in good agreement with observations. Congestus outflow is triggered by the Melting Level Stability Anomaly (MLSA). The MLSA is an anomalous variation in lapse rate near the melting level caused by a rapid increase in the stratiform downdraft mass flux. The model also simulates the observed early morning peak in total rainfall over the ocean, and the late afternoon peak in congestus rainfall. The amplitude of the diurnal variation in rainfall over the ocean can be attributed to the weak buoyancies of convective updrafts.
Mesoscale convective systems and critical clusters
Size distributions and other geometric properties of mesoscale convective systems (MCS), identified as clusters of adjacent pixels exceeding a precipitation threshold in satellite radar images, are examined. Based on percolation theory, cluster properties are expected to be highly sensitive to changes in the rainfall probability, and thereby to changes in the water vapor. To confirm this we categorize clusters by their prevalent water vapor. As expected, mean cluster size and radius of gyration strongly increase as a critical water vapor is approached from below. In the critical region we find scale-free size distributions spanning several orders of magnitude. Large clusters are typically from the critical region: at low water vapor most clusters are small, and super-critical water vapor values are too rare to contribute much. The perimeter of the clusters confirms previous observations in field- and model-data of robust non-trivial scaling. The well-known area-perimeter scaling is fully compatible with the quantitative prediction from the plausible null-model of gradient percolation, where the accessible hull is a fractal object with dimension 4/3.
Critical Roles of the Stratiform Rainfall in Sustaining the Madden-Julian Oscillation: GCM Experiments
This study assesses the impact of stratiform rainfall (a. k. a., large-scale rainfall) in the development and
maintenance of Madden-Julian Oscillation (MJO) in a contemporary general circulation model: ECHAM-4 AGCM
and its coupled version. To examine how the model MJO would change as the stratiform proportion (the ratio
of the stratiform versus total rainfall) varies, a suite of sensitivity experiments has been carried out under a
weather forecast setting and with three 20-year free integrations. In these experiments, the detrainment rates of
deep/shallow convections that function as water supply to stratiform clouds were modified, which results in
significant changes of stratiform rainfall.
Both the forecast experiments and long-term free integrations indicate that only when the model produces a
significant proportion (0.3) of stratiform rainfall, a robust MJO can be sustained. When the stratiform rainfall
proportion becomes small, the tropical rainfall in the model is dominated by drizzle-like regimes with neither
eastward- propagating nor northward-propagating MJO being sustained.
It is found that the latent heat release of stratiform rainfall significantly warms up the upper troposphere. The
co-variability between the heating and positive temperature anomaly produces eddy available potential energy
that sustains the MJO against dissipation and also allows the direct interaction between the precipitation
heating and large-scale low-frequency circulations, which is critical to the development and maintenance of the
MJO. This finding calls for better representations of stratiform rainfall and its connections with convective
components in GCMs in order to improve their simulation and prediction of MJO.
Effects of Entrainment on Closure Assumptions in Convection Parameterization
This study investigates the effect of entrainment dilution on CAPE and closure assumptions in convection parameterization using the sounding data from three Intensive Observation Periods. It shows that entrainment of the environmental air has a strong dilution effect on CAPE, and this effect depends on the degree of subsaturation of the entrained air: the drier the entrained air, the larger the effect. For CAPE-based closure assumptions, the dilute CAPE has a moderate correlation with convective removal of CAPE. For quasi- equilibrium-based closures, while the free tropospheric quasi-equilibrium assumption is a superior closure for convection when undiluted CAPE is used, both the Arakawa-Schubert quasi-equilibrium closure and the free tropospheric quasi-equilibrium closure work well for dilute CAPE in all three IOPs studied. It is further shown from the CAPE definition and the large-scale temperature budget equation that for undiluted CAPE the free tropospheric large-scale CAPE change and precipitation are approximately linearly related. The most important effect of entrainment dilution on CAPE and convection parameterization closure assumptions is to enhance the role of free tropospheric humidity, thereby diminishing the overwhelming control of boundary layer air on undiluted CAPE and its variation. Single column model tests will be performed to evaluate the effect of the incorporation of entrainment in convection parameterization on model simulation.
The transition to strong convection in observations and models
An overview will be provided of recent observational results for the statistics of the onset of strong tropical deep convection. This will include properties for which forced tracer advection problems appear to provide a good prototype, results that indicate a strong analogy between this onset and the properties of a continuous phase transition, as well as diagnostics of temporal relationships among tropospheric water vapor, temperature and precipitation. These observational results based on combinations of data from Tropical Rainfall Measuring Mission, AQUA, and the Atmospheric Radiation Measurement program, provide a baseline for comparison of initial results for corresponding quantities in re-analysis, climate model and mesoscale model data sets.