The heat balance of the equatorial trough zone, revisted: Part II (Invited)
More than fifty years ago the seminal study of Riehl and Malkus provided estimates of the energy balance of the equatorial low pressure trough zone in coordinates following the trough around the globe. Out of that study evolved the concept of convective hot towers and their fundamental role in the heat balance of this zone among other topics. Twenty years later Riehl and Malkus (Simpson) revisited that study using updated global data that existed at that time noting some significant differences in the radiation balance between the two studies. Since then dramatic improvements in our ability to observe the global atmosphere and create globally consistent data records of the atmosphere have occurred with new satellite observations and re-analysis appearing at the apex of this improvement. This talk revisits the study of the heat balance of the equatorial trough incorporating these new and improved observational resources. The heat balance of the region will be documented, including the nature of convection and the frequency of hot towers, the radiation balance and the influence of different cloud types on this balance and the latent heating characteristics of the trough zone.
Large-scale tropical transients in aquaplanet simulations with zonally symmetric sea surface temperature distributions (Invited)
Spectral analyses of sub-seasonal variations of tropical convection revealed features such as convectively coupled equatorial waves (CCEW) and the Madden-Julian Oscillations (MJO) over a red noise background. In this work, the super-parameterized Community Atmosphere Model (SPCAM) is used in aquaplanet experiments forced with zonally symmetric sea surface temperature distributions to investigate the roles of various processes in shaping the tropical spectra. Control experiments with the SPCAM model were able to produce the red noise background spectrum, CCEWs, and in some Intertropical Convergence Zone (ITCZ) configurations, “MJO-like” disturbances. To unravel the roles of various processes, experiments with simplified dynamics/settings are performed. In experiments where the large-scale dynamics in the model is largely linearized and with no feedbacks from radiative heating or surface sensible/latent heat and momentum fluxes, the spectra of large-scale tropical convectively coupled transients are dominated by the CCEWs, in ways generally consistent with results from the simple model of Andersen and Kuang (2008), and there are no red noise background spectra. Additional experiments show that the red noise aspect of the spectrum is mostly due to eddy stirring of the moisture field across its meridional gradient at the edge of the ITCZ, in particular the deep dry intrusions from the subtropics to the tropics. We will also discuss the effects of surface friction and idealized moist static energy sources, and use a simple model to understand these behaviors. It is hoped that through these and additional idealized studies, the various mechanisms that shape the tropical spectra can be elucidated. Ref:Andersen, J. A., Z. Kuang, A toy model of the instability in the equatorially trapped convectively coupled waves on the equatorial beta plane, Journal of Atmospheric Sciences, 65, 3736-3757, (2008)
A Multi-Scale Interaction Model for Madden-Julian Oscillation
Madden-Julian oscillation (MJO) is an equatorial, planetary scale circulation system coupled with a multi-scale convective complex. The nature and roles of multi-scale interaction (MSI) on MJO dynamics has not been well understood. Here we formulate a prototype theoretical model to advance our understanding the MSI in MJO. The model integrates three essential elements: a) large scale equatorial wave dynamics driven by boundary layer frictional convergence instability (FCI), b) effects of multi-cloud heating and an instability arising from synoptic system-induced convective momentum transfer (CMT), and c) interaction between the planetary and synoptic systems. We show that the CMT mechanism tends to yield a growing stationary mode with a quadrupole-vortex horizontal structure (enhanced Rossby wave component); whereas the FCI favors a fast eastward-moving mode with a Gill-Pattern structure (enhanced Kelvin wave response). The MSI instability can stem from either FCI or CMT mechanisms or both, depending on the ratio of deep convective versus stratiform/congestus heating. With increasing stratiform/congestus heating, the FCI weakens while the CMT becomes more effective. A growing MSI mode has a mixed horizontal structure of CMT and FCI and prefers slow eastward propagation. The FCI sets the eastward propagation, and CMT plays a vital role in slowing down the propagation speed. These results encourage further observational diagnosis of multi-cloud structure and heating profiles in the MJO convective complex and improvement of models’ capability in reproducing correct partitioning of cloud amounts between deep convective and stratiform/congestus clouds.
Leading modes of submonthly tropical convective activity
An Empirical Orthogonal Function (EOF) analysis is undertaken of global tropical (20S-20N) brightness temperature data filtered to retain fluctuations on various synoptic (<10 day) to submonthly (<30 day) time scales. This filtering specifically removes signals in deep convection associated with the Madden-Julian Oscillation (MJO), but retains most of the power associated with other convectively coupled equatorial waves (CCEWs). The analysis can also be further restricted to westward or eastward propagating modes using space-time filtering techniques. The leading modes broadly correspond to known CCEW disturbances, although in some cases the analysis reveals a mixture of such waves. One example of this mixture is associated with the leading mode of <10 fluctuations in ITCZ/SPCZ convection over the west Pacific warm pool, where antisymmetric (out of phase) variations in convection on either side of the equator propagate poleward in both hemispheres over time. Spectral analysis suggests that this mode is comprised of mixed Rossby-gravity (MRG) and eastward inertio-gravity (EIG) waves, and this structure is confirmed by projecting the signals of dynamical fields onto the principal components. In contrast, the leading mode of <10 westward variance is characterized by mixed Rossby-gravity waves that originate near the dateline, which then transform into easterly waves as they approach the region of New Guinea. Slower westward modes can be identified as Equatorial Rossby waves or "Rossby gyres" confined to one hemisphere. The latter are common over southeast and south central Asia during the northern summer monsoon season. In general many of the modes also have strong extratropical signals associated with them, and extratropical forcing of the equatorial wave activity is unambiguous based on their lead-lag relationships. The principal component time series generated allows the analysis of temporal fluctuations in "wave activity", and the associated basic state changes accompanying these changes. While the Madden-Julian Oscillation (MJO) is prominent in modulating equatorial wave activity in many cases, there are other intraseasonal changes to especially the zonal wind that appear to be even more important.
Multi-scale energy conversion during composite Madden-Julian Oscillation
Kinetic and potential energy conversions during composite Madden-Julian Oscillation (MJO) are analyzed in a multi-scale framework. Based on wavenumber spectrum, all quantities are decomposed into three separable scales: zonal mean, MJO scale with wavenumber 1-4, and small scale with wavenumber larger than 4. Hence, the energy cascade related to the composite MJO event can be explicitly resolved. The energy equations with the three scales are derived, focusing on the MJO scale. All variables used for calculation are obtained from NCEP reanalysis, with an exception of convective heating which is obtained from YOTC products. Results show that there are three major processes which contribute to the enhanced kinetic energy during MJOs. One is the energy gain from the potential energy on the MJO scale, the second one is the influences from the zonal boundaries, and the last one is horizontal advection by zonal mean currents. As a sink of kinetic energy on the MJO scale, there is significant energy transfer from the MJO scale to small scale perturbations in the west of the convection centers during MJOs, which indicates that MJOs (as the environment of convection) provide energy to small scale convection. For potential energy on the MJO scale, convective heating contributes to the increase of potential energy, which then converts to the kinetic energy of MJOs straightforwardly. Usually, there are two convection centers during MJOs, one in the central Indian Ocean and the other one in the western Pacific Ocean. Based on our analysis, the energy budget for the two convection centers are different, which implies that different mechanisms dominate the two convection centers during MJOs.
PV generation for the MJO, convectively coupled Rossby and Kelvin waves
One distinction between the MJO, convectively coupled Rossby wave and Kelvin wave is that the potential vorticity (PV) of the MJO and Rossby waves is very strong but nearly zero for the Kelvin wave. To investigate under what circumstances convection would lead to initiation of one of these tropical disturbances, this study examines the PV generation during their initiation stages using recent global reanalyses (MERRA and ERA-Intrim). Both vertical and horizontal distributions of PV generation are compared for the three types of tropical disturbances. It is found that PV generation by diabatic heating is not negligible for the Kelvin wave but it is canceled by other processes (e.g., PV advection). For the MJO and Rossby wave, PV generation by diabatic heating is much stronger. It starts from the low levels and is quickly elevated as the heating evolves from being shallow to deep. The major difference between PV generation for the MJO and Rossby wave is that there is a strong equatorial component of PV generation for the MJO, which is absent for the Rossby waves.
ARM Data sets for the Year of Tropical Convection (YOTC)
The Atmospheric Radiation Measurement (ARM) Climate Research Facility operates three measurement sites in the Tropical Western Pacific (TWP) region at Manus, Papua New Guinea (2.06° S, 147.43° E), Nauru Island (0.52° S, 166.92° E), and Darwin, Australia (12.42° S, 130.89° E). These well-instrumented ground sites provide continuous, 24-hour measurements of thermodynamic, cloud, aerosol, and radiation parameters beginning in 1996. Here, we present an overview of the measurements and value-added products (VAPs) available from the ARM tropical sites during the Year of Tropical Convection (YOTC) period. These data sets include twice-daily rawinsondes, profiles of cloud occurrence from radar and lidar measurements, and estimates of shortwave and longwave cloud radiative forcing from surface broadband radiometers. In addition, we describe where to find information on the YOTC data sets on the ARM website and how to download YOTC data sets from the ARM archive.
Investigation of the physical mechanisms responsible for the recent MJO forecast improvements in the ECMWF model during the YoTC period
The MJO is the major mode of intraseasonal variability in the tropics. It encompasses interactions between sub-grid scale convective processes and the large-scale dynamical circulation as well as between the sea-surface and the atmospheric boundary-layer. The MJO has significant influence on other components of the global system and therefore provides an important source of atmospheric predictability. Improving the representation of the MJO, and the teleconnection patterns associated with it, is an important aim for weather centres such as ECMWF as it enables improvements in forecast skill to be made on seasonal and sub-seasonal timescales. Recent modifications were made to the convection scheme in the ECMWF Integrated Forecasting System (IFS). A comparison of forecasts made over the YoTC period using the IFS with the old and new versions of the convection scheme shows a clear improvement in the representation of the MJO. One consequence of the convection scheme change is that the mid-tropospheric humidity, thought to play an important role in preconditioning the atmosphere to allow for the eastward propagation of convection, has changed from being anomalously dry to anomalously moist. Corresponding differences can be seen in the vertical structure of cloud. The old convection scheme produces a bimodal distribution of cloud with thick layers in the upper and lower troposphere whereas the new convection scheme produces a more trimodal structure with a third layer of cloud at midlevels. Current work involves validating these differences against CloudSat satellite data.
Evaluating the Community Atmospheric Model (CAM) against satellite data during YOTC
We examine short-term forecasts along the GCSS Pacific cross-section to evaluate parameterizations in the Community Atmospheric Model versions 3, 4 and 5 and highlight major improvements in the recent releases. Climate models are routinely validated against various time-mean statistics based on observations. However, it is difficult using this validation method to determine whether a reasonable climate simulation is achieved due to the correct interaction of processes or due to compensating errors, which are difficult to untangle in time-mean diagnostics. It is also problematic when trying to attribute any mean biases to a particular parameterized process. An innovative method to evaluate parameterizations in climate models is to use the Cloud-Associated Parameterization Testbed (CAPT) approach where the state of the atmosphere is initialized using realistic conditions and the model is run in a series of short-term forecasts. This approach allows a direct comparison of the parameterized variables (e.g. clouds, precipitation, radiation) with the time varying observations. Therefore, it is possible to gain insight into parameterization deficiencies and to diagnose the processes responsible for the drift away from observed. The YOTC period is particularly relevant for such a study because of the availability of state-of-the-art analyses to initialize the model and a wealth of integrated observational datasets to evaluate the forecasts. The forecasts are initialized from analyses (ECMWF-YOTC and GMAO-MERRA) and the CAM is run for 5-day forecasts throughout the whole period and evaluated against A-train, AIRS, TRMM, SSM/I, ISCCP and CERES satellite-based products. Our focus is the GCSS cross-section case study region that runs from the stratocumulus regions off the coast of California, across the shallow convection dominated trade winds, to the deep convection regions of the ITCZ. It is particularly relevant for such a comparison because it includes several important cloud regimes and their interactions through the large-scale circulation. The mean forecast biases grow very quickly along the cross-section, and after 5 days, the error pattern is very similar to the mean climate error. The CAM3 significantly overestimates the temperature along the Pacific cross-section. These warm biases are attributable to deep convection errors and their propagation to the other cloud regimes. In CAM4, the local tropical errors are reduced and grow much more slowly as a result of a modified deep convection scheme. This leads to a dramatic improvement of the precipitation in the ITCZ region. In association with the adjusted atmospheric state, the biases are also reduced away from the region of significant deep convection (i.e. in the shallow cumulus and stratocumulus regimes). This clearly illustrates the interaction between the tropical convection and the large-scale circulation. Similar forecasts with CAM5, which includes a two-moment microphysics scheme, show further reduction in upper troposphere errors. Additionally, a more realistic representation of cloud-topped boundary layers deepens the boundary layer and produces a more accurate location for low-level clouds.