Year of Tropical Convection (YOTC): A Joint WWRP and WCRP Activity to Address the Challenges of Multi-Scale Organized Convection
The realistic representation of tropical convection in our global atmospheric models is a long-standing grand challenge for numerical weather prediction and climate projection. To address this challenge, WCRP and WWRP/THORPEX have proposed a Year of coordinated observing, modeling and forecasting of organized tropical convection and its influences on predictability. This effort is intended to exploit the vast amounts of existing and emerging observations, the expanding computational resources and the development of new, high-resolution modeling frameworks, with the objective of advancing the characterization, diagnosis, modeling, parameterization and prediction of multi-scale convective/dynamic interactions, including the two- way interaction between tropical and extra-tropical weather/climate. This activity and its ultimate success will be based on the coordination of a wide range of ongoing and planned international programmatic activities (e.g., GEWEX/CEOP/GCSS, THORPEX, EOS, AMY), strong collaboration among the operational prediction, research laboratory and academic communities, and the construction of a comprehensive data base consisting of satellite data, in-situ data sets and global/high-resolution forecast and simulation model outputs relevant to tropical convection. The target time frame for scientific focus is May 2008 to October 2009, and was chosen as a period that would leverage the most benefit from recent investments in Earth Science infrastructure and overlapping programmatic activities (e.g., AMY, T-PARC). Specific areas of emphasis identified in YOTC are: 1) MJO and convectively coupled waves, 2) diurnal cycle, 3) easterly waves and tropical cyclones, 4) tropical-extratropical interactions, and 5) monsoon. This presentation will describe the development of this activity, its current status and planned programmatic framework and research agenda.
Multiscale Organization of Equatorial Waves
This contribution will discuss some observational aspects of the organization of equatorial rainfall systems. It is well-known that convective disturbances in the tropics occur over a very broad spectrum of scales, ranging from individual cumulus cells to planetary scale features such as the Madden-Julian Oscillation (MJO). It is also observed that the larger scale features are composed of smaller scale equatorial waves, so that for example the "envelope" of the MJO is often comprised of eastward propagating Kelvin, and westward inertio- gravity waves. The envelopes of these waves are in turn are comprised of a broad spectrum of mesoscale features, which are predominantly westward propagating. While it is certainly evident that the larger envelopes must be creating a favorable environment for the higher frequency activity within them, the precise mechanisms for this modulation are still a subject of debate, as is the inverse role of the mesoscale disturbances in the upscale transfer of energy. Interestingly, a census of a large number of individual MJOs shows that they are comprised of a wide variety of smaller scale disturbance types from case to case, suggesting that parameterization of their upscale impacts of might be feasible. A majority of the westward propagating features move much too quickly (greater than 20 m/s) to be explained solely by advection. A space-time spectrum of high resolution satellite cloudiness data shows an overall dominance of westward over eastward power, especially at higher zonal wavenumbers and frequencies. In particular, a spectral peak extends from the previously well-documented large scale westward inertio-gravity peak into the westward portion of the mesoscale region, with a dispersion relationship representative of pure gravity waves. These westward gravity waves are strongly modulated by the diurnal cycle, especially over the continents. Understanding the precise role of these scale interactions is likely a crucial step towards the improved simulation of equatorial disturbances in models.
Tropical-extratropical interaction during YOTC
Tropical-extratropical interaction determines the mid-latitude (30-60 degrees) atmospheric transport of mass, momentum and energy. Coherent atmosphere and ocean phenomena on all time scales contribute to the interaction and to a portion of the transport. A survey of phenomena producing tropical-extratropical interaction is presented with preliminary results on their behavior during the early part of the YOTC year. Phenomena include ENSO, the Madden-Julian oscillation, the global wind oscillation, mid-latitude wave energy dispersion, and others. The talk will focus on the extratropical transition of west Pacific typhoons, the interaction of global teleconnection patterns and the role of wave breaking and extreme weather events. The zonal mean budget of atmospheric angular momentum will be used to measure the impact of the interaction and the role of different physical processes. Ultimately the prediction and simulation of these phenomena by global weather and climate models should be examined during YOTC.
Observations of Atmospheric Rivers with CloudSat CPR and Aqua AMSR-E
"Atmospheric rivers" (ARs) are filamentary water vapor structures, occurring primarily over oceans, thousands of kilometers long that form along the leading edge of cold fronts. ARs are an important link between weather and climate by transporting large amounts of moisture, on the order of 108kgs-1, through the middle latitude regions and causing heavy precipitation events along coastal regions. The CloudSat satellite, launched 28 April 2006, is designed to measure vertical cloud structure and fill a long-existing gap in satellite observations. Instruments onboard the CloudSat and Aqua satellites observed 22 AR events (with multiple overpasses for each AR) over a period from November 2006 to April 2007. CloudSat Cloud Profiling Radar (CPR) observations of cloud location and cloud type are used along with moisture observations from Aqua Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) to create a preliminary average profile of vertical cloud structure within ARs. The CloudSat observations (using Aqua moisture and precipitation measurements as references) are first presented for case studies of two AR events out of the total 22 events. The CloudSat measurements are then combined into composite frequency plots to show the typical cloud locations within and near the AR with respect to the AR's water vapor structure. Frequency plots are presented for all 92 overpasses along with categories of overpasses based on time of occurrence within the ARs' life spans to give a preliminary time evolution of cloud structure.
Cross-Scale Evolution of Tropical Diabatic Heating
Dominant structures and variability in tropical diabatic heating derived from sounding observations of several field experiments, global reanalyses, and TRMM retrievals are identified. Two rotated EOF leading modes, one deep, one shallow, explain up to 90 percent of the total variance. Based on these two modes, composite profiles of diabatic heating relevant to the large-scale circulation are reconstructed. A statistical calculation leads to most probable transitions from no convection to shallow, deep, then stratiform heating profiles, and back to the no convection situation. Such a structural transition cycle exists in heating data of different temporal resolutions (6 hourly, daily, and pentad), suggesting a 'self- similarity' in heating evolution across a wide range of timescales. The evolution of diabatic heating associated with the MJO shows that shallow heating leads deep heating during their eastward propagation over the Indian and western Pacific Oceans. A steady-state linear model forced by the dominant heating profiles produced multiple overturning structures that suggest more than one baroclinic modes in the dynamic field.
The role of surface fluxes in tropical intraseasonal oscillations
We will present evidence, much of it old but some of it new, for the hypothesis that interactive feedbacks
involving surface heat fluxes are important to the dynamics of tropical intraseasonal oscillations. Both cloud-
radiative feedbacks and surface turbulent flux feedbacks are probably important. Evidence in favor of this
hypothesis includes the observed spatial distribution of intraseasonal variance in precipitation and outgoing
longwave radiation, the observed relationship between intraseasonal latent heat flux and precipitation
in regions where intraseasonal variability is strong, and sensitivity experiments performed with a small
number of general circulation and idealized models. We will argue that it would be useful to assess the
importance of surface fluxes to intraseasonal variability in a larger number of comprehensive numerical
models, including the new generation of global high-resolution nonhydrostatic models. Such an assessment
could provide insight into the
relevance of interactive surface fluxes to real intraseasonal variability, perhaps making it possible to rule out
either theoretical explanations in which surface fluxes are crucial, or those in which they are not.
Dynamical and Thermodynamical Controls on Tropical and Subtropical Convective Activity Inferred from Three Dimensional Latent Heating Distributions with TRMM SLH Data
Tropical and subtropical latent heating associated with precipitation and its relationship with large-scale environments are studied. Three dimensional apparent heat source minus radiative heating (Q1-QR) data obtained with TRMM Spectral Latent Heating (SLH) algorithm, which evaluates the convective rain heating and the stratiform rain heating separately, are utilized for the analysis. The analysis region is 36N-36S, and period is 1998-2007. There are three characteristic peaks in the average Q1-QR profiles, at ~2km, ~5km, and at ~8km. The former two attribute to the convective rain, while the last one is from the deep stratiform rain. The lowest peak at ~2km corresponds to the heating associated with shallower isolated rain, mostly cumulus congestus, with top height at ~4km. The highest 8 km peak well represents the organized deep precipitation. Shallow congestus rain heating is significant over oceans but not over land. We next compare the oceanic congestus rain and deep rain heating with the large-scale environmental conditions. As a result, congestus rain heating strongly correlates with the sea surface temperature (SST), while deep rain behaves like feeling SST as merely a threshold value. It is indicated that even for the same SST, deep convection is effectively suppressed with a large-scale subsidence represented by dp/dt at 500 hPa. It is indicated that this relationship does not exist over land. It is quantitatively suggested that for a climate model simulation, a reproduction of realistic subsidence is essential for realistic distributions of tropical and subtropical precipitation.
Multi-scale structure of Madden-Julian Oscillation revealed by two-dimensional wavelet analysis
To advance our understanding of the multi-scale nature of Madden-Julian oscillation (MJO), we introduce a new tool, the two-dimensional wavelet transform (2D WT) and investigated the case studied by Nakazawa (1988). 2D global wavelet spectrum (scalogram) provides a lucid picture of the spectral power distribution as a function of wavelength, periodicity, and the direction and speed of phase propagation. The 2D WT has the capability of reconstruction and wave filtering, providing an intuitive representation of the wavenumber- frequency power spectrum and most importantly an effective way to study multi-scale interaction. With 2D WT filtering, the equatorial convective activity is composed of convectively coupled, eastward propagating MJO and Kelvin waves, westward propagating inertio-gravity (WIG) waves, mixed-Rossby gravity (MRG) waves, and equatorial Rossby waves, which together account for about 60% of the total variance at a three-hour resolution. One of the most interesting findings is the difference in the hierarchical structure between the Kelvin wave (super cloud cluster) and the MJO. Both the Kelvin wave and MJO consist of and modulate WIG waves. However, the WIG waves move slower and have shorter wavelengths in the MJO, but move faster and have longer wavelengths in the Kelvin waves. In addition, the MJO has a distinctive tri-pole meridional structure and may be viewed as a convectively coupled Kelvin-Rossby wave packet rather than the envelope of a convectively active region. Given the different meridional structure, the Kelvin waves modulate the westward propagating MRG waves over the western Pacific while the MJO does not. The Kelvin waves tend to be strong to the east of MJO while weak to the west of the MJO. The strong Kelvin waves to the east of MJO may precondition the environment for an eastward propagating MJO to develop. The MJO in turn seems to consume most of the available convective energy so that the Kelvin waves coming from its west cannot fully develop. The possible gupscale feedbackh of the WIG wave behavior on MJO and Kelvin waves is also discussed.
On the Low-level Moisture Preconditioning of the Madden-Julian Oscillation
The Atmospheric Infrared Sounder (AIRS) data have revealed the low-level moisture preconditioning of the Madden-Julian Oscillation (MJO), but the cause of this preconditioning remains unknown. The present study addresses this issue by analyzing the latest satellite-based hydrological cycle data, i.e., moisture profiles from AIRS, rainfall from TRMM, surface evaporation from OAFlux, and total-column moisture transport from combined observations of surface wind vector by QuikSCAT, cloud drift wind vector by MISR and NOAA geostationary satellites, and precipitable water by SSM/I. Our analysis indicates that the low-level moisture preconditioning of the MJO is due primarily to the moisture convergence anomaly instead of surface evaporation anomaly associated with the MJO. Furthermore, the moisture convergence anomaly is mainly from the zonal rather than meridional component and should also be largely in the lower troposphere. These satellite-based results are consistent with the frictional wave-CISK theory in which the low-level zonal moisture convergence associated with convectively coupled equatorial waves plays a central role in organizing the low-level moisture preconditioning. The wind-evaporation feedback may be of secondary importance.
Tropical Penetrative Deep Convection as Depicted by CloudSat and MODIS
Collocated observations of cloud-top height and cloud-profiling information from CloudSat and cloud-top temperature from MODIS are analyzed to determine where convective cloud top occurs in relation to the cold point tropopause and to characterize the internal vertical structure of these deep convective clouds. Three types of penetrating convection identified as cold-low (CL), cold-high (CH), and warm-high (WH) are defined according to the cloud-top temperature and height in comparison to the cold point tropopause height and temperature: It is suggested that CL, CH and WH types correspond to, respectively, the incipient, mature and dissipating stage of the convective lifecycle. Multiple lines of evidence, including characteristics of CloudSat radar profiles, test against the undiluted ascent hypothesis, and examination of convective system size, all lend support to this interpretation.