Overview of Recent Cloud and Radiation Satellite Observations for Studying Tropical Climate Variability (Invited)
This presentation provides an overview of recent satellite observations for studying clouds, radiation and climate variability at various time and space scales. We focus on data products from the Clouds and the Earth’s Radiant Energy System (CERES) and a new merged data product that combines CALIPSO, CloudSat, CERES, and MODIS (C3M). These data products provide unprecedented accuracy in top-of-atmosphere, within-atmosphere, and surface radiative fluxes, profiles of radiative heating rates, and coincident cloud and aerosol properties over a range of temporal and spatial scales. We show examples using these data highlighting the distribution of various properties of deep convective clouds and their relationship to atmospheric state, and illustrate how these data can be used to evaluate cloud-radiative feedbacks in climate models.
Year of Tropical Convection (YOTC): Status and Research Agenda (Invited)
The realistic representation of tropical convection in our global atmospheric models is a long-standing grand challenge for numerical weather forecasts and global climate predictions. Our lack of fundamental knowledge and practical capabilities in this area leaves us disadvantaged in modeling and predicting prominent phenomena of the tropical atmosphere such as the ITCZ, ENSO, monsoons and their active/break periods, the MJO, subtropical stratus decks, near-surface ocean properties, tropical cyclones, and even the diurnal cycle. To address this the challenge of tropical convection, WCRP and WWRP/THORPEX are conducting a joint research activity consisting coordinated observing, modeling and forecasting of organized tropical convection. The timing, focus year approach and integrated framework of this effort is intended to exploit the vast amounts of existing 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. The target time frame for scientific focus is May 2008 to April 2010, 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 current status of YOTC’s science and implementation plans, available resources and research agenda.
Impact of Deep Convection on ENSO Prediction Skill (Invited)
Results are described from a large sample of coupled ocean-atmosphere retrospective forecasts during 1982-1998. The intent is to examine how improvement in the parameterization of deep convection affects ENSO forecast skill. The two prediction systems presented here are the National Center for Atmospheric Research (NCAR) Community Climate System Model version 3 (CCSM3.0) and version 3.5 (CCSM3.5). These versions of model are selected because of their differences in the parameterized deep convection. In all other aspects, the models are identical. In the forecasts, a state-of-the-art ocean data assimilation system made available by the National Ocean and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory (GFDL) is used to initialize the ocean component model. The retrospective forecasts are initialized each January, April, July and November of each year, and ensembles of six forecasts are run for each initial month, yielding a total of 408 1-yr predictions. In generating the ensemble members, perturbations are added to the atmospheric initial state only. The skill of the prediction systems are compared from both a deterministic and a probabilistic perspective. In analyzing the two sets of forecast specific attention is given to representation of tropical precipitation anomalies and the associated wind stress anomalies. The forecasts are also compared in terms of how the differences in the parameterization of deep convection affects the predicted coupled air-sea feedbacks.
Connecting Increased Intra-seasonal Variability and ENSO Changes Associated with Deep Convection Modifications in CCSM3
Previous studies have speculated on the role of intraseasonal variability associated with the Madden Julian Oscillation (MJO) in both modulating and preceding ENSO events. Observations of individual ENSO events and in particular the strong 1997/98 event provided strong evidence of a causal relationship between strong MJO activity and the initiation of an El Nino warming. In models this relationship has been difficult to diagnose due to the weak MJO activity seen in most models. The Community Climate System Model (CCSM3) suffers both from weak MJO activity and an El Nino mode that is too regular and occurs too frequently. Motivated by a desire to improve the deep convective sensitivity to tropospheric humidity changes were made that enhanced the entrainment characteristics of the convective plumes. The response to this change and the inclusion of convective momentum transports, constituting the major atmospheric physics enhancements in CCSM3.5, lead to tropical climate coupling changes across scales. Variability on all scales is enhanced through the convecting tropics. This is embedded within an increase in variability associated with the MJO. An analysis of enhanced MJO activity leading ENSO events is inconclusive. However, it is clear that MJO activity extending into the central Pacific during a developing ENSO event is able to maintain warm SSTs through Winter and into Spring. This signal is in much better agreement with observations compared with CCSM3. This talk will focus on changes in the surface flux relationships that lead directly to the improvements in the strength of the simulated MJO and the upscale effects on ENSO.
The Role of Atmosphere Feedbacks During ENSO
Although most current coupled general circulation models (GCMs) exhibit some sort of ENSO signal, there are still many areas for improvement. For example, the models generally simulate El Niño events with frequencies which are too high, structures which extend too far to the west, and a large diversity of amplitudes. Moreover, simulating the correct ENSO properties with the right balance of mechanisms and feedbacks is still a challenge. Several recent studies using ocean-atmosphere GCMs suggest that the atmospheric component, and in particular the deep convection scheme, plays a dominant role in the modeled ENSO. To help elucidate these findings, the two main atmosphere feedbacks relevant to ENSO, the Bjerknes positive feedback (μ) and the heat flux feedback (α), are here analysed in 12 coupled GCMs from the CMIP3 database. We find that the models generally underestimate both feedbacks, leading to an error compensation. The strength of α is inversely related to the ENSO amplitude in the models and the latent heat and shortwave flux components of this feedback dominate. While the latent heat feedback is primarily responsible for this inverse relationship, errors in the shortwave flux feedback are the main cause of the model diversity in the overall α. In the tropical Pacific, the shortwave flux feedback is intrinsically linked to the large-scale vertical motion, with SST anomalies in the East Pacific coupled to changes in the amount of deep convection/subsidence and cloud cover. We thus propose that an improved atmosphere-ocean heat flux feedback in the models can only be achieved by correcting the errors in the convection/cloud physics responsible for the biases in the shortwave flux feedback.
The Intertropical Convergence Zone, equatorial westerlies and the creation of climatically sensitive zones in the ocean (Invited)
In each of the tropical oceans, sea-surface temperature (SST) distributions lead to cross-equatorial pressure gradients (CEPG) that may promote regional inertial instability through the advection of “wrong signed” absolute vorticity across the equator. We show how the magnitude of the CEPG determines the latitude and strength of convection. If the CEPG is strong, the intertropical convergence zone (ITCZ) is located off the equator with convection collocated with equatorial westerlies that are distinctly sub-geostrophic. If the CEPG is weak, mean convection is less intense and winds asymptote into the convection as easterlies. Instabilities (manifested as westward propagating waves) occur at the inertial frequency of the latitude of the mean ITCZ. The equatorial westerlies enhance the curl of the wind stress shallowing the thermocline and rendering regions climatically sensitive to strong wind anomalies. One such region exists in the South Indian Ocean during the boreal summer and has been tied to variations in cyclone formation and African rainfall. In this manner, large scale convection and wind fields set up by slow cross-equatorial forcing by the ocean, feed back onto the ocean structure itself.
Coupled Feedback and Ocean Heat Transport in the Simulation of ITCZ
This study investigates the coupled atmosphere-ocean feedback and the role of ocean dynamic heat transport in the formation of double ITCZ over the tropical Pacific in the NCAR CCSM3. A hierarchy of coupling strategy is employed for this purpose. A slab ocean model is coupled with the atmospheric component of the model, CAM3, to investigate the local feedback between the atmosphere and the ocean. To understand the role of ocean heat transport, the fully coupled CCSM3 model is used. The analysis of CCSM3 simulations shows that the altered ocean dynamic heat transport when different convection schemes are used plays a major role in the simulation of sea surface temperature (SST) in the southern ITCZ region, although surface energy flux also contributes to seasonal variation. Results suggest that the unrealistic simulation of convection over the southern ITCZ region in the standard CCSM3 leads to the double ITCZ bias through complex coupled interactions among atmospheric convection, surface winds, latent heat flux, cloud radiative forcing, SST, and upper ocean circulations. The double ITCZ bias can be mitigated by altering this chain of interactions.
The double-ITCZ syndrome in coupled general circulation models: the role of large-scale vertical circulation regimes
The double-intertropical convergence zone (DI) systematic error, affecting state-of-the-art coupled general circulation models (CGCM) is examined in the multi-model Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) ensemble of simulations of the twentieth-century climate. Aim of this study is to quantify the DI error on precipitation in the tropical Pacific, with a specific focus on the relationship between the DI error and the representation of large-scale vertical circulation regimes in climate models. The DI rainfall signal is analysed using a regime sorting approach for the vertical circulation regimes. This methodology allows to partition the precipitation into deep and shallow convective components. Also, it makes possible to decouple the error on the magnitude of precipitation associated with individual convective events, from the error affecting the frequency of occurrence of convective regimes. It is shown that, despite the existing large intra-model differences, CGCMs can be ultimately grouped into a few homegenous clusters, each featuring a well defined rainfall-vertical circulation pattern in the DI region. A critical parameter controlling the strength of the DI is identified in the SST threshold leading to the onset of deep convection (THR), combined with the mean SST in the south-eastern Pacific. The models featuring a THR which is systematically colder (warmer) than their mean surface temperature are more (less) prone to exhibit a spurious southern ITCZ.