Year of Tropical Convection (YOTC): Status and Research Agenda
The realistic representation of tropical convection in global models is a long-standing challenge for numerical weather prediction and an emerging grand challenge for climate prediction in respect to its physical basis. Insufficient knowledge and practical capabilities in this area disadvantage the modeling and prediction of prominent multi-scale phenomena such as the ITCZ, ENSO, monsoons and their active/break periods, the MJO, subtropical stratus decks, near-surface ocean properties, and tropical cyclones. Science elements include the diurnal cycle of precipitation, multi-scale convective organization, the global energy and water cycle, and interaction between the tropics and extra-tropics which interact strongly on timescales of weeks-to-months: the intersection of weather and climate. To address such challenges, the WCRP and WWRP/THORPEX are conducting a joint international research project, the Year of Tropical Convection (YOTC) which is a coordinated observing, modeling and forecasting project. The focus-year and integrated framework is intended to exploit the vast observational datasets, the modern high-resolution modeling frameworks, and theoretical insights. The over-arching objective is to advance the characterization, diagnosis, modeling, parameterization and prediction of multi-scale organized tropical phenomena and their interaction with the global circulation. The “Year” (May 2008 - April 2010) is intended to leverage recent major investments in Earth Science infrastructure and overlapping observational activities, e.g., Asian Monsoon Years (AMY) and the THORPEX Pacific Asian Regional Campaign (T-PARC). The research agenda involves phenomena and scale-interactions that are problematic for prediction models and have important socio-economic implications: MJO and convectively coupled equatorial waves; easterly waves and tropical cyclones; the monsoons including their intraseasonal variability; the diurnal cycle of precipitation; and two-way tropical-extratropical interaction. This presentation will summarize the status of the above.
Excursions into the strong precipitation regime; long tails in water vapor and other tracers (Invited)
From previous work, we know that conditional averages of precipitation undergo rapid transition to a strongly convective regime above a critical value of water vapor for a given temperature. Here we examine two aspects of how one gets into this regime. 1) Temporal relations for the water vapor, for instance preceding a high water vapor/convective state, using Tropical Rainfall Measuring Mission, AQUA, and Atmospheric Radiation Measurement program data. Both synoptic scale and mesoscale increases of water vapor prior to precipitation events are noted. 2) The probability distribution for column integrated water vapor (CWV) has a Gaussian core but an approximately exponential tail under precipitating conditions. Idealized passive tracer forced-advection-diffusion problems can produce such distributions---but are simple prototypes relevant to three-dimensional atmospheric transports with complex sources and sinks? And can we trust the tails estimated from retrievals under strongly precipitating conditions? We show that such tails are found in observed, model, and reanalysis data sets for column integrated tracers under a wide set of circumstances, including for important tracers with anthropogenic sources, namely CO and new retrievals of CO2. The long tails in water vapor are associated with vertical transport and can occur independent of a local precipitation sink. In addition to the importance of CO and CO2 distributions in their own right, the multi-tracer comparison boosts confidence in the long tails in the water vapor distribution that lead to relatively frequent excursions into heavy precipitation conditions.
Understanding the MJO via data assimilation
The Madden-Julain Oscillation (MJO) is well observed, and has large space and time scales well within the resolved range in global models, yet is poorly simulated. Analysis tendencies in a data assimilation process contain important information about how model physics is failing in these cases. This information is brought out from the MERRA reanalysis, and interpreted in terms of MERRA's model physics (the GEOS-5 model). Result: shallow-deep convection transitions are problematic, and the handling of precipitation (melting and re-evaporation) exhibits systematic errors. Can fixing these errors lead to a demonstrably better-performing model?
Vertical cloud water structures of the boreal summer intraseasonal variability based on CloudSat observations and ERA-Interim reanalysis
The boreal summer intraseasonal variability (BSISV), which is characterized by pronounced meridional propagation from equatorial zone to the Indian Continent, exerts significant modulation of the active/break phase of the south Asian monsoon. This form of variability provides a primary source for subseasonal predictive skills of the Asian summer monsoon. Unfortunately, the current general circulation models display large deficiencies in representing this variability. The new cloud observations made available by the CloudSat mission provide unprecedented opportunity to advance our understanding of the BSISV. In this study, the vertical structures of cloud water content and cloud types associated with the BSISV over the Indian Ocean and subcontinent are analyzed based on the CloudSat observations from 2006 to 2009. These cloud water structures are also compared to their counterparts as derived from the recently released ERA-Interim reanalysis. Results based on both datasets suggest that during the northward propagation of the BSISV, while the cloud ice water content (IWC) in upper troposphere are largely in phase with (or slightly lags) the convection, a marked vertical tilting structure is evident in cloud liquid water content (LWC). Increased LWC tends to appear to the north of rainfall maximum, i.e., leads the convection, particularly in the lower troposphere. This northward shift of increased LWC, which is in accord with local enhanced moisture as previously documented, could be fundamental responsible for the northward propagation of the BSISV. Nevertheless, some differences in the cloud water structures between the CloudSat and ERA-Interim are also noted, particularly in the amplitudes of IWC and LWC fields. Further analysis based on CloudSat data indicates that IWC variability of the BSISV is largely associated with deep convective clouds. While LWC is mainly linked to non-precipitating alto-cumulus at mid-level and drizzling stratocumulus cloud at low-level. These aforementioned results would provide valuable information for climate modeling efforts in describing subseasonal variability of tropical convection.
Variance scaling and moist conserved variables from AIRS/CloudSat and comparisons to ECMWF
The simultaneous vertical profiling of cloud condensate, temperature and water vapor from simultaneous A-train instruments are expected to be useful for evaluating and improving subgrid-scale climate model cloud parameterizations and thus their realism. The Atmospheric Infrared Sounder (AIRS) is used to obtain a climatology of height-resolved variance scaling of tropospheric temperature and water vapor. The magnitudes and gradients of temperature and water vapor-derived power law exponents have large but opposite latitudinal dependences in the tropics and extratropics, two large-scale regions dominated by convective and baroclinic processes, respectively. The exponents are not only consistent to previous aircraft campaigns, numerical modeling, and theoretical studies, but they also provide a highly detailed and global view not obtained from previous observational investigations. This methodology has been extended to the ECMWF analysis and forecast fields during YOTC, where similarities and differences to AIRS exponents are highlighted. Furthermore, we will discuss progress on using temperature and water vapor derived from AIRS, combined with cloud condensate obtained from the 94GHz CloudSat radar, to derive moist conserved variables that are not obtainable from individual satellite sensors.
Atmospheric Diabatic Heating Distributions Derived from a Combination of Satellite Sensor Data
Satellite estimates of atmospheric latent+eddy heating (Q1-QR) and radiative heating (QR) are combined to yield estimates of the total diabatic heating, or apparent heat source (Q1). The latent+eddy and radiative heating estimates rely on cloud and precipitation information from TRMM Microwave Imager (TMI) data, with additional cloud information supplied by the TRMM Visible and Infrared Scanner (VIRS) and clear-air environmental properties from NCEP reanalyses. Comparisons of the diabatic heating estimates to those derived primarily from the TRMM Precipitation Radar (PR) and from rawinsonde diagnostic budgets are favorable, although some biases due to differences in sampling and the limited sensitivity of the TMI are noted. Recently, a ten-year database of diabatic heating has been constructed using TRMM observations from 1998-2007, as part of NASA’s Energy and Water cycle Study (NEWS) program. Initial applications of this dataset have been the delineation of the seasonal cycle in the tropics/subtropics, the distribution of heating anomalies associated with the phases of ENSO, and the progression of heating in the Madden Julian Oscillation. Preliminary comparisons of satellite heating estimates versus model-based heating and dynamical fields from the Modern Era Retrospective-Analysis for Research and Applications (MERRA) will be presented at the conference.
Easterly Wave Activity And Its Modulation By The Larger Scales During The YOTC Time Period Of Focus: May 2008 - Oct 2009
The Year of Tropical Convection (YOTC) is a joint WCRP-WWRP/THORPEX international initiative focused on organized tropical convection, its prediction, and predictability. Its aim is to “advance basic knowledge, diagnosis, modeling, parameterization, and prediction of multi-scale tropical convection and two-way interaction between the tropics and extra-tropics with emphasis on the intersection between weather and climate”. The YOTC has selected the May 2008 through October 2009 time period as a focus for analysis in an effort to contain the myriad of possible research contributions as well as to take advantage of specific field campaign activities and related modeling efforts. As a contribution to this “virtual field campaign” we analyze easterly wave track statistics across the tropics and associated Atlantic and east Pacific hurricane initiation events related to these disturbances using a combination of ECMWF Interim reanalyses and observations. The background state supporting the observed easterly wave activity is also analyzed, as are larger scale intraseasonal variations in low-level atmospheric winds associated with the Madden-Julian oscillation (MJO) and the Caribbean low-level jet (CLLJ). The goal is to provide a metric of easterly wave activity for the YOTC that can be used to compare against model analyses, as well as to advance our understanding of the interactions between the synoptic and larger-scales influencing convective organization in the tropics with a focus on the Intra-Americas Sea region.
On the occurrence and properties of middle tropospheric stable layers over the tropical western Pacific
Recent observational and modeling studies of the meteorological environment in the Tropical Western Pacific (TWP) have noted a mid-tropospheric stable layer closely associated with the melting level and a local maximum in relative humidity. This stable layer may have an impact on the convective dynamics and radiation budget in the region, thereby partially regulating the characteristics of the Walker circulation. In this paper, we present results from an observational study that documents the frequency and properties of mid-tropospheric stable layers using data from Atmospheric Radiation Measurement (ARM) Program sites located in the TWP: Darwin, Australia; Manus Island, Papua New Guinea; and Nauru Island, Republic if Nauru. We use upper air soundings to examine the prevalence of mid-tropospheric stable layers at each TWP ARM site and examine how the frequency of occurrence changes in each season and with time of day. We employ surface observations and estimates of cloud fraction from the MMCR and MPL instruments to explore composite characteristics of clouds and precipitation properties associated with the presence of a stable layer. We find that melting-level stable layers are common in soundings from all three TWP ARM sites, though the seasonal variability varies by location. Stable layers are common year-round at Manus, but occur primarily during the inactive phase of the North Australian Monsoon (NAM) at Darwin, and are most commonly observed during the boreal summer and winter months at Nauru. All three sites exhibit nearly equal occurrence of stable layers in 0000 UTC vs. 1200 UTC soundings, an indication that the diurnal cycle may have little influence on the frequency of occurrence. Composite precipitation, longwave radiation and cloud fraction suggest that there is a complex relationship between the presence of deep convection and the formation and maintenance of the melting-level stable layer. In our presentation, we will explore the implications of stable layer characteristics and frequency for improved understanding of the interplay between mid-level stable layers and deep convection, and suggest how this information can be used to improve cloud parameterizations in global climate models.