Leveraging A-Train Satellite Observations To Develop New Insights, Diagnostics And Constraints For Model Representations Of Clouds And Convection
The breadth and advances associated with the A-Train framework and its suite of sensors in terms of atmospheric hydrological processes is unprecedented, providing new prospects for characterizing cloud and convective processes, including morphology, cloud water content, particle/hydrometeor size(s), radiative and thermodynamic interactions. When augmented with dynamical quantities from the latest atmospheric analyses (e.g, MERRA; ECMWF-YOTC), such data present an altogether new opportunity to describe and understand convection/clouds, from their microphysical details to their connections to large-scale dynamics. In this presentation, we will discuss our work with a new, unique, comprehensive, and CloudSat-centric, A-Train data set, constructed on behalf of the Year of Tropical Convection (YOTC) that is aimed at formulating carefully posed observational constraints and diagnoses to address a number of present-day model shortcomings. Among the applications to be highlighted are: 1) characterizing and developing diagnoses and constraints on model partitions between “suspended” ice/liquid in clouds versus precipitating hydrometeors; 2) quantifying and evaluating the radiative impacts of precipitating hydrometeors which are often ignored in GCM cloud representations; 3) evaluating and constraining the assumptions concerning entrainment and its interactions with microphysics in moist plume models seeking bulk relationships that can inform model parameterizations of convection.
Use of A-Train data to estimate convective buoyancy and entrainment rate
A new method has been developed that uses CloudSat and MODIS data to estimate convective buoyancy and entrainment rate. The key observational requirements are 1) independent measurements of cloud-top height (CTH) and cloud-top temperature (CTT), 2) internal vertical structure of convection, and 3) ambient sounding data. Examples and global statistics will be shown. We focus on tropical convection for now. Satellite information on convective buoyancy and entrainment rate opens up a new door toward using new-generation satellite data to effectively evaluate GCM cumulus parameterization. We will discuss this new framework.
UTLS Signatures in MLS Fields during the YOTC May-July 2008 Large-Scale Convective Event
The YOTC Implementation Plan identifies a period of interest from late May through mid July 2008 during which a large-scale convective event propagated eastward from the Indian Ocean eastward to Central America. At times, the event could be characterized as an MJO event of modest strength and at other times as a convectively-coupled Kelvin wave. Over the course of its propagation, it influenced a rich variety of processes including the onset of the Indian Monsoon, convection over the warm pool, the strong modulation the Eastern Pacific ITCZ, the initiation of several tropical cyclones, and the modulation of the North American Monsoon. This work examines the signatures of these events in upper tropospheric and lower stratospheric water vapor, cloud ice, temperature and geopotential height fields from the Microwave Limb Sounder on Aura, and other A-Train data, and in the corresponding ECMWF analysis fields. Special consideration will be given to remote and extra-tropical signatures during this period as they are observed in the upper troposphere and lower stratosphere.
Alternative Representations of Convective Processes in the NASA GEOS-5 AGCM
The gap in explicit resolution of phenomena between global climate models and cloud resolving models is shrinking at a steady pace with global integrations of several km in resolution now practical for at least some time scales. In moving toward finer resolution the long standing problem of convective parameterization is being examined along with the assumption of convective quasi-equilibrium and how this can be reconciled with the stochastic and intermittent nature of convection. In this context we examine the nature of parameterized convection in the NASA Goddard Earth Observing System (GEOS-5) Atmospheric General Circulation Model. Our analysis uses both coarse (2.5 degree) and fine scale 0.25 degree spatial resolution integrations. Two basic formulations are compared: the default option is the Relaxed Arakawa-Schubert (RAS) scheme which invokes a sequence of linearly entraining plumes and quasi-equilibrium closure. An optional modification of this method (the “Stochastic Tokioka” constraint) places a random lower limit on plume entrainment. An alternative representation is the Kain-Fritsch parameterization which was originally developed for mesoscale numerical modeling strategies. Here entrainment is determined by a crude buoyancy sorting approach that allows the plume spectrum to be more responsive to ambient vertical stratification of moisture. Diagnostics of the model behavior are referenced to recent observational evidence of continuous phase transition behavior. In particular we examine the relationship between column water vapor and probablility of convective presence and intensity. Sensitivity of the statistics of convective behavior to parcel mixing / entrainment formulations and parcel initial thermodynamics are considered. Observational statistics from A-Train and TRMM sensors provide validation of the model integrations.
Mechanisms affecting the transition from shallow to deep convection over land: Inferences from observations collected at the ARM Southern Great Plains site
11 years of summertime observations at the Atmospheric Radiation Measurement (ARM) Climate Research Facility Southern Great Plains (SGP) site are used to investigate mechanisms controlling the transition from shallow to deep convection over land. A more humid environment above the boundary layer favors the occurrence of late-afternoon heavy precipitation events. The higher moisture content is brought by wind from south. Greater boundary layer inhomogeneity in moist static energy (MSE) is correlated to larger rain rates at the initial stage of precipitation. MSE inhomogeneity is attributed to both moisture and temperature fields, and is correlated with westerly winds. In an examination of afternoon rain statistics, higher relative humidity above the boundary layer is correlated to an earlier onset and longer duration of precipitation, while greater boundary layer inhomogeneity and atmospheric instability are positively correlated to the total rain amount and the maximum rain rate. On balance, these observations favor theories for the transition that involve a moist free troposphere and boundary layer heterogeneity in preference to those that involve convective available potential energy or convective inhibition. Thus the evidence presented here supports the current emphasis in the modeling community on the entraining nature of convection and the role of boundary layer cold pools in triggering new convection.
Aspects of orogenic mesoscale convective organization admitted by a global multi-scale climate modeling framework
Multi-scale modeling frameworks (MMFs) constitute a new class of global climate model (GCM) in which embedded, idealized cloud resolving models (CRMs) are used in place of statistical parameterizations of subgrid cloud and boundary layer processes. Due to two severe idealizations in the assumed geometry of the CRM subdomains in MMFs - reduced dimensionality and lateral periodicity - MMFs may be unable to simulate orogenic mesoscale convection. Indeed, over the Central United States, MMF simulations do not exhibit the tell-tale eastward propagating diurnal packets of surface rainfall that are in nature the signature of orogenic mesoscale convection. However, close inspection of an MMF simulation shows that despite its idealizations several important aspects of orogenic mesoscale convection are resolved. When synoptic and low-level moisture transport are conducive, orogenesis of mesoscale diurnal convective heating (Q1) and moistening (Q2) anomalies is triggered. Subsequent advection of these CRM-induced heating anomalies by strong synoptic winds on the large scale grid connects adjacent CRM cells, creating condensate metastructures in the MMF, which precipitate primarily as virga in the model. The vertical morphology of these MMF simulated cloud metastructures is consistent with the lifecycle of mesoscale convective complexes in nature. In light of these findings, the fact that MMFs do not simulate the correct diurnally propagating surface precipitation signature of mesoscale convection over the Central United States may be more readily explained by surmountable imperfections in its microphysics (precipitation not reaching the surface) than by a fundamental design flaw that denies orogenic mesoscale organization.
Relationships between Moisture Surges and Mesoscale- to Large-Scale Convection from Multi-year Satellite Imagery and North American Regional Reanalysis Data
This study consists of a comprehensive climatology of moisture surges (MSs) in the North American monsoon (NAM) core region using a multiyear (1990-2006) set of MSs, surface, upper-air soundings (RAOB and pibal), historical satellite-estimated mesoscale convective systems (MCSs), and the North American Regional Reanalysis (NARR). Climatological composites of MSs are first created with respect to MCS occurrence. MS’s are further stratified based on the presence of tropical synoptic-scale disturbances (such as Tropical Easterly Waves and Tropical Storms/Tropical Cyclones) and intraseasonal variations (30-60 day Madden-Julian Oscillation variability). Our results show that over the GoC coastal plain, MCSs modulate the diurnal cycle of the low-level circulation around the GoC during “major MS”, “minor MS”, and “non-MS” environments. We found that MCS activity enhances the offshore flow along the eastern GoC coast, which then enhances the GoC LLJ. On the other hand, immediately before major MSs onset, the occurrence of MCSs over the southern GoC is associated with more intense MSs. It is also shown that this relationship holds regardless of the intensity and type of tropical synoptic-scale disturbance forcing the MS. At larger scales, an MJO active phase over the eastern Pacific affects the overall NAM variability by enhancing convective activity, which in turn is statistically connected with an up to 120% increase in MS frequency. Although the MJO active phase is also connected with a large increase in frequency of TSs/TCs activity (triples - quadruples), the maximum in TSs/TCs activity occurs 5-10 days before the MS frequency maximum.
Relative Role of Convection and Large-Scale Flow in Controlling Upper Tropospheric Humidity
The controls on the subtropical Upper Tropospheric Humidity (UTH) are examined using satellite measurements of UTH from the Atmospheric Infrared Sounder (AIRS), meteorological analyses, and trajectory-based water vapor simulations. The subtropics is much drier than the tropics through most of the troposphere, but there are significant zonal variations that are related to regional variability in the processes that determine subtropical UTH. Both observations and trajectory model calculations show that there are intermittent high and low UTH values that are due to intrusions of high PV air into the subtropical eastern Pacific and Atlantic Ocean. On the other hand, the humidity distribution over the Indian Ocean and western Pacific is more closely linked to the location and strength of subtropical anticyclones. In these regions there are eastward propagating features in the subtropical UTH that are out of phase with tropical UTH, which appear to be linked to the intraseasonal tropical convection variability (e.g. Madden Julian Oscillation). Clustering analysis shows that coherent variations of trajectory clusters with longitude are also consistent with these transient intraseasonal convections. The dominant trajectory patterns differ between convective and non-convective regions, indicating that the transport processes are different in these regions. Furthermore, the variability of relative humidity (RH) can also be partially explained by the mean RH of the trajectory clusters. The trajectory-based water vapor simulations and clustering analysis are used to understand the relative contribution of convection, subsidence and horizontal transport of the exchanges of air between the tropics and subtropics. In addition to helping to explain the variability of RH associated with different physical processes, our analysis also helps to quantify the difference in transport pathways and origins of moisture.
Vertical structure of convective heating over the large-scale subsidence region in the CMIP3 models
Vertical structure of tropical heating associated with convective activities is known to play an important role in determining a large-scale circulation (Hartmann et al. 1984; Wu 2003). Takayabu et al. (AGU2008) showed heating over the large-scale subsidence region is suppressed in the lower troposphere using TRMM data. They suggested a dry layer in the lower troposphere is important for the suppression of deep convection. In this study, apparent heat source (Q1) associated with convective activities in 22 20C3M model runs from the World Climate Research Programme's (WCRP's) Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model dataset is analyzed over the subsidence region (monthly dp/dt > 0) in September-November seasons of 1979-2000. A multi-model ensemble average of Q1-QR, where radiative heating (QR) is obtained from JRA reanalysis data, well reproduces the suppressed heating profile over the subsidence region as observed by TRMM. Then we examine the impact of the lower-level dry layer to the strength of the suppression represented by a ratio of Q1-QR at 500hPa and 850hPa (Q1-QR500/850). Each model shows a decrease of Q1-QR500/850 as the relative humidity at 600hPa (RH600) decreases in the region RH600 < 35%. This relationship is identified in most of the models, JRA, and ERA reanalysis data, which means in each model, the dryness of the lower troposphere suppresses the deep convection effectively. Although most of the models reproduce the suppressed structure of the heating and its relationship with RH600, the strength of the suppression varies largely among the models. We speculate that the large diversity among models is influenced by cumulus parameterization schemes. When we compare six models with a similar parameterization scheme based on Arakawa and Schubert (1974), we find relatively converged relationships between RH600 and the suppression of deep convection. We will further discuss the scheme dependency at the poster. This study is financially supported by the Global Environment Research Fund (S-5-2) of the Ministry of the Environment, Japan.
Diurnal Cycle: The Major Source of Error in ECMWF 1 to 10 Days Forecasts During YOTC 2008
European Centre for Medium-Range Weather Forecasts (ECMWF) model generated 10-day long predictions during June to September of the Year of Tropical Convection (YOTC) 2008 were used to study precipitation forecast skills of the model over the tropics. Three hourly, high spatial resolution data available from the Tropical Rainfall Measuring Mission (TRMM) 3B42 products were used as observational counterpart. It was seen that the ECMWF model was able to capture the overall features of tropical convection reasonably. Northward propagation of convective bands over the Bay of Bengal was predicted realistically up to 5 days in advance. However, large errors exist in the daily data sets especially for longer lead times over smaller domains. For shorter lead times (less than 4 to 5 days), forecast errors are much smaller over the oceans as that over land. Moreover, the rate of increase of errors with lead-time is rapid over the oceans and is confined to the regions where observed precipitation has high daily standard deviation. An analysis with 2-meter air temperature shows that this rapid error growth over the oceans is related to the error in spatial pattern of near-surface air temperature. This is probably due to the absence of air-sea interaction in the atmosphere-only model used for forecasting. While the prescribed surface temperature over the oceans remains realistic at shorter lead times, the pattern and hence the gradient of the surface temperature does not change with the changes in atmospheric parameters like precipitation and evaporation at longer lead times. However, over land, due to errors in the land surface model and calculated energy fluxes at the surface, errors are higher than that over ocean initially, and does not change much with lead time, but the spatial pattern shows a much better correspondence with the observation due to the presence of feedback mechanism. An analysis of diurnal cycle of precipitation shows that a major source of error in the ECMWF forecasts is the errors in the phase and amplitude of the diurnal cycle of precipitation over the south Asian monsoon region. The error in monthly mean 3-hourly precipitation forecasts is about 2 to 4 times of the error in the daily mean data sets. An improvement in the phase and amplitude of the diurnal cycle of precipitation forecasts is necessary to improve the overall skill of the ECMWF model.
Global Observations of the Madden-Julian Oscillations from Multi-Satellite Data
The Madden-Julian Oscillation (MJO) is the dominant form of intraseasonal variability in the tropical atmosphere, usually observed as eastward propagating wave activity over the tropical Indian Ocean and the western Pacific. There are still a number of features associated with the MJO that need explanation. For example, Tian et al. [J. Atmos. Sci, 63, 2462, 2006] notes large water vapor anomalies associated with the MJO in the lower troposphere over the eastern Pacific Ocean and even a westward propagating feature over the central Pacific Ocean. In this work, we further examine the MJO in (a) 10 years of satellite-tracked winds from the Atmospheric Motion Vectors (AMVs); (b) 22 years of satellite estimates of total precipitable water, liquid water content and rain from SSM/I; (c) 24 years of cloud-top pressure, cloudiness, shortwave and longwave fluxes from ISCCP; and, (d) 4 years of tropospheric ozone column measured by the Thermal Emission Spectrometer (TES). Our goal is to elucidate the causes for the large water vapor anomalies in the eastern Pacific and the nature of the westward propagating component.
The global circulation response to diabatic heating associated with the Madden-Julian Oscillation
Composite maps of the life cycle of the MJO during the northern winter using the NCEP reanalysis and OLR data show statistically significant upper- and lower-tropospheric circulation response to the tropical diabatic heating in the MJO. To examine the detailed dynamical mechanisms of the global circulation response, a series of GFDL dynamical core model (R30L20) simulations have been carried out. The nonlinear integrations show the upper-level streamfunction anomalies very consistent with the observations, especially, when enhanced convection fully develops over the central Indian Ocean and reduced convection over the Maritime Continents. Results show that much of the tropical and extratropical circulation patterns are determined by the interplay of the equatorally trapped Rossby and Kelvin waves and the Rossby wave propagation associated with two forcing anomalies. Sensitivity test shows the strength of the streamline anomalies are quasi-linearly proportional to the magnitude of the forced diabatic heating, implying the observed circulation anomalies are the direct response to the MJO diabatic heating. The response to the different peak heating level and vertical gradient of heating, and basic wind will be discussed.
HIRS Upper Tropospheric Water Vapor Dataset for Tracking Tropical Convections and Waves
The High-Resolution Infrared Radiation Sounder (HIRS) onboard polar orbiting satellites provide global sounding measurement of the atmosphere. Among the HIRS water vapor channels, channel 12 observes the upper tropospheric water vapor (UTWV). Daily measurements are processed from multiple NOAA and METOP satellites. Intersatellite calibration is performed to bring the data from multi-satellites to a base satellite to form a temporal homogeneous data record. A new dataset, which removes only the clouds in the upper troposphere, is developed for monitoring the variation of water vapor in the upper troposphere. Using the intersatellite calibrated UTWV data, tropical convective events are tracked. Relationship of the UTWV data with several major tropical indexes is examined. The variability of UTWV during major tropical events such as El Niño and La Niña are analyzed. To track and monitor tropical waves, time-longitude section of UTWV data near the equator are examined. The analysis shows that UTWV data have the advantage of providing around-the-globe coverage of equatorial waves. A number of tropical waves, including the Madden-Julian oscillations, Kelvin waves, and equatorial Rossby waves, can be computed by wave-number frequency analysis and monitored on a daily basis. These satellite observations contribute to a better understanding of the tropical variability and the changes in intensity and frequency of tropical convective events.
Comparing the Strength of Eyewall, Inner Rainband, and Outer Rainband Convection Using 10 years of TRMM Data
The Tropical Rain Measuring Mission (TRMM) satellite has passed over more than 900 tropical cyclones since its launch in 1997. Previous studies have established that the intensity of convection in the eyewall (<50~100-km radius from the storm center) is greater than that in the rainbands, and stronger convection often accompanies more intense storms (i.e., defining intensity of the tropical cyclone by its maximum wind speed). However, the question remains on how the strength of convection in the eyewall or rainbands varies as a function of storm intensity and intensity change. This study is aimed at answering this question with 10 years of TRMM observations. Over 0.1 million Tropical Cyclone related Precipitation Features (TCPFs) have been identified from the 10 year University of Utah TRMM Precipitation Feature (PF) database. The TCPF database includes TRMM tropical cyclone observations from six basins: Atlantic (ATL), east+central Pacific (EPA), northwest Pacific (NWP), north Indian Ocean (NIO), south Indian Ocean (SIO), and south Pacific (SPA). In this study, we categorize eyewall, inner rainband and outer rainband PFs subjectively based on the horizontal fields of radar reflectivity and 85-GHz ice scattering. Properties of these PFs related to convective intensity including radar reflectivity profile, maximum height of 20 dBZ echo, minimum brightness temperature in IR and passive microwave channels (37 and 85 GHz), flash rate, and others are examined. First, we establish the composite distributions of these properties for eyewalls and rainbands as a function of storm intensity and intensity change. Then, we examine these distributions for different basins and over-land vs over-ocean. Finally, we show examples of extremely intense convective events, compare them to the composite distributions, and explore their potential significance to the evolution of the storms in which they appear.
NASA Goddard Giovanni Support for YOTC
GIOVANNI is NASA’s GES-DISC (Goddard Earth Sciences Data and Information Services Center) Interactive Online Visualization ANd aNalysis Infrastructure tool for visualization and analysis of very large, global datasets. The current GIOVANNI analytical and statistical tools have been expanded to support the Year of Tropical Convection (YOTC) Satellite Datasets. YOTC provides a unique and comprehensive multi-sensor satellite and model data set that is designed to facilitate the study and model improvement of “tropical convection” and its multi-scale organization. The data target period, May 01, 2008-April 30, 2010, is long enough to encompass a number of scientifically challenging cases of tropical convection with significant human impact. This includes mesoscale and synoptic variability, easterly waves and hurricanes, convectively coupled waves, the MJO and the culmination of these in terms of the monsoon, their interactions with the extra-tropics, and mean characteristics such as tropical-to-subtropical transitions. The YOTC time period and length are driven in part by the following: 1) keeping the multi-sensor/multi-platform and model-analyses data sets and associated infrastructure manageable, 2) facilitating a focused effort by the research and operational communities on a specific scientific problem, and 3) capitalizing on the recent key additions to the armada of satellites (e.g., CloudSat and CALIPSO). It is worth noting that in regards to item 3), the NASA EOS constellation of research satellites is at its peak maturity level with respect to satellite observations of tropical convection.
Cirrus Clouds and Deep Convection in the Tropics: Insights from CALIPSO and CloudSat
Using a 2-y dataset of combined lidar and cloud radar measurements from the CALIPSO and CloudSat satellites, the occurrence of tropical cirrus and deep convective clouds is studied. The cloud identification algorithm takes advantage of the ability of the radar to probe deep precipitating clouds and the lidar to sample even subvisual cirrus clouds. Examined are the frequency of occurrence and the geographical distribution of these clouds, and their apparent interconnections. There is a strong apparent diurnal variability in tropical cirrus mainly over land, with significantly more cirrus detected at night compared to day, but no clear diurnal pattern in deep convective activity. CALIPSO daylight signal noise effects do not appear to be not responsible for the diurnal cirrus pattern, because high, thin tropopause transitional layer (TTL) cirrus do not show a clear diurnal effect. Stratifying the global results by estimated visible cloud optical depth τ, we find that most of the planets subvisual (τ<~0.03) cirrus clouds occur in the tropics and are more frequent at night and over ocean; thin (~0.03<τ<~0.3) cirrus have their highest global frequencies over equatorial land masses and in the West Pacific region, and are also more frequent at night but occur mainly over land; and opaque (~0.3<τ<~3.0) cirrus are spread globally and tend to occur during the day over ocean. Although it is unknown which of the several proposed cirrus cloud formation mechanisms are key in the tropics, the close association of cirrus with convective clouds implies that tropical cirrus are linked to deep convective activity, with the likely exception of TTL cirrus clouds.
Testing a Rossby-wave theory of the MJO
Madden-Julian Oscillation (MJO) is the planetary scale, intraseasonal variability in the tropical Indian and western Pacific Oceans. It propagates eastward at about 5 m/s. Although it has a large impact on global climate systems, some basic questions about the MJO remain, such as why the MJO propagates ~ 5 m/s, why it lasts for 30 - 60 days, and what provides energy. We propose that the MJO is composed of the Mixed Rossby Gravity (MRG) waves, and it propagates with the MRG wave group velocity (about 5 m/s). Simulation with GFDL spectral dynamical core Flexible Modeling System shows that the MRG waves can be forced independently, and the MRG wave packet can last long enough to form the intraseasonal variability. In other words, the MJO could be an atmospheric response to the independent forcing. Two-dimensional Fourier analysis and filtering have been carried out to analyze the NOAA Outgoing Long-wave Radiation (OLR) dataset. The results indicate that the power of the MRG waves is concentrated in the Indian and western Pacific Oceans, where the MJO usually happens. We will present further statistical analysis of the OLR data set to see if there is a relation between the amplitude of the MRG waves and that of the MJO.
The Madden-Julian oscillation wind-convection coupling and the role of moisture processes in the MM5 model
The Madden-Julian oscillation (MJO) produced by a mesoscale model is investigated using standardized statistical diagnostics. Results show that upper- and lower-level zonal winds display the correct MJO structure, phase speed (8 m/s) and space-time power spectrum. However, the simulated free atmosphere moisture, outgoing longwave radiation and precipitation do not exhibit any clear MJO signal. Yet, the boundary layer moisture, moist static energy and atmospheric instability, measured using a moist static energy instability index, have clear MJO signals. A significant finding is the ability of the model to simulate a realistic MJO phase speed in the winds without reproducing the MJO wind-convection coupling or a realistic propagation in the free atmosphere water vapor. This study suggests that the convergence of boundary layer moisture and the discharge and recharge of the moist static energy and atmospheric instability may be responsible for controlling the speed of propagation of the MJO circulation.
Numerical Modelling of the Observed Diurnal Cycle of Indian Monsoon Rainfall
We have analysed the diurnal cycle of rainfall over the Indian region (10S-35N, 60E-100E) using both satellite and in-situ data, and found many interesting features associated with this fundamental, yet under-explored, mode of variability. Since there is a distinct and strong diurnal mode of variability associated with the Indian summer monsoon rainfall, we evaluate the ability of the Weather Research and Forecasting Model (WRF) to simulate the observed diurnal rainfall characteristics. The model (at 54km grid-spacing) is integrated for the month of July, 2006, since this period was particularly favourable for the study of diurnal cycle. We first evaluate the sensitivity of the model to the prescribed sea surface temperature (SST), by using two different SST datasets, namely, Final Analyses (FNL) and Real-time Global (RTG). It was found that with RTG SST the rainfall simulation over central India (CI) was significantly better than that with FNL. On the other hand, over the Bay of Bengal (BoB), rainfall simulated with FNL was marginally better than with RTG. However, the overall performance of RTG SST was found to be better than FNL, and hence it was used for further model simulations. Next, we investigated the role of the convective parameterization scheme on the simulation of diurnal cycle of rainfall. We found that the Kain-Fritsch (KF) scheme performs significantly better than Betts-Miller-Janjić (BMJ) and Grell-Devenyi schemes. We also studied the impact of other physical parameterizations, namely, microphysics, boundary layer, land surface, and the radiation parameterization, on the simulation of diurnal cycle of rainfall, and identified the “best” model configuration. We used this configuration of the “best” model to perform a sensitivity study on the role of various convective components used in the KF scheme. In particular, we studied the role of convective downdrafts, convective timescale, and feedback fraction, on the simulated diurnal cycle of rainfall. The “best” model simulations, in general, show a good agreement with observations. Specifically, (i) Over CI, the simulated diurnal rainfall peak is at 1430 IST, in comparison to the observed 1430-1730 IST peak; (ii) Over Western Ghats and Burmese mountains, the model simulates a diurnal rainfall peak at 1430 IST, as opposed to the observed peak of 1430-1730 IST; (iii) Over Sumatra, both model and observations show a diurnal peak at 1730 IST; (iv) The observed southward propagating diurnal rainfall bands over BoB are weakly simulated by WRF. Besides the diurnal cycle of rainfall, the mean spatial pattern of total rainfall and its partitioning between the convective and stratiform components, are also well simulated. The “best” model configuration was used to conduct two nested simulations with one-way, three-level nesting (54-18-6km) over CI and BoB. While, the 54km and 18km simulations were conducted for the whole of July, 2006, the 6km simulation was carried out for the period 18 - 24 July, 2006. The results of our coarse- and fine-scale numerical simulations of the diurnal cycle of monsoon rainfall will be discussed.
Vertical Structure of the Madden-Julian Oscillation from AIRS and CloudSat
The Madden-Julian Oscillation (MJO) is the dominant form of intraseasonal variability in the tropical atmosphere and characterized by slowly eastward-propagating, large-scale oscillations in tropical deep convection and baroclinic wind field, especially during the boreal winter (November-April) over the warmest tropical waters in the equatorial Indian Ocean and western Pacific. The MJO has been shown to have important influences on various global weather and climate phenomena. However, the MJO is still not well understood nor well represented in global climate models. The recent available satellite data provide an excellent opportunity to study the MJO, especially its vertical structure. For example, Tian et al. [2006 J. Atmos. Sci, 63, 2462] documented the vertical moist thermodynamic structure of the MJO using the first three-year (2002-2005) of Atmospheric Infrared Sounder (AIRS) data. In this paper, we will reexamine the vertical moist thermodynamic structure of the MJO using the current available 7-year AIRS data (2002-2009). Meanwhile, we will examine the vertical cloud structure of the MJO using the liquid and ice water content from CloudSat. We will also compare our results from AIRS and CloudSat to a new reanalysis, the ECMWF interim reanalysis.
Monthly climatology of the equatorial hydrological cycle as observed from satellite and its connection to major equatorial climate indices
A climatology of the hydrological cycle is presented for the ±30 latitude band between Sep 2002 and Aug 2008 using Sea Surface Temperature, total water vapor, rain rate and Liquid Water Paths from AMSR-E as well as Temperature, water vapor, and RH profiles from AIRS. The annual cycle as well as the interannual variabiliby and anomalies are shown and compared to the timing of the climate signals of El Niño 2007, La Niña 2008, and to the QBO between 2002 and 2008. The comparison focuses on looking for temporal-lag correlations as a function of latitude, longitude, and height.
Observed Variability of the East Pacific ITCZ
The location and extent of the ITCZ has been identified for 30 years of satellite data using a statistical model. Satellite 3 hourly IR, daily visual and 12 hourly total precipitable water are used as inputs to the model. The information has been used to produce a full climatology of ITCZ occurrence and variability on synoptic time scales. The ITCZ labels are identified by LIMA (Labeling the ITCZ using a Markov-random field Algorithm), a new probability model which uses satellite data from a given location and information from neighboring pixels (in space and time) to inform the decision on whether a given pixel is classified as ITCZ or non-ITCZ. By this method the algorithm is able to emulate what a human would `see' as organized cloud belonging to the convergence zone and does not include isolated convection which would be picked up from thresholding techniques. This method of identification is particularly useful for examining the dynamical aspects of the observed ITCZ as it can pick up the shape and undulations of the different parts of its lifecycle. The overall climatology of the ITCZ and its intraseasonal characteristics are also obtained and have been compared to alternative identification techniques for validation. A separate tracking algorithm has been created to identify clouds associated with tropical cyclone activity. The information has been used to garner statistics on the interaction between ITCZ and tropical cyclones as well extracting information on the cloud characteristics of the cyclones themselves. The LIMA labels of ITCZ have been used to investigate interaction between the Pacific convective region and other aspects of tropical variability. The impacts of tropical features such as the MJO on the ITCZ shape, extent and convection strength will be shown alongside timeseries analysis for the full length dataset. In addition, interactions between ITCZ and ENSO will be shown along with diagnostics for climatic trends in the structure and occurrence of the convergence zone. The method of detection has the advantage that variability within the labeled zone can be extracted, thus the diurnal cycle of convection as well as relationship to sea surface temperatures have been obtained and will be presented alongside the seasonal variability.
Shading is IR satellite for June 7 2002, 06 UTC, resolution of 0.5 degrees, obtained from the HURSAT data base. Black contour is the LIMA label of the location of the ITCZ, showing undulation of the feature. The tropical cyclone is identified by a separate tracking algorithm at a later stage.
Asian Monsoon and Western Pacific Precipitation Variability in the Super-Parameterized Community Climate System Model
Climate simulations using a multi-scale modeling framework (MMF), in which traditionally parameterized effects of cumulus convection are explicitly simulated with multiple realizations of a cloud resolving model, may offer new insights into the role of convection on large-scale climate variability. Analysis of MMF simulations of the Community Atmospheric Model (CAM, v3.0) with prescribed sea surface temperatures indicates that the "super-parameterized" CAM, or SpCAM, dramatically improves variability of precipitation across a broad range of temporal scales (from diurnal to intraseasonal) at a variety of locations compared to the traditional CAM. These improvements in variability, however, are accompanied by overly intense tropical precipitation at some locations, notably the Indian and Asian summer monsoon regions, and extending into the western Pacific Ocean. Hypotheses for the anomalously high precipitation include the cloud-resolving model's cyclic boundary condition leading to a positive feedback between the convective environment and precipitation production, and the non-interaction of convection with sea surface temperatures (SSTs) in this region. In this study, we compare standard and super-parameterized versions of CAM (CAM, SpCAM) with coupled versions of the Community Climate System Model (CCSM, SpCCSM). In addition to an improved Madden-Julian Oscillation, ENSO, and reduced double-ITCZ, the SpCCSM also improves the intensity and geographic distribution of precipitation in the Indian-Asian monsoon region and western Pacific Ocean compared to CCSM. Coupling improves monsoonal rainfall intermittency in SpCCSM, and produces more realistic seasonal mean interactions between SST and precipitation anomalies for this region. Intermittency improvements are shown to arise from previously identified improvements in temporal and spatial variability of SpCAM rainfall. Improvements in seasonal-mean SST-precipitation relationship are a new result, the reasons for which are not yet completely understood. The elimination of the western Pacific high rainfall bias in SpCCSM is substantial, and we continue to explore the role of SST-precipitation interactions in this region via analysis of daily mean precipitation and SST anomalies.
The Modulation of the Extratropical Atmosphere in the Pacific Basin in Response to the Madden Julian Oscillation
ECMWF Re-analysis data are combined with a number of novel climatologies to conduct a comprehensive examination of the response of the subtropical and extratropical atmosphere over the Pacific basin to an evolving Madden Julian Oscillation (MJO) event. The adopted approach constitutes a symbiosis of a climatological analysis during the Northern Hemisphere winter from 1979-2002 and a case study analysis of a distinct MJO event that occurred in January-February 1993. The former is designed to obtain the general characteristics observed during a composite MJO lifecycle, while the latter is used to provide insight into the instantaneous mechanisms responsible for the observed composite evolution. A primary component of the study involves the diagnosis of anomalous wave breaking activity in response to MJO forcing in the form of tropical convection and/or upper-level divergence. Wave breaking events are separated by their characteristic life cycles: LC1 (anticyclonic) and LC2 (cyclonic) events. Statistically significant anomalies in wave breaking activity are found to be prevalent during the composite MJO event. Furthermore, the dynamical distinction between LC1 and LC2 wave breaking is useful in that the two different characteristic lifecycles exhibit significantly different anomalous behavior during the MJO. Statistically significant variability is also identified in both the subtropical and extratropical flow and atmospheric blocking and surface cyclone frequency. These data, taken in conjunction with the observed evolution of the 1993 MJO event, provide a relatively coherent picture of the response of the atmosphere to MJO forcing. A schematic representation of the evolution is provided.
Seasonal Variation and Spatial Extent of Tropical Intraseasonal Oscillations
The intraseasonal oscillations and seasonally persisting patterns of precipitation in the tropical atmosphere are examined in this study. The nonlinear oscillatory and persisting modes of variability are obtained in a data-adaptive manner by applying multi-channel singular spectrum analysis to TRMM precipitation. The leading oscillatory modes are in the timescales of 45 days and 30 days and show different zonal and meridional propagation features. The seasonal variation of the leading oscillation is investigated to find the relation to the South Asian monsoon’s active-break cycle and the Madden-Julian oscillation (MJO). The spatial extent of the intraseasonal oscillations is found to span a large area and vary in a coherent manner. The leading persisting mode varies in a coherent manner very slowly, and is shown to be an atmospheric component of El Niño-Southern Oscillation (ENSO). The relation between the intraseasonal oscillations (MJO and monsoon) and the ENSO mode is also examined. The characteristics of these coherent intraseasonal oscillations and ENSO mode during the YOTC period are discussed.
Water vapor recycling induced by the MJO convection: Application of water isotope tracers to the study of tropical convection
Here, we newly introduce water isotope tracers to understand tropical convection system, which has been a major challenging subject in meteorology, and consider what new findings they can provide us. Water has several naturally occurring isotopes (H216O, HDO, H218O etc.), and in the atmosphere, the behavior of isotopic ratio of them in the atmosphere is intimately linked to the transportation of atmospheric water due to significant vertical profile that isotopic ratio progressively decrease with altitude and cloud physics due to the fractionation, which occurs during phase change in the cloud. For example, because heavy isotopes are enriched in the condensation phase, vapor in the cloud is more depleted in heavy isotopes as amount of precipitation increase. In this study, using newly obtained observational data from satellite (TES/AURA) and MISMO campaign program carried out in the tropical Indian Ocean in 2006, we examine the relationship between the isotopic composition of water and convective processes, and then identify the key process governing the isotopic behavior in the tropics using single column model including the Arakawa-Schubert cumulus parameterization. New findings of this study are that 1) minimum isotope peaks in the intrapersonal isotopic variation almost overlap with the active convection period includes the MJO. 2) During the active convection, eastward-propagating precipitation system was observed frequently, and by repeatedly passing of rainfall systems, isotopic composition of water in the lower atmosphere has gradually decreased. This isotopic shift from inactive to active convection period is well reproduced by the model. Model result suggests that in active convection period, due to successive convective circulation, water vapor in the atmosphere is well mixed. Because isotopic composition of atmospheric water gradually decreases toward high altitude, active vertical circulation results in lower isotopic composition in the lower atmosphere. That means isotopic composition in the lower atmosphere can be considered as the indicator of the intensity of downdraft and subsidence. The strong negative peak of isotopic composition associated with MJO reflects high contribution of downdraft and subsidence.
Linear response functions of a cumulus ensemble to temperature and moisture perturbations and implication to the dynamics of convectively coupled waves
In order to better bridge numerical simulations and conceptual/toy models of large-scale convectively coupled waves, we construct linear response functions of a cumulus ensemble to temperature and moisture perturbations using a cloud system-resolving model (CSRM). The cumulus ensemble is assumed to be in statistical equilibrium with the large-scale sounding. For a sufficiently complete set of perturbations, these linear response functions provide an adequate parameterization of the cumulus ensemble for investigating large-scale convectively coupled waves under a reference mean state. An approach for constructing these response functions is presented and applied to two different mean state conditions, where the CSRM, when coupled with 2D gravity waves, exhibits interestingly different behaviors. The linear response functions were able to reproduce these behaviors of the full CSRM with good quantitative accuracy. Comparing the response functions with the treatment of convection in the simple model of Kuang (2008) indicates general consistency, lending confidence that the instability mechanisms identified in that model provide the correct explanation to the instability seen in the CSRM simulations. Physical interpretations of these linear response functions will be explored. Reference: Kuang, Z., A moisture-stratiform instability for convectively coupled waves, Journal of Atmospheric Sciences, 65, 834-854, (2008).
The Role of Extratropical Controls in Northward Moisture Surges of the North American Monsoon
Advanced understanding of the dynamics of the North American monsoon (NAM) is critical to the moisture-sensitive desert southwest region of the United States where upwards of 50% of the annual rainfall is contributed by the monsoon season of June through September. A key determinant in interannual summer precipitation variability across the central and southern interior western United States are the northward surges of subtropical moisture that penetrate into the midlatitudes. Prior research into initiation mechanisms for these NAM-associated moisture surges has been limited to dynamical mechanisms inherent to tropical and sub-tropical latitudes, such as easterly waves. A novel approach is considered in this study by examining the role of extratropical features in both initiating and enabling NAM surge events. Observational evidence indicates that surge events can be triggered through an extratropical-tropical feedback process initiated by Rossby wave breaking east of the upper-tropospheric monsoonal ridge. In the wake of wave breaking, a positive vorticity eddy, or tropical upper tropospheric trough (TUTT), is injected into the subtropical latitudes, thereby invoking a mechanism for the return flow of moisture into midlatitudes across the south and central interior western United States. Results also suggest that the northward surge of subtropical moisture associated with the proposed Rossby wave breaking-TUTT triggering mechanism is further sensitive to the midlatitude background flow, providing additional evidence that the extratropics play a decidedly vital function in the NAM system.