Tropical Warm Pool International Cloud Research Articles

The Role of Entrainment in the Diurnal Cycle of Continental Convection

Abstract In continental convective environments, general circulation models typically produce a diurnal cycle of rainfall that peaks close to the noon maximum of insolation, hours earlier than the observed peak. One possible reason is insufficient sensitivity of their cumulus parameterizations to the state of the environment due to weak entrainment. The Weather Research and Forecasting (WRF) model, run at cloud-resolving (600 and 125 m) resolution, is used to study the diurnal transition from shallow to deep convection during the monsoon break period of the Tropical Warm Pool–International Cloud Experiment. The WRF model develops a transition from shallow to deep convection in isolated events by 1430–1500 local time. The inferred entrainment rate weakens with increasing time of day as convection deepens. Several current cumulus parameterizations are tested for their ability to reproduce the WRF behavior. The Gregory parameterization, in which entrainment rate varies directly with parcel buoyancy and inversely as the square of the updraft speed, is the best predictor of the inferred WRF entrainment profiles. The Gregory scheme depends on a free parameter that represents the fraction of buoyant turbulent kinetic energy generation on the cloud scale that is consumed by the turbulent entrainment process at smaller scales. A single vertical profile of this free parameter, increasing with height above the boundary layer but constant with varying convection depth, produces entrainment rate profiles consistent with those inferred from the WRF over the buoyant depth of the convection. Parameterizations in which entrainment varies inversely with altitude or updraft speed or increases with decreasing tropospheric relative humidity do not perform as well. Entrainment rate at cloud base decreases as convection depth increases; this behavior appears to be related to an increase in vertical velocity at downdraft cold pool edges.

Observed Large-Scale Structures and Diabatic Heating and Drying Profiles during TWP-ICE

Abstract This study documents the characteristics of the large-scale structures and diabatic heating and drying profiles observed during the Tropical Warm Pool–International Cloud Experiment (TWP-ICE), which was conducted in January–February 2006 in Darwin during the northern Australian monsoon season. The examined profiles exhibit significant variations between four distinct synoptic regimes that were observed during the experiment. The active monsoon period is characterized by strong upward motion and large advective cooling and moistening throughout the entire troposphere, while the suppressed and clear periods are dominated by moderate midlevel subsidence and significant low- to midlevel drying through horizontal advection. The midlevel subsidence and horizontal dry advection are largely responsible for the dry midtroposphere observed during the suppressed period and limit the growth of clouds to low levels. During the break period, upward motion and advective cooling and moistening located primarily at midlevels dominate together with weak advective warming and drying (mainly from horizontal advection) at low levels. The variations of the diabatic heating and drying profiles with the different regimes are closely associated with differences in the large-scale structures, cloud types, and rainfall rates between the regimes. Strong diabatic heating and drying are seen throughout the troposphere during the active monsoon period while they are moderate and only occur above 700 hPa during the break period. The diabatic heating and drying tend to have their maxima at low levels during the suppressed periods. The diurnal variations of these structures between monsoon systems, continental/coastal, and tropical inland-initiated convective systems are also examined.

Evaluating regional cloud‐permitting simulations of the WRF model for the Tropical Warm Pool International Cloud Experiment (TWP‐ICE), Darwin, 2006

Data from the Tropical Warm Pool International Cloud Experiment (TWP‐ICE) were used to evaluate Weather Research and Forecasting (WRF) model simulations with foci on the performance of three six‐class bulk microphysical parameterizations (BMPs). Before the comparison with data from TWP‐ICE, a suite of WRF simulations were carried out under an idealized condition, in which the other physical parameterizations were turned off. The idealized simulations were intended to examine the interaction of BMP at a “cloud‐resolving” scale (250 m) with the nonhydrostatic dynamic core of the WRF model. The other suite of nested WRF simulations was targeted on the objective analysis of TWP‐ICE at a “cloud‐permitting” scale (quasi‐convective resolving, 4 km). Wide ranges of discrepancies exist among the three BMPs when compared with ground‐based and satellite remote sensing retrievals for TWP‐ICE. Although many processes and associated parameters may influence clouds, it is strongly believed that atmospheric processes fundamentally govern the cloud feedbacks through the interactions between the atmospheric circulations, cloudiness, and the radiative and latent heating of the atmosphere. Based on the idealized experiments, we suggest that the discrepancy is a result of the different treatment of ice‐phase microphysical processes (e.g., cloud ice, snow, and graupel). Because of the turn‐off of the radiation and other physical parameterizations, the cloud radiation feedback is not studied in idealized experiments. On the other hand, the “cloud‐permitting” experiments engage all physical parameterizations in the WRF model so that the radiative heating processes are considered together with other physical processes. Common features between these two experiment suites indicate that the major discrepancies among the three BMPs are similar. This strongly suggests the importance of ice‐phase microphysics. To isolate the influence of cloud radiation feedback, we further carried out an additional suite of simulations, which turns off the interactions between cloud and radiation schemes. It is found that the cloud radiation feedback plays a secondary, but nonnegligible role in contributing to the wide range of discrepancies among the three BMPs.

Exploring the Transient Behavior of Z–R Relationships: Implications for Radar Rainfall Estimation

Abstract The objective of this study is to characterize the signature of dynamical microphysical processes on reflectivity–rainfall (Z–R) relationships used for radar rainfall estimation. For this purpose, a bin model with explicit microphysics was used to perform a sensitivity analysis of the shape parameters of the drop size distribution (DSD) as a function of time and rainfall regime. Simulations show that coalescence is the dominant microphysical process for low to moderate rain intensity regimes (R < 20 mm h−1) and that the rain rate in this regime is strongly dependent on the spectral properties of the DSD (i.e., the shape). The time to equilibrium for light rainfall is at least twice as long as in the case of heavy rainfall (1 h for stratiform vis-à-vis 30 min for thunderstorms). For high-intensity rainfall (R > 20 mm h−1), collision–breakup dynamics dominate the evolution of the raindrop spectra. The time-dependent Z–R relationships produced by the model converge to a universal Z–R relationship for heavy intensity rainfall (A = 1257; b ∼ 1) centered on the region of Z–R space defined by the ensemble of over 100 empirical Z–R relationships. Given the intrinsically transient nature of the DSD for light rainfall, it is proposed that the vertical raindrop spectra and corresponding rain rates should be modeled explicitly by a microphysical model. A demonstration using a multicolumn simulation of a Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) overpass over Darwin for a stratiform event during the Tropical Warm Pool–International Cloud Experiment (TWP-ICE) is presented.

Imaging of atmospheric gravity waves in the stratosphere and upper mesosphere using satellite and ground‐based observations over Australia during the TWPICE campaign

During the Tropical Warm Pool International Cloud Experiment (TWPICE) an intense tropical low was situated between Darwin and Alice Springs, Australia. Observations made on 31 January 2006 by the Atmospheric Infrared Sounder instrument on the NASA Aqua satellite imaged the presence of atmospheric gravity waves (AGWs), at approximately 40 km altitude, with horizontal wavelengths between 200 and 400 km that were originating from the region of the storm. Airglow images obtained from Alice Springs (about 600 km from the center of the low) showed the presence of similar waves with observed periods of 1 to 2 h. The images also revealed the presence of 30‐ to 45‐km‐horizontal‐wavelength AGWs with shorter observed periods of near 15 to 25 min. Ray tracing calculations show that (1) some of the long wavelength waves traveled on rays, without ducting, to the altitudes where the observations were obtained, and (2) shorter‐period waves rapidly reached 85 km altitude at a horizontal distance close to the storm, thus occurring over Alice Springs only if they were trapped or ducted. The mesospheric inversion layer seen in the measured temperature data almost forms such a trapped region. The winds therefore critically control the formation of the trapped region. Wind profiles deduced from the available data show the plausibility for the formation of such a trapped region. Variations in the wind, however, would make ideal trapped region conditions short‐lived, and this may account for the sporadic nature of the short‐period wave observations.

Testing ice microphysics parameterizations in the NCAR Community Atmospheric Model Version 3 using Tropical Warm Pool–International Cloud Experiment data

Cloud properties have been simulated with a new double‐moment microphysics scheme under the framework of the single‐column version of NCAR Community Atmospheric Model version 3 (CAM3). For comparison, the same simulation was made with the standard single‐moment microphysics scheme of CAM3. Results from both simulations compared favorably with observations during the Tropical Warm Pool–International Cloud Experiment by the U.S. Department of Energy Atmospheric Radiation Measurement Program in terms of the temporal variation and vertical distribution of cloud fraction and cloud condensate. Major differences between the two simulations are in the magnitude and distribution of ice water content within the mixed‐phase cloud during the monsoon period, though the total frozen water (snow plus ice) contents are similar. The ice mass content in the mixed‐phase cloud from the new scheme is larger than that from the standard scheme, and ice water content extends 2 km further downward, which is in better agreement with observations. The dependence of the frozen water mass fraction on temperature from the new scheme is also in better agreement with available observations. Outgoing longwave radiation (OLR) at the top of the atmosphere (TOA) from the simulation with the new scheme is, in general, larger than that with the standard scheme, while the surface downward longwave radiation is similar. Sensitivity tests suggest that different treatments of the ice crystal effective radius contribute significantly to the difference in the calculations of TOA OLR, in addition to cloud water path. Numerical experiments show that cloud properties in the new scheme can respond reasonably to changes in the concentration of aerosols and emphasize the importance of correctly simulating aerosol effects in climate models for aerosol‐cloud interactions. Further evaluation, especially for ice cloud properties based on in situ data, is needed.

WRF and GISS SCM simulations of convective updraft properties during TWP‐ICE

The Weather Research and Forecasting (WRF) model, running at cloud‐resolving model resolution (1.3 km and 0.6 km), is used to simulate cumulus updraft speeds associated with three distinct convective regimes sampled during the intensive observing period of the Tropical Warm Pool–International Cloud Experiment (TWP‐ICE) near Darwin, Australia. The WRF model produces strong updrafts during a monsoon break period and weaker updrafts during an active monsoon period, consistent with observational proxies of convective strength. It also captures the observed feature of midlevel convection during a suppressed monsoon period. The ability of the WRF model to differentiate the updraft speeds among three subperiods is robust to changes in its microphysics and turbulence schemes, resolution, and forcing procedure. For comparison to the parameterized diagnostic updraft speeds in the Goddard Institute for Space Studies Single Column Model (GISS SCM), we define an equivalent mean updraft speed for deep convection in the WRF simulation as the ratio of the domain average upward flux of hydrometeors to the domain average hydrometeor water content. Parameterized convective updraft speeds diagnosed from the thermodynamic structure in the SCM can reproduce the WRF difference between the active and break period updraft strength and the shallower suppressed monsoon convection, but only if a free parameter that regulates entrainment strength is allowed to vary. SCM updraft speeds are consistently too strong in the upper troposphere compared with the WRF. Hydrometeor profiles in both the WRF and the SCM are sensitive to assumptions about the ice phase microphysics.

Single‐scattering properties of aggregates of plates

AbstractDuring the 2006 Tropical Warm Pool International Cloud Experiment (TWP‐ICE) sponsored by the Atmospheric Radiation Measurement programme, the Scaled Composites Proteus aircraft executed spiral profiles and flew horizontal legs through aging anvils, fresh anvils, and cirrus of unknown origin in the vicinity of Darwin, Australia. Pristine ice crystals including both plates and bullet rosettes, their aggregates, and unclassifiable ice crystals were observed by a Cloud Particle Imager. The widths of observed plates ranged between 80 µm and 200 µm. When a fresh dissipating anvil was observed on 2 February, aggregates of plates contributed 46.2% of the total area of measured ice crystals with maximum dimensions greater than 200 µm, while it was only 7.2% and 1.0% for 27 and 29 January, respectively, when aged cirrus was sampled. Because aggregates of plates have been observed to make large contributions to projected ice crystal area near convection during TWP‐ICE and other projects, their single‐scattering properties that have not been previously examined are investigated here.The dependence of the scattering phase function P11, asymmetry parameter g, and single‐scattering albedo ω0 on three parameters (the area ratio AR, normalized projected area An, and a newly defined aggregation index AI which varies between 0 and 1 with ice crystals with more compact shape having lower AI) defining the three‐dimensional shapes of aggregates of plates were calculated using a geometric ray‐tracing code at wavelengths λ of 0.55, 1.38 and 2.13 µm. The scattering properties depended on the crystal morphology with, for example, the g of an aggregate of plates with a high AI of 0.818 differing by + 6.89% (+6.44%; + 4.55%) from that with a lower AI of 0.378 at λ of 0.55 µm (1.38 µm; 2.13 µm), but by only + 0.29% (+0.25%; − 0.03%) from those of the component plates. The ω0 at absorbing λ increased with AI, 1 − AR, and An. Adding ray distortion to the aggregates caused a decrease in forward scattering and an increase in lateral and backward scattering, decreasing g, an effect that weakened with λ due to absorption. The aggregates of plates with low AI were more influenced by ray distortion compared to those with high AI. The dependence of scattering properties on crystal morphology noted here should be considered when computing bulk scattering properties of ice clouds to determine its importance for climate and remote‐sensing studies. Copyright © 2009 Royal Meteorological Society

A study of the triggering mechanisms for deep convection in the Tropics using a Mesoscale Model: Hector events during SCOUT-O 3 and TWP-ICE campaigns. (3.10)

One of the purposes of SCOUT-O 3 (Stratosphere–Climate Links with emphasis on the Upper Troposphere and Lower Stratosphere) campaign was to improve our understanding of the interaction between convection and the tropical tropopause layer. Within the framework of SCOUT-O 3 project a study on Hector events has been carried out. Hector is a deep convective cell developing on Tiwi Islands during the pre-monsoon period and break monsoon period. In this study two Hector events are investigated: 30 November 2005 a multicells event occurred during SCOUT-O 3 campaign, and a single cell developed on 6 February 2006 during the TWP-ICE (Tropical Warm Pool International Cloud Experiment) campaign. The first event was characterized by two cells: the first cell downdraft triggering the second cell. The second Hector event was characterized by a cell triggered by the interaction of a previous convective cell and the south breeze front. The dynamics and thermodynamics of both events have been analyzed using the mesoscale model MM5V3 (Fifth–Generation Mesoscale Model Version 3). A few preliminary experiments have been performed to the aim of tuning the MM5 for the tropics; to this purpose two parameters are considered as the mass flux in the convective scheme and the condensation nuclei in the microphysical parameterization. Satellite and Radar images are used together with MM5V3 results to investigate the triggering mechanisms of the two Hector events. The MM5 results for the Hector event of Nov 30, 2005 shows a slightly lower Hector first cell, whereas the second one is very well reproduced, suggesting a stronger triggering acting in this second case. Moreover, the structure and the precipitation produced are in good agreement with the observation. For Feb 6, 2006 MM5 shows a shortcoming in the timing of maximum development of Hector (1 h earlier), but the structure and the precipitation are in good agreement with the observations. Finally, numerical experiments are performed to the aim of investigating the triggering factor for the two Hector events, which allows for assessing type A for the first cell of November 30, 2005 and type B for the second cell of November 30, 2005 and February 6, 2006 event.

Supplement to The Tropical Warm Pool International Cloud Experiment

One of the most comprehensive data sets of tropical cloud systems and their environmental setting and impacts ever sampled has been collected during the Tropical Warm Pool International Cloud Experiment in the area around Darwin, Northern Australia in January and February of 2006. The experiment design utilized permanent observational facilities in Darwin which include a polarimetric weather radar operated by the Australia Bureau of Meteorology (BOM) and a suite of cloud remote sensing instruments operated by the DOE Atmospheric Radiation Measurement (ARM) program. A dense network of observations added for the experiment included ocean observations and a dense balloon-borne sounding network. An integral factor in the design was to provide boundary conditions and validation data sets for a range of modelling activities and cloud retrieval development. A fleet of five research aircraft were deployed including two high altitude aircraft for characterizing cloud properties and the atmospheric state, a plane carrying airborne cloud radar and lidar and two aircraft sampling the boundary layer in great detail including fluxes, aerosols and chemistry.

The Tropical Warm Pool International Cloud Experiment

A comprehensive dataset describing tropical cloud systems and their environmental setting and impacts has been collected during the Tropical Warm Pool International Cloud Experiment (TWPICE) and Aerosol and Chemical Transport in Tropical Convection (ACTIVE) campaign in the area around Darwin, Northern Australia, in January and February 2006. The aim of the experiment was to observe the evolution of tropical cloud systems and their interaction with the environment within an observational framework optimized for a range of modeling activities with the goal of improving the representation of cloud and aerosol process in a range of models. The experiment design utilized permanent observational facilities in Darwin, including a polarimetric weather radar and a suite of cloud remote-sensing instruments. This was augmented by a dense network of soundings, together with radiation, flux, lightning, and remote-sensing measurements, as well as oceanographic observations. A fleet of five research aircraft, including ...

Development and Evaluation of a Simple Algorithm to Find Cloud Optical Depth with Emphasis on Thin Ice Clouds

An algorithm is presented here for determining cloud optical depth, � , using data from shortwave broadband ir- radiances, focusing on the case of optically thin clouds. This method is empirical and consists of a one-line equation. This method is applied to cirrus clouds observed at the Atmospheric Radiation Measurement Program Climate Research Facil- ity (ACRF) at Darwin, Australia, during the Tropical Warm Pool International Cloud Experiment (TWP-ICE) campaign and cirrus clouds observed at the ACRF Southern Great Plains (SGP) site. These cases were chosen because independent verification of cloud optical depth retrievals was possible. For the TWP-ICE case, the calculated optical depths agree to within 1 unit withcalculated from a vertical profile of ice particle size distributions obtained from an aircraft sounding. For the SGP case, the results from the algorithm correspond reasonably well withvalues obtained from an average over other retrieval methods, some of which have been subject to independent verification. The medians of the two time series are 0.79 and 0.81, for the empirical and averaged values, respectively. Because such close agreement is likely to be fortui- tous and therefore not truly represent the performance of our method, � values derived from our method were compared to values obtained from lidar data. Over a three year period, the difference in median values between the two methods is about 0.6, with the lidar optical depths being larger. This tool may be applied wherever measurements of the direct, dif- fuse, and total components of the shortwave broadband flux are available at 1- to 5-minute resolution. Because these measurements are made across the world, it then becomes possible to estimate optical depth for both liquid water and ice clouds at many locations.

Anvil Characteristics as Seen by C-POL during the Tropical Warm Pool International Cloud Experiment (TWP-ICE)

Abstract The Tropical Pacific Warm Pool International Cloud Experiment (TWP-ICE) took place in Darwin, Australia, in early 2006. C-band radar data were used to characterize tropical anvil (i.e., thick, nonprecipitating cloud associated with deep convection) areal coverage, height, and thickness during the monthlong field campaign. The morphology, evolution, and longevity of the anvil were analyzed, as was the relationship of the anvil to the rest of the precipitating system. The anvil was separated into mixed (i.e., echo base below 6 km) and ice-only categories. The average areal coverage for each anvil type was between 4% and 5% of the radar grid. Ice anvil thickness averaged 2.8 km and mixed anvil thickness averaged 6.7 km. Areal peaks show that mixed anvil typically formed out of the stratiform rain region. Peak production in ice anvil usually followed the mixed anvil peak by 1–3 h. Anvil typically lasted 4–10 h after the initial convective rain area peak. TWP-ICE experienced three distinct regimes: an active monsoon, a dry monsoon, and a break period. During the experiment (except the active monsoon period) there was a strong negative correlation between ice anvil thickness and ice anvil height, a strong positive correlation between ice anvil area and thickness, and a greater variance in ice anvil bottom than ice anvil top. These results have important implications for understanding how anvil affects the tropical atmosphere.

The CO<sub>2</sub> tracer clock for the Tropical Tropopause Layer

Abstract. Observations of CO2 were made in the upper troposphere and lower stratosphere in the deep tropics in order to determine the patterns of large-scale vertical transport and age of air in the Tropical Tropopause Layer (TTL). Flights aboard the NASA WB-57F aircraft over Central America and adjacent ocean areas took place in January and February, 2004 (Pre-AURA Validation Experiment, Pre-AVE) and 2006 (Costa Rice AVE, CR-AVE), and for the same flight dates of 2006, aboard the Proteus aircraft from the surface to 15 km over Darwin, Australia (Tropical Warm Pool International Cloud Experiment, TWP-ICE). The data demonstrate that the TTL is composed of two layers with distinctive features: (1) the lower TTL, 350–360 K (potential temperature(θ); approximately 12–14 km), is subject to inputs of convective outflows, as indicated by layers of variable CO2 concentrations, with air parcels of zero age distributed throughout the layer; (2) the upper TTL, from θ=~360 K to ~390 K (14–18 km), ascends slowly and ages uniformly, as shown by a linear decline in CO2 mixing ratio tightly correlated with altitude, associated with increasing age. This division is confirmed by ensemble trajectory analysis. The CO2 concentration at the level of 360 K was 380.0(±0.2) ppmv, indistinguishable from surface site values in the Intertropical Convergence Zone (ITCZ) for the flight dates. Values declined with altitude to 379.2(±0.2) ppmv at 390 K, implying that air in the upper TTL monotonically ages while ascending. In combination with the winter slope of the CO2 seasonal cycle (+10.8±0.4 ppmv/yr), the vertical gradient of –0.78 (±0.09) ppmv gives a mean age of 26(±3) days for the air at 390 K and a mean ascent rate of 1.5(±0.3) mm s−1. The TTL near 360 K in the Southern Hemisphere over Australia is very close in CO2 composition to the TTL in the Northern Hemisphere over Costa Rica, with strong contrasts emerging at lower altitudes (<360 K). Both Pre-AVE and CR-AVE CO2 observed unexpected input from deep convection over Amazônia deep into the TTL. The CO2 data confirm the operation of a highly accurate tracer clock in the TTL that provides a direct measure of the ascent rate of the TTL and of the age of air entering the stratosphere.

Importance of small ice crystals to cirrus properties: Observations from the Tropical Warm Pool International Cloud Experiment (TWP‐ICE)

During the Department of Energy Atmospheric Radiation Measurement Program (DOE ARM) sponsored Tropical Warm Pool International Cloud Experiment (TWP‐ICE), ice crystals with maximum dimensions (D) < 50 μm were measured in aged cirrus and fresh anvils by a Cloud and Aerosol Spectrometer (CAS) and a Cloud Droplet Probe (CDP). The CAS/CDP ratio of the number concentrations of droplets with 3 < D < 50 μm, N3−50, averaged 0.98 ± 0.69 in liquid clouds. However, N3−50, measured by the CAS averaged 91 ± 127 times larger than N3−50 from the CDP in ice clouds. The CAS/CDP N3−50 ratio had a correlation coefficient of 0.387 with the concentration of particles with D > 100 μm measured by the Cloud Imaging Probe, suggesting that ice crystals may have been shattering or bouncing on the CAS inlet or protruding airflow shroud enhancing N>3−50,CAS. During the Costa Rica Aura Validation Experiment N3−50,CAS measured by a CAS without an airflow shroud were an order of magnitude less than those observed during TWP‐ICE. This, and estimates of the maximum shattering based on the inlet and shroud sizes, suggest that the airflow shroud used during TWP‐ICE was responsible for much of the shattering or bouncing.

Experiment to characterize tropical cloud systems

A major experiment to study tropical convective cloud systems and their impacts will take place around Darwin, northern Australia, in January and February 2006. The Tropical Warm Pool International Cloud Experiment (TWP‐ICE) aims to study cloud systems and their radiative, chemical, and moisture budget impacts from the initial convection through to the decaying, convectively generated cirrus.This broad aim is being addressed by a dense observational network that includes two sites equipped with cloud radar and lidar, a polarimetric (dual‐polarization) weather radar and another operational Doppler radar in the area, wind profilers, infrared interferometers, flux stations, lightning interferometers, and oceanographic measurements. Also, an array of five radiosonde sites arranged in a ring with a diameter of about 250 km around Darwin will conduct soundings every three hours for a period of 24 days.