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Sandra E. Yuter and Wendy S. Parker

Abstract

Fifteen rain measurement instruments were deployed on the National Oceanic and Atmospheric Administration Ship Ronald H. Brown during the 1997 Pan American Climate Studies (PACS) Tropical Eastern Pacific Process Study (TEPPS). To examine differences in rainfall catchment related to instrument design, three types of disdrometers, an optical rain gauge, a ship rain gauge, and a siphon gauge were clustered in one location to ensure similar exposure. To address exposure effects, eight siphon rain gauges were deployed on different sides of the ship and on several different levels.

Cross-ship differences in hourly rainfall accumulation were negligible when relative wind speeds were less than 3 m s−1 and became significant at greater than 5 m s−1, especially when the relative wind direction was 20° or greater from the bow. Instruments with both horizontal and vertical catchment surfaces yielded a measurable collection advantage over instruments with only horizontal catchment surfaces.

Analysis of data collected during TEPPS using a multiple-instrument, multiple-location approach yields the following recommendations for reducing uncertainty in rain measurement at sea. The first two of the four recommendations apply to rain measurements on buoys as well as on ships. 1) Deploy experimental rain measurement instrumentation paired with a baseline minimum siphon gauge or other trusted instrument. Comparison of the rain-rate time series between the baseline gauge measurements and the experimental instrument data permits detection of erratic behavior and bias. 2) Apply an appropriate wind correction. To do this step properly, both a wind correction formula derived for the specific gauge type and a nearby measurement of relative wind are needed. These features are already incorporated into the ship rain gauge. 3) Locate gauges where distortion of the airflow by the ship is locally minimized and relative wind speeds are as low as possible. This analysis confirms previous recommendations for placement of rain instrumentation at lower locations as long as the location is protected against direct spray from the sea without being shadowed by higher objects. 4) Place instrumentation on both sides of ship and along centerline. Airflow distortion by the ship itself can induce significant differences between port and starboard accumulations at high wind speeds and high angle of wind attack to the bow. Multiple locations aid in constraining error, because relative wind direction and speed vary during a cruise and there is no one perfect location on ship for rain instrumentation.

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S. S. Chen, R. A. Houze Jr., B. E. Mapes, S. R. Brodzik, and S. E. Yuter

Satellite data summary images and analysis plots from the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE), which were initially prepared in the field at the Honiara Operations Center, are now available on the Internet via World Wide Web browsers such as Mosaic. These satellite data summaries consist of products derived from the Japanese Geosynchronous Meteorological Satellite IR data: a time–size series of the distribution of contiguous cold cloudiness areas, weekly percent high cloudiness (PHC) maps, and a five-month time–longitude diagram illustrating the zonal motion of large areas of cold cloudiness. The weekly PHC maps are overlaid with weekly mean 850-hPa wind calculated from the ECMWF global analysis field and can be viewed as an animation loop. These satellite summaries provide an overview of spatial and temporal variabilities of the cloud population and a large-scale context for studies concerning specific processes of various components of TOGA COARE.

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Kimberly K. Comstock, Christopher S. Bretherton, and Sandra E. Yuter

Abstract

Observations from the East Pacific Investigation of Climate (EPIC) 2001 field campaign are well suited for exploring the relationships among the diurnal cycle, mesoscale (10–100 km) structure, and precipitation in the stratocumulus region in the southeast Pacific. Meteorological time series and observations from a scanning C-band radar, vertically pointing cloud radar, and ceilometer, as well as satellite data, are used to show that drizzle is associated with increased variability in cloud and boundary layer properties compared to nondrizzling periods. The stratocumulus-topped boundary layer is typically well mixed at night, transitioning to less well mixed in the afternoon, with drizzle most frequently occurring in the early morning. Coherent patches of drizzle, or “cells,” can have large areas with radar reflectivities of greater than 5 dBZ of up to about 100 km2. Individual cells have long lifetimes, up to 2 h, and appear to be replenished by moisture in the boundary layer.

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Robert A. Houze Jr., Shuyi S. Chen, David E. Kingsmill, Yolande Serra, and Sandra E. Yuter

Abstract

Deep convection over the western tropical Pacific warm pool is analyzed in terms of its relation to the atmospheric Kelvin–Rossby wave, which dominates the large-scale flow during the austral summer. The study uses Doppler radar data collected by aircraft and ship radars during different time periods in the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment to characterize the mesoscale circulations of organized convective cloud systems occurring throughout the season. The study focuses on convection in two contrasting phases of the wave: the “westerly onset region” just west of the point within the wave where low-level easterlies change to westerlies, and the “strong westerly region” (or “westerly wind burst”) lying between the large-scale counterrotating gyres of the Kelvin–Rossby wave.

In the westerly onset region the zonal wind component had midlevel easterlies overlying low-level westerlies. In the strong westerly region a deep layer of westerlies extended from the surface up to the upper troposphere, with a maximum of westerly component at about the 850-mb level. The different vertical shear of the zonal wind in these two regions of the wave led to different momentum transport by the mesoscale circulations that develop into very large “super convective systems” (cloud tops colder than −65°C over regions of ∼300 km or more in lateral dimension). The super convective systems developed strong midlevel inflow jets. The direction of the jet was determined by the environmental shear, which in turn was determined by the dynamics of the large-scale wave. In the westerly onset region, the large-scale shear determined that the jet had an easterly component. In the strong westerly region, the jet had a westerly component. In both cases, the inflow intensified within the cloud system as the convective cells of the super convective system filled a broad region with a deep stratiform ice cloud, from which ice particles fell. Evidently, as the particles sublimated and melted, they cooled the air at midlevels in the cloud system. The cooling evidently modified the mesoscale pressure field in the system so as to accelerate the flow of ambient air into the system and to encourage the inflow to subside. In this way, the mesoscale inflow to super convective systems transported easterly momentum downward in the westerly onset region and westerly momentum downward in the strong westerly region, so that the mesoscale momentum feedback of the mesoscale inflow jets were negative in the westerly onset region and positive in the strong westerly region (accelerating the westerly wind burst). These momentum transports by the broad mesoscale midlevel inflow of super convective systems affected broad horizontal regions and were sometimes different in sign from the momentum transports of individual convective-scale cells in the same system.

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Adam H. Sobel, Sandra E. Yuter, Christopher S. Bretherton, and George N. Kiladis

Abstract

An overview of the large-scale behavior of the atmosphere during the Tropical Rainfall Measuring Mission (TRMM) Kwajalein Experiment (KWAJEX) is presented. Sounding and ground radar data collected during KWAJEX, and several routinely available datasets including the Geostationary Meteorological Satellite (GMS), NOAA outgoing longwave radiation (OLR), the Special Sensor Microwave Imager (SSM/I), and ECMWF operational analyses are used. One focus is on the dynamical characterization of synoptic-scale systems in the western/central tropical Pacific during KWAJEX, particularly those that produced the largest rainfall at Kwajalein. Another is the local relationships observed on daily time scales among various thermodynamic variables and areal average rain rate. These relationships provide evidence regarding the degree and kind of local thermodynamic control of convection.

Although convection in the Marshall Islands and surrounding regions often appears chaotic when viewed in satellite imagery, the largest rain events at Kwajalein during the experiment were clearly associated with large-scale envelopes of convection, which propagated coherently over several days and thousands of kilometers, had clear signals in the lower-level large-scale wind field, and are classifiable in terms of known wave modes. Spectral filtering identifies mixed Rossby–gravity (MRG) and Kelvin waves prominently in the OLR data. “Tropical depression–type” disturbances are also evident. In some cases multiple wave types may be associated with a single event. Three brief case studies involving different wave types are presented.

Daily-mean sounding data averaged over the five sounding sites show evidence of shallow convective adjustment, in that near-surface moist static energy variations correlate closely with lower-tropospheric temperature. Evidence of thermodynamic control of deep convection on daily time scales is weaker. Upper-tropospheric temperature is weakly correlated with near-surface moist static energy. There are correlations of relative humidity (RH) with deep convection. Significant area-averaged rainfall occurs only above a lower-tropospheric RH threshold of near 80%. Above this threshold there is a weak but significant correlation of further lower-tropospheric RH increases with enhanced rain rate. Upper-tropospheric RH increases more consistently with rain rate. Lag correlations suggest that higher lower-tropospheric RH favors subsequent convection while higher upper-tropospheric RH is a result of previous or current convection. Convective available potential energy and surface wind speed have weak negative and positive relationships to rain rate, respectively. A strong relationship between surface wind speed (a proxy for latent heat flux) and rain rate has been recently observed in the eastern Pacific. It is suggested that in the KWAJEX region, this relationship is weaker because there are strong zonal gradients of vertically integrated water vapor. The strongest surface winds tend to be easterlies, so that strong surface fluxes are accompanied by strong dry-air advection from the east of Kwajalein. These two effects are of opposite sign in the moist static energy budget, reducing the tendency for strong surface fluxes to promote rainfall.

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David B. Mechem, Carly S. Wittman, Matthew A. Miller, Sandra E. Yuter, and Simon P. de Szoeke

Abstract

Marine boundary layer clouds are modified by processes at different spatial and temporal scales. To isolate the processes governing aerosol–cloud–precipitation interactions, multiday synoptic variability of the environment must be accounted for. Information on the location of low clouds relative to the ridge–trough pattern gives insight into how cloud properties vary as a function of environmental subsidence and stability. The technique of self-organizing maps (SOMs) is employed to objectively classify the 500-hPa geopotential height patterns for 33 years of reanalysis fields (ERA-Interim) into pretrough, trough, posttrough, ridge, and zonal-flow categories. The SOM technique is applied to a region of prevalent marine low cloudiness over the eastern North Atlantic Ocean that is centered on the Azores island chain, the location of a long-term U.S. Department of Energy observation site. The Azores consistently lie in an area of substantial variability in synoptic configuration, thermodynamic environment, and cloud properties. The SOM method was run in two ways to emphasize multiday and seasonal variability separately. Over and near the Azores, there is an east-to-west sloshing back and forth of the western edge of marine low clouds associated with different synoptic states. The different synoptic states also exhibit substantial north–south variability in the position of high clouds. For any given month of the year, there is large year-to-year variability in the occurrence of different synoptic states. Hence, estimating the climatological behavior of clouds from short-term field campaigns has large uncertainties. This SOM approach is a robust method that is broadly applicable to characterizing synoptic regimes for any location.

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Kimberly K. Comstock, Sandra E. Yuter, Robert Wood, and Christopher S. Bretherton

Abstract

Drizzling marine stratocumulus are examined using observations from the 2001 East Pacific Investigation of Climate Stratocumulus (EPIC Sc) field experiment. This study uses a unique combination of satellite and shipborne Doppler radar data including both horizontal and vertical cross sections through drizzle cells. Stratocumulus cloud structure was classified as closed cellular, open cellular, or unclassifiable using infrared satellite images. Distributions of drizzle cell structure, size, and intensity are similar among the cloud-structure categories, though the open-cellular distributions are shifted toward higher values. Stronger and larger drizzle cells preferentially occur when the cloud field is broken (open-cellular and unclassifiable categories). Satellite observations of cloud structure may be useful to indicate the most likely distribution of rain rates associated with a set of scenes, but infrared data alone are not sufficient to develop routine precipitation retrievals for marine stratocumulus. Individual drizzle cells about 2–20 km across usually showed precipitation growth within the cloud layer and evaporation below, divergence near echo top, and convergence below cloud base. Diverging flow near the surface was also observed beneath heavily precipitating drizzle cells. As the cloud field transitioned from a closed to an open-cellular cloud structure, shipborne radar revealed prolific development of small drizzle cells (<10 km2) that exceeded by over 5 times the number of total cells in either the preceding closed-cellular or following open-cellular periods. Peak area-average rain rates lagged by a few hours the peak in total number of drizzle cells. Based on observations from EPIC Sc, the highest stratocumulus rain rates are more likely to occur near the boundary between closed and open-cellular cloud structures.

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S. E. Yuter, R. A. Houze Jr., B. F. Smull, F. D. Marks Jr., J. R. Daugherty, and S. R. Brodzik

An electronic atlas of research aircraft missions in TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment) has been prepared and is available on the Internet via World Wide Web browsers such as Mosaic. These maps are in the form of time sequences of color imagery assembled using the NCAR Zebra software. Initial versions of these maps were prepared in the field at the TOGA COARE Honiara Operations Center to aid in the evaluation of each aircraft mission immediately after it was flown. The maps prepared in the field have been updated, corrected, and remapped at standard scales and with common color schemes. They show the meteorological setting of sampling by all seven aircraft participating in TOGA COARE—the two NOAA WP-3D aircraft, the NCAR Electra, the FIAMS C-340, the UK C-130, and the NASA DC-8 and ER-2—by overlaying flight tracks, GMS satellite infrared data, and NOAA WP-3D airborne radar images. The map sequences are combined with text of scientists' notes and other background information on the research flights to form a summary of each aircraft mission. The resulting aircraft mission summaries are intended as a road map to the COARE aircraft dataset. They indicate where and when data were collected and the meteorological context for those data. As an electronic document, the atlas of aircraft mission summaries is available on demand, and it is dynamic: as further information becomes available, the mission summaries will continue to be added to and updated as appropriate, and new releases will be issued periodically.

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Christopher S. Bretherton, Taneil Uttal, Christopher W. Fairall, Sandra E. Yuter, Robert A. Weller, Darrel Baumgardner, Kimberly Comstock, Robert Wood, and Graciela B. Raga

Overlaying the cool southeast Pacific Ocean is the most persistent subtropical stratocumulus cloud deck in the world. It produces a profound affect on tropical climate by shading the underlying ocean and radiatively cooling and stirring up turbulence in the atmosphere. In October 2001, the East Pacific Investigation of Climate undertook an exploratory cruise from the Galapagos Islands to Chile. The cruise gathered an unprecedented dataset, integrating radiosonde, surface, cloud remote sensing, aerosol, and ocean measurements. Scientific objectives included measuring the vertical structure of the ABL in this region, understanding what physical processes are determining the stratocumulus cloud albedo, and understanding the fluxes of heat and water that couple the atmosphere and ocean in this region.

An unexpectedly well-mixed stratocumulus-capped boundary layer as a result of a strong inversion was encountered throughout. A strong diurnal cycle was observed, with thicker clouds and substantial drizzle (mainly evaporating above the sea surface) during the late night and early morning. This was driven in part by local diabatic processes, and was reinforced by a surprisingly pronounced diurnal cycle of vertical motion. The vertical motion appears to be an inertia-gravity wave driven by daytime heating over South America that propagates over 1000 km offshore. Much more nocturnal drizzle and pronounced mesoscale cellularity were observed in “clean” conditions when cloud droplet concentrations and aerosol concentrations were low. Entrainment of dry, warm air is inferred to be the primary regulator of cloud thickness in this region, but drizzle also appears to have a large indirect impact by inhibiting and changing the spatial organization of turbulence.

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C. R. Mechoso, R. Wood, R. Weller, C. S. Bretherton, A. D. Clarke, H. Coe, C. Fairall, J. T. Farrar, G. Feingold, R. Garreaud, C. Grados, J. McWilliams, S. P. de Szoeke, S. E. Yuter, and P. Zuidema

The present paper describes the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study (VOCALS), an international research program focused on the improved understanding and modeling of the southeastern Pacific (SEP) climate system on diurnal to interannual time scales. In the framework of the SEP climate, VOCALS has two fundamental objectives: 1) improved simulations by coupled atmosphere–ocean general circulation models (CGCMs), with an emphasis on reducing systematic errors in the region; and 2) improved estimates of the indirect effects of aerosols on low clouds and climate, with an emphasis on the more precise quantification of those effects. VOCALS major scientific activities are outlined, and selected achievements are highlighted. Activities described include monitoring in the region, a large international field campaign (the VOCALS Regional Experiment), and two model assessments. The program has already produced significant advances in the understanding of major issues in the SEP: the coastal circulation and the diurnal cycle, the ocean heat budget, factors controlling precipitation and formation of pockets of open cells in stratocumulus decks, aerosol impacts on clouds, and estimation of the first aerosol indirect effect. The paper concludes with a brief presentation on VOCALS contributions to community capacity building before a summary of scientific findings and remaining questions.

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