Sandra E. Yuter

Convection over the Pacific Warm Pool in relation to the Atmospheric Kelvin-Rossby Wave*

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...

Physical Characterization of Tropical Oceanic Convection Observed in KWAJEX

The Tropical Rainfall Measuring Mission (TRMM) Kwajalein Experiment (KWAJEX) was designed to obtain an empirical physical characterization of precipitating convective clouds over the tropical ocean. Coordinated datasets were collected by three aircraft, one ship, five upper-air sounding sites, and a variety of continuously recording remote and in situ surface-based sensors, including scanning Doppler radars, profilers, disdrometers, and rain gauges. This paper describes the physical characterization of the Kwajalein cloud population that has emerged from analyses of datasets that were obtained during KWAJEX and combined with long-term TRMM ground validation site observations encompassing three rainy seasons. The spatial and temporal dimensions of the precipitation entities exhibit a lognormal probability distribution, as has been observed over other parts of the tropical ocean. The diurnal cycle of the convection is also generally similar to that seen over other tropical oceans. The largest precipitating cloud elements—those with rain areas exceeding 14 000 km 2 —have the most pronounced diurnal cycle, with a maximum frequency of occurrence before dawn; the smallest rain areas are most frequent in the afternoon. The large systems exhibited stratiform rain areas juxtaposed with convective regions. Frequency distributions of dual-Doppler radar data showed narrow versus broad spectra of divergence in the stratiform and convective regions, respectively, as expected because strong up- and downdrafts are absent in the stratiform regions. The dual-Doppler profiles consistently showed low-level convergence and upper-level divergence in convective regions and midlevel convergence sandwiched between lower- and upper-level divergence in stratiform regions. However, the magnitudes of divergence are sensitive to assumptions made in classifying the radar echoes as convective or stratiform. This sensitivity implies that heating profiles derived from satellite radar data will be sensitive to the details of the scheme used to separate convective and stratiform rain areas. Comparison of airborne passive microwave data with ground-based radar data indicates that the pattern of scattering of 85-GHz radiance by ice particles in the upper portions of KWAJEX precipitating clouds is poorly correlated with the precipitation pattern at lower levels while the emission channels (10 and 19 GHz) have brightness temperature patterns that closely correspond to the lower-level precipitation structure. In situ ice particle imagery obtained by aircraft at upper levels (11 km) shows that the concentrations of ice particles of all densities are greater in the upper portions of active convective rain regions and lower in the upper portions of stratiform regions, probably because the active updrafts convey the particles to upper levels, whereas in the stratiform regions sedimentation removes the larger ice particles over time. Low-level aircraft flying in the rain layer show similar total drop concentrations in and out of convective cells, but they also show a sudden jump in the concentration of larger raindrops at the boundaries of the cells, indicating a discontinuity in growth processes such as coalescence at the cell boundary.

Ocean–Cloud–Atmosphere–Land Interactions in the Southeastern Pacific: The VOCALS Program

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 issue...

The 1997 Pan American Climate Studies Tropical Eastern Pacific Process Study. Part I : ITCZ region

Abstract The Pan American Climate Studies Tropical Eastern Pacific Process Study (TEPPS) obtained a comprehensive set of observations of the structure of clouds and precipitating storms over the eastern tropical Pacific from 28 July to 6 September 1997. The TEPPS data can address a wide range of problems involving tropical oceanic clouds and precipitation. The main goal of the project was to understand why passive microwave satellite algorithms indicate an E—W gradient in the precipitation pattern in the tropical Pacific with heavier rainfall in the east while infrared satellite algorithms indicate heavier rainfall in the west. Satellite—derived precipitation estimates are based on characteristics of the vertical structure of precipitating clouds: in the case of infrared methods, cloud—top temperature, and in the case of microwave methods, the vertically integrated ice scattering and/or water absorption determined by the vertical profile of hydrometeors. The premise of the expedition was that by obtaining...

Radar Data Processing and Visualization over Complex Terrain

MountainZebra is a data flow configuration that processes and displays radar data over complex terrain. The system combines three elements: the data stream from an operational radar, 3D topographical information, and the NCAR Zebra data visualization and integration software. MountainZebra operates routinely on a 3D data stream from the National Weather Service Weather Surveillance Radar-1988 Doppler (WSR-88D) at Camano Island, Washington (near Seattle). The WSR-88D data are continuously acquired, archived, formatted, and interpolated for multidimensional display. The three-dimensional topographical information in MountainZebra can be automatically underlaid on any horizontal or vertical display of the radar data. This system allows radar data and other geophysical fields to be analyzed in precise relation to the underlying terrain. Terrain-based visualization facilitates radar data analysis by identifying terrain clutter and shadowing and by identifying orographic precipitation mechanisms. The utility of MountainZebra is illustrated in the investigation of stable orographic enhancement over the windward slopes of the Cascade Mountains of the Pacific Northwest and an orographically enhanced squall line to the lee of the European Alps.

The 1997 Pan American Climate Studies Tropical Eastern Pacific Process Study. Part II: Stratocumulus region

Abstract An ad hoc experiment in the marine stratocumulus region to the west of Mexico was conducted from 29 August to 6 September 1997 as part of the Pan American Climate Studies Tropical Eastern Pacific Process Study cruise on the National Oceanic and Atmospheric Administration ship Ronald H. Brown after a medical emergency cut short the planned time in the eastern Pacific ITCZ. The joint variation of cloud structure, drizzle, and tropospheric stratification was documented by a combination of three hourly upper air soundings, scanning C—band radar, hourly cloud photography, and visual observation. The sensitive C—band Doppler radar mounted on the ship was able to obtain observations of drizzle cells with regions of greater than 10 dBZ of 2—3—km scale in the horizontal and peak reflectivities of greater than 25 dBZ.

TOGA COARE Aircraft Mission Summary Images: An Electronic Atlas

Abstract 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...

TOGA COARE satellite data summaries available on the world wide web

Abstract 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...

Comment on “Partitioning tropical oceanic convective and stratiform rains by draft strength” by David Atlas et al.

[1] The crux of this comment is to clarify attribution. If someone looks at a published graph and has a different interpretation from that of the original authors, that new interpretation should not be attributed to the original authors. [2] The references given by Atlas et al. [2000] regarding distinct convective and stratiform reflectivity Z to rain rate R relations attributed to Yuter and Houze [1997] are incorrect. The convective and stratiform Z-R relations labeled as those of Yuter and Houze in Table 3 (incorrectly referred to there as 1996) and mentioned in the text on pp. 2262, 2264, and 2265 of Atlas et al. [2000] do not appear anywhere in the Yuter and Houze [1997] paper. The Yuter and Houze [1997] study used two-dimensional (2-D) particle probe drop spectra collected by the National Center for Atmospheric Research Electra aircraft during the Tropical Ocean-Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE) to compute Z and R values over 6 s intervals of flight track ( 1400 L sample volumes). These data were classified into convective and stratiform subsets using radar data obtained by the National Oceanic and Atmospheric Administration WP-3D airborne radar. Segments of the Electra flight track were categorized by noting when the Electra flew within regions of convective and stratiform precipitation classified according to a radar reflectivity texture algorithm based on the paper by Steiner et al. [1995]. The main conclusion of the Yuter and Houze [1997] study was that the populations of radarclassified convective and stratiform Z-R points overlapped in dbZ-log R space and did not form two statistically distinct populations. [3] The previous statements do not rule out the possibility that different physical processes can yield different drop size distributions (DSD) [e.g., Braun and Houze, 1994, pp. 2749–2750]. However, in the large areas of radar-classified convective and stratiform precipitation examined in this study, the physical processes mixed, exhibited natural variation, and were subject to sampling error to such a degree that the Z and Rvalues associated with the DSD samples in the convective and stratiform populations did not form distinct populations. A classification method that can truly distinguish between dominant precipitation growth by vapor deposition versus accretion (riming and collection/coalescence) would have the property of yielding distinct populations in lowrain-rate regions of dbZ-log R space where both processes are known to occur. 2. Follow-up