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Alexandre O. Fierro, Edward J. Zipser, Margaret A. LeMone, Jerry M. Straka, and Joanne (Malkus) Simpson

Abstract

This paper addresses questions resulting from the authors’ earlier simulation of the 9 February 1993 Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Research Experiment (TOGA COARE) squall line, which used updraft trajectories to illustrate how updrafts deposit significant moist static energy (in terms of equivalent potential temperature θe) in the upper troposphere, despite dilution and a θe minimum in the midtroposphere. The major conclusion drawn from this earlier work was that the “hot towers” that Riehl and Malkus showed as necessary to maintain the Hadley circulation need not be undilute. It was not possible, however, to document how the energy (or θe) increased above the midtroposphere. To address this relevant scientific question, a high-resolution (300 m) simulation was carried out using a standard 3-ICE microphysics scheme (Lin–Farley–Orville).

Detailed along-trajectory information also allows more accurate examination of the forces affecting each parcel’s vertical velocity W, their displacement, and the processes impacting θe, with focus on parcels reaching the upper troposphere. Below 1 km, pressure gradient acceleration forces parcels upward against negative buoyancy acceleration associated with the sum of (positive) virtual temperature excess and (negative) condensate loading. Above 1 km, the situation reverses, with the buoyancy (and thermal buoyancy) acceleration becoming positive and nearly balancing a negative pressure gradient acceleration, slightly larger in magnitude, leading to a W minimum at midlevels. The W maximum above 8 km and concomitant θ e increase between 6 and 8 km are both due to release of latent heat resulting from the enthalpy of freezing of raindrops and riming onto graupel from 5 to 6.5 km and water vapor deposition onto small ice crystals and graupel pellets above that, between 7 and 10 km.

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Joanne Simpson, G. Roff, B. R. Morton, K. Labas, G. Dietachmayer, M. McCumber, and R. Penc

Abstract

A waterspout funnel and spray ring were observed under a cumulus line over the Great Salt Lake for about 5 min shortly after sunrise on 26 June 1985. Videotaped features strongly suggested that the funnel rotation was anticyclonic, These observations have been used as the basis for a study of the initiation and evolution of waterspouts through a series of numerical experiments at two scales, that of a cloud and a waterspout.

The cloud scale has been simulated using an improved Goddard-Schlesinger model with nearby Salt Lake City soundings. The main model improvements have been 1) a parameterized, three-class ice phase and 2) a line initialization in addition to the more common axisymmetric buoyant bubble. Cloud-scale vortex pairs developed for each mode of initiation, but a much stronger, more upright, low-level anticyclonic vortex grew from the line initiation than from the bubble. However, cumulus-scale vortices are common while waterspouts are rare, and the real test of a model is whether a waterspout can develop in the limited cumulus lifetime.

The 600-m horizontal grid of the cloud model cannot resolve waterspouts, and a modified Monash high-resolution axisymmetric vortex model with vertical domain and small section has been “embedded” at selected positions and initiated at selected times in the computed flow field of the cloud. Many experiments have been carried out with the vortex model. In the most important series, the boundary conditions were changed with the fields of the model cumulus as it evolved, and the time at which the vortex was started was varied through the lifetime of the parent cloud. Results showed that for each mode of cloud initiation, the vortex that started at the anticyclonic center grew faster than those started at other centers. This result fits with the observed anticyclonic rotation of the waterspout, strongly suggesting that the cloud vorticity was important in its initiation. The greatest azimuthal speed for the bubble-initiated cloud was 11 ms−1 when the vortex model was started at 28 min cloud time with time-varying boundary conditions, whereas it was 21 m s−1 when started at 12 min in the line-initiated cloud. Speeds were comparable when the inner domain moved with the anticyclonic cloud center. These speeds are close to the spray-ring threshold azimuthal velocity of roughly 22 m s−1 estimated by Golden from photographs.

Together, these model results support the hypothesis that, at least in some circumstances, cloud processes alone can produce waterspouts in the absence of external vorticity sources such as surface convergence lines or other shear features.

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Brian D. Scannell, Tom P. Rippeth, John H. Simpson, Jeff A. Polton, and Joanne E. Hopkins

Abstract

The combination of acoustic Doppler current profilers and the structure function methodology provides an attractive approach to making extended time series measurements of oceanic turbulence (the rate of turbulent kinetic energy dissipation ε) from moorings. However, this study shows that for deployments in the upper part of the water column, estimates of ε will be biased by the vertical gradient in wave orbital velocities. To remove this bias, a modified structure function methodology is developed that exploits the differing length scale dependencies of the contributions to the structure function resulting from turbulent and wave orbital motions. The success of the modified method is demonstrated through a comparison of ε estimates based on data from instruments at three depths over a 3-month period under a wide range of conditions, with appropriate scalings for wind stress and convective forcing.

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Anne M. Thompson, Wei-Kuo Tao, Kenneth E. Pickering, John R. Scala, and Joanne Simpson

Theoretical studies, aircraft, and space-borne measurements show that deep convection can be an effective conduit for introducing reactive surface pollutants into the free troposphere. The chemical consequences of convective systems are complex. For example, sensitivity studies show potential for both enhancement and diminution of ozone formation. Field observations of cloud and mesoscale phenomena have been investigated with the Goddard Cumulus Ensemble and Tropospheric Chemistry models. Case studies from the tropical ABLE 2, STEP, and TRACE-A experiments show that free tropospheric ozone formation should increase when deep convection and urban or biomass burning pollution coincide, and decrease slightly in regions relatively free of ozone precursors (often marine). Confirmation of post-convective ozone enhancement in the free troposphere over Brazil, the Atlantic, and southern Africa was a major accomplishment of the September–October 1992 TRACE-A (Transport and Atmospheric Chemistry near the Equator—Atlantic) aircraft mission. A flight dedicated to cloud outflow showed that deep convection led to a factor of 3–4 increase in upper tropospheric ozone formation downwind. Analysis of ozonesondes during TRACE-A was consistent with 20%–30% of seasonally enhanced ozone over the South Atlantic being supplied by a combination of biomass burning emissions, lightning, and deep convection over South America. With the Tropics the critical region for troposphere-to-stratosphere transfer of pollutants, these results have implications for the total ozone budget. Cloud-scale analyses will guide the development of more realistic regional and global chemical-transport models to assess the full impact of deep convection on atmospheric chemical composition.

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R. David Baker, Barry H. Lynn, Aaron Boone, Wei-Kuo Tao, and Joanne Simpson

Abstract

Idealized numerical simulations of Florida convection are performed with a coupled atmosphere–land surface model to identify the roles of initial soil moisture, coastline curvature, and land-breeze circulations on sea-breeze-initiated precipitation. The 3D Goddard Cumulus Ensemble cloud-resolving model is coupled with the Goddard Parameterization for Land–Atmosphere–Cloud Exchange land surface model, thus providing a tool to simulate more realistically land surface–atmosphere interaction and convective initiation. Eight simulations are conducted with either straight or curved coastlines, initially homogeneous soil moisture or initially variable soil moisture, and initially homogeneous horizontal winds or initially variable horizontal winds (land breezes). An additional simulation is performed to assess the role of Lake Okeechobee on convective development.

All model simulations capture the diurnal evolution and general distribution of sea-breeze-initiated precipitation over central Florida. The distribution of initial soil moisture influences the timing and location of subsequent precipitation. Soil moisture acts as a moisture source for the atmosphere, increases the convectively available potential energy, and thus preferentially focuses heavy precipitation over existing wet soil. Soil moisture–induced mesoscale circulations do not produce heavy precipitation. Coastline curvature has a major impact on the timing and location of precipitation. Earlier low-level convergence occurs inland of convex coastlines, and subsequent heavy precipitation occurs earlier in simulations with curved coastlines. Early-morning land breezes influence the timing of precipitation by modifying low-level convergence. Because of nonlinear interaction between coastline curvature and soil moisture, the highest peak accumulated rainfall and highest peak rain rates occur in simulations with both coastline curvature and initial soil moisture variations. Lake Okeechobee influences the timing and location of precipitation because of strong lake-breeze circulations.

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Joanne Simpson, Robert F. Adler, and Gerald R. North

The Tropical Rainfall Measuring Mission (TRMM) satellite is planned for an operational duration of at least three years, beginning in the mid-1990's. The main scientific goals for it are to determine the distribution and variability of precipitation and latent-heat release on a monthly average over areas of about 105 km2, for use in improving short-term climate models, global circulation models and in understanding the hydrological cycle, particularly as it is affected by tropical oceanic rainfall and its variability.

The TRMM satellite's instrumentation will consist of the first quantitative spaceborne weather radar, a multichannel passive microwave radiometer and an AVHRR (Advanced Very High Resolution Radiometer). The satellite's orbit will be low altitude (about 320 km) for high resolution and low inclination (30° to 35°) in order to visit each sampling area in the tropics about twice daily at a different hour of the day. A strong validation effort is planned with several key ground sites to be instrumented with calibrated multiparameter rain radars.

Mission goals and science issues are summarized. Research progress on rain retrieval algorithms is described. Radar and passive microwave algorithms are discussed and the use of radiative models in conjunction with cloud dynamical-microphysical models is emphasized especially. Algorithms are being and will continue to be tested and improved using microwave instruments on high-altitude aircraft overflying precipitating convective systems, located in the range of well-calibrated radars.

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Zhaoxia Pu, Wei-Kuo Tao, Scott Braun, Joanne Simpson, Yiqin Jia, Jeffrey Halverson, William Olson, and Arthur Hou

Abstract

This paper assesses the impact of TRMM Microwave Imager (TMI) derived surface rainfall data on the numerical simulation of Supertyphoon Paka (1997). A series of mesoscale numerical simulations of Supertyphoon Paka is performed during its mature stage by using the Pennsylvania State University–National Center for Atmospheric Research (Penn State–NCAR) Mesoscale Model (MM5). The model initial and boundary conditions were derived from Goddard Earth Observing System (GEOS) global analyses with and without assimilation of the TMI surface rainfall data. Simulation results clearly demonstrate that the GEOS analysis with TMI rainfall data leads to an improved simulation of Supertyphoon Paka in terms of its intensity and kinematical and precipitation structures, because rainfall assimilation modifies the environment of the storm such that the initial conditions are more favorable for development of the storm.

Since a bogus vortex is often necessary for initialization of typhoon (hurricane) simulations, additional numerical experiments are also performed by introducing mesoscale bogus vortices into GEOS analysis at the initial time using a four-dimensional variational technique. Simulation results indicate that a well-designed bogus vortex can play a dominant role in the improvements of forecasts of typhoon intensity and track. However, incorporation of the TMI data with a bogus vortex is still beneficial because it improves the simulation of the asymmetric storm structure.

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Arthur Y. Hou, Sara Q. Zhang, Arlindo M. da Silva, William S. Olson, Christian D. Kummerow, and Joanne Simpson

As a follow-on to the Tropical Rainfall Measuring Mission (TRMM), the National Aeronautics and Space Administration in the United States, the National Space Development Agency of Japan, and the European Space Agency are considering a satellite mission to measure the global rainfall. The plan envisions an improved TRMM-like satellite and a constellation of eight satellites carrying passive microwave radiometers to provide global rainfall measurements at 3-h intervals. The success of this concept relies on the merits of rainfall estimates derived from passive microwave radiometers. This article offers a proof-of-concept demonstration of the benefits of using rainfall and total precipitable water (TPW) information derived from such instruments in global data assimilation with observations from the TRMM Microwave Imager (TMI) and two Special Sensor Microwave/Imager (SSM/I) instruments.

Global analyses that optimally combine observations from diverse sources with physical models of atmospheric and land processes can provide a comprehensive description of the climate systems. Currently, such data analyses contain significant errors in primary hydrological fields such as precipitation and evaporation, especially in the Tropics. It is shown that assimilating the 6-h-averaged TMI and SSM/I surface rain rate and TPW retrievals improves not only the hydrological cycle but also key climate parameters such as clouds, radiation, and the upper-tropospheric moisture in the analysis produced by the Goddard Earth Observing System Data Assimilation System, as verified against radiation measurements by the Clouds and the Earth's Radiant Energy System instrument and brightness temperature observations by the Television Infrared Observational Satellite Operational Vertical Sounder instruments.

Typically, rainfall assimilation improves clouds and radiation in areas of active convection, as well as the latent heating and large-scale motions in the Tropics, while TPW assimilation leads to reduced moisture biases and improved radiative fluxes in clear-sky regions. Ensemble forecasts initialized with analyses that incorporate TMI and SSM/I rainfall and TPW data also yield better short-range predictions of geopotential heights, winds, and precipitation in the Tropics.

These results were obtained using a variational procedure based on a 6-h time integration of a column model of moist physics with prescribed dynamical and other physical tendencies. The procedure estimates moisture tendency corrections at observation locations by minimizing the least square differences between the observed TPW and rain rates and those generated by the column model over a 6-h analysis window. These tendency corrections are then applied during the assimilation cycle to compensate for errors arising from both initial conditions and deficiencies in model physics. Our results point to the importance of addressing deficiencies in model physics in assimilating data types such as precipitation, for which the forward model based on convective parameterizations may have significant systematic errors.

This study offers a compelling illustration of the potential of using rainfall and TPW information derived from passive microwave instruments to significantly improve the quality of four-dimensional global datasets for climate analysis and weather forecasting applications.

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Richard Goody, James Anderson, Thomas Karl, Roberta Balstad Miller, Gerald North, Joanne Simpson, Graeme Stephens, and Warren Washington

A successful global climate monitoring system must fulfill clear societal objectives. For some aspects of climate monitoring, the societal goals are understood and are clearly stated, but long-term, decadal/centennial climate predictions have, in the past, been judged more in terms of curiosity-led criteria. A curiosity-led climate program is not, however, the effective way to achieve the required societal objective, which is to produce the best possible long-term climate projections. In terms of the universal use of numerical models for climate projections, this leads to the need for monitoring programs that provide data to test model output against reliable observations. This requires an operational climate model (which the United States does not now have), and observations that emphasize accurate and reproducible data designed to provide critical tests of model output. The priorities for specific monitoring programs can be formulated in terms of these requirements, which can also provide metrics of progress. A schematic program that combines monitoring, modeling, and research is described. This program has, as its end point, the provision of demonstrably improved long-term climate projections. Europe appears to be advancing more rapidly along this path than is the United States.

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Joanne Simpson, William L. Woodley, Howard A. Friedman, Thomas W. Slusher, R. S. Scheffee, and Roger L. Steele

Abstract

The development, testing and use of an airborne pyrotechnic cloud seeding system is described. Pyrotechnic flares producing 50 gm of silver iodide smoke each were developed by two industrial corporations and laboratory tested for nucleation effectiveness in the Colorado State University cloud chamber. A delivery rack and firing system were developed, under ESSA supervision, and installed on its B-57 jet aircraft. Night flight tests were made of reliability, burn time and flare trajectory.

The flare system was used in a Florida cumulus seeding experiment in May 1968 conducted jointly by ESSA and the Naval Research Laboratory, with the participation of the U.S. Air Force, the University of Miami Radar Laboratory, and Meteorology Research, Inc. A randomized seeding scheme was used on 19 supercooled cumuli, of which 14 were seeded and 5 were studied identically as controls. Of the 14 seeded clouds, 13 grew explosively. Seeded clouds grew 11,400 ft higher than the controls, with the difference significant at better than the 0.5% level. Rainfall from seeded and control clouds was compared by means of calibrated ground radars. Large increases in rainfall were found from seeded clouds, but at a significance level ranging from 5–20% depending on the statistical test used. A single successful repeat of the experiment could result in rainfall differences significant at the 3% level with the most stringent test.

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