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John S. Theon

Abstract

By 1984, more than a decade had passed since the National Aeronautics and Space Administration (NASA) weather and climate program had won approval for a new research mission. There was concern that it would be difficult to justify the budget of the program, so ideas were requested for a new research mission aimed at advancing our understanding of the weather and/or climate. More than a dozen proposals were submitted, including one by North, Wilheit, and Thiele for a mission to observe rainfall directly from space. They called it the Tropical Rainfall Measuring Mission (TRMM).

Studies were conducted to demonstrate that the proposal was feasible by deploying airborne versions of the proposed precipitation radar, microwave radiometer, and visible-infrared radiometer over carefully documented ground-based observations of rainfall. Sampling studies were undertaken to assure that one satellite could adequately sample precipitation events, and advanced mission studies were undertaken to define the mission as well as its cost.

When it became obvious that the cost of the mission would severely limit chances of winning approval, it was decided to invite an international partner to share the cost. With the support of Dr. Bert Edelson, the NASA associate administrator, and through the cooperation of Dr. Nobuyoshi Fugono of Japan, it was possible to study the mission as a joint enterprise. Although the one-year joint mission study concluded that the mission was feasible, obtaining the funding in both countries was anything but simple. When Dr. North decided to leave NASA, Dr. Simpson was suggested as his successor as project scientist. Dr. Simpson's energy and determination were key to winning approval of TRMM by the U.S. Congress. Dr. Simpson had, as President of the American Meteorological Society, briefed Congressman Green of New York on the enormous potential scientific benefits of TRMM. The fiscal year 1991 NASA budget was amended, mandating a new start for TRMM. Once NASA had approval for the mission, Japan agreed to share the costs, and the rest is history. TRMM was launched in 1997 and continues to acquire unprecedented rainfall data on a global scale.

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William M. Gray

Abstract

This paper uses extensive aircraft, composited rawinsonde data, and an idealized hurricane structure model to analyze the physical processes that maintain the transverse circulation of the steady-state hurricane. It is shown that convective available potential energy (CAPE) or processes other than frictional forcing plays an important role in maintaining the hurricane's inner-core (radius < 60 km) in-up-and-out radial circulation. But this is not true at outer radii (60–250 km or 250–700 km) where surface friction forcing is dominant and larger than the resulting upward vertical motion.

Overall, there is less vertical motion within the hurricane's 0–250-km area than that specified by frictional forcing and, overall, CAPE or buoyancy plays a negative role in enhancing vertical motion. But this is not true of the inner-core eye-wall cloud region where nonfrictionally driven eye-wall vertical motion has an important buoyant contribution and a strong ocean-to-air energy flux is present. Frictionally forced vertical motion resulting from low-level relative vorticity is typically not balanced locally. Quasi balance between frictional forcing and vertical motion is observed only for the larger-scale vortex (approximately 0°–3° radius) as a whole.

PROLOGUE

Joanne Simpson tells the story that in the mid-1940s, when she was a young (and precocious!) graduate student at the University of Chicago, she told Carl Rossby that she wanted to study clouds and that he responded by saying that that was a good subject for a girl. We now more fully appreciate the role of clouds as the fundamental component of the hydrologic cycle. Most of us would agree that understanding the physics behind cumulus convection is a fundamental challenge for all, girl or boy. Joanne's choice of cloud studies as a career endeavor was a wiser choice than most meteorologists of that day (and many of this day) realized. Attention in the 1940s and 1950s had been focused more on the requirements of wind for the transfer of energy from the tropical to the polar regions. There is no doubt that horizontal transport of energy is a fundamental ingredient of the general circulation. But vertical energy transport to balance the troposphere's continuous radiational cooling of ∼1°C per day is more important. Globally averaged, the required vertical transport of energy from the surface up into the troposphere is about four times larger than the required horizontal transport. It is this vertical energy transport that is so messy and so difficult to understand, and so hard to treat in a realistic and quantitative fashion. Many modelers and theoreticians have chosen to neglect the many hydrologic cycle complications (by assuming that the troposphere's radiational cooling is balanced by condensation warming) and to concentrate only on the horizontal energy imbalances. This has been the approach of the dishpan or annulus experiments. But this is not satisfactory for a full understanding of how the troposphere really functions. We have to face up to the need for the development of a realistic quantitative treatment of the globe's hydrologic cycle. The cumulus convection schemes in current GCMs are still inadequate. It is this continuing need to better understand the full range of cloud processes that has made Joanne's decision in the mid-1940s to concentrate on clouds such a wise one. She has since made many contributions to the understanding of the role of clouds. The paper she wrote with Herbert Riehl in 1958 (Riehl and Malkus) had much influence on the thinking of the important role of cumulus convection. Her recent work with the Tropical Rainfall Measuring Mission (TRMM) experiment is an example of her continuing drive to better understand clouds and the hydrologic cycle.

I first met and worked with Joanne in the late 1950s when I was a graduate student of Herbert Riehl's at the University of Chicago. I participated in the study she was directing on the variations of tropical Pacific cloudiness from aircraft time-lapse photography. This was before the satellite and the computer. We had more time to think and to speculate in those days. I have been most grateful to both Joanne and Bob Simpson for their interest and encouragement of my research efforts since that time.

It is a pleasure to make a contribution to this symposium honoring Joanne. The paper to follow has many similarities to the early and original paper of Joanne in 1958 titled “The Structure and Maintenance of the Mature Hurricane Eye.”

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Eric A. Smith and Throy D. Hollis

Abstract

Currently, satellite algorithms are the methodology showing most promise for obtaining more accurate global precipitation estimates. However, a general problem with satellite methods is that they do not measure precipitation directly, but through inversion of radiation–rain relationships. Because of this, procedures are needed to verify algorithm-generated results. The most common method of verifying satellite rain estimates is by direct comparison with ground truth data derived from measurements obtained by rain gauge networks, ground-based weather radar, or a combination of the two. However, these types of comparisons generally shed no light on the physical causes of the differences. Moreover, ground validation measurements often have uncertainty magnitudes on the order of or greater than the satellite algorithms, motivating the search for alternate approaches. The purpose of this research is to explore a new type of approach for evaluating and validating the level-2 Tropical Rainfall Measuring Mission (TRMM) facility rain profile algorithms. This is done by an algorithm-to-algorithm intercomparison analysis in the context of physical hypothesis testing.

TRMM was launched with the main purpose of measuring precipitation and the release of latent heat in the deep Tropics. Its rain instrument package includes the TRMM Microwave Imager (TMI), the Precipitation Radar (PR), and the Visible and Infrared Scanner (VIRS). These three instruments allow for the use of combined-instrument algorithms, theoretically compensating for some of the weaknesses of the single-instrument algorithms and resulting in more accurate estimates of rainfall. The focus of this research is on the performance of four level-2 TRMM facility algorithms producing rain profiles using the TMI and PR measurements with both single-instrument and combined-instrument methods.

Beginning with the four algorithms' strengths and weaknesses garnered from the physics used to develop the algorithms, seven hypotheses were formed detailing expected performance characteristics of the algorithms. Procedures were developed to test these hypotheses and then applied to 48 storms from all ocean basins within the tropical and subtropical zones over which TRMM coverage is available (∼35°N–35°S). The testing resulted in five hypotheses verified, one partially verified, and one inconclusive. These findings suggest that the four level-2 TRMM facility profile algorithms are performing in a manner consistent with the underlying physical limitations in the measurements (or, alternatively, the strengths of the physical assumptions), providing an independent measure of the level-2 algorithms' validity.

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W-K. Tao, J. Halverson, M. LeMone, R. Adler, M. Garstang, R. Houze Jr., R. Pielke Sr., and W. Woodley

Abstract

Dr. Joanne Simpson's nine specific research contributions to the field of meteorology during her 50-year career—1) the hot tower hypothesis, 2) hurricanes, 3) airflow and clouds over heated islands, 4) cloud models, 5) trade winds and their role in cumulus development, 6) air–sea interaction, 7) cloud–cloud interactions and mergers, 8) waterspouts, and 9) TRMM science—will be described and discussed in this paper.

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Ken'ichi Okamoto

Abstract

The Tropical Rainfall Measuring Mission (TRMM) satellite carried aboard the world's first spaceborne precipitation radar (PR). This paper describes a short history of the TRMM PR. It describes the Communications Research Laboratory's (CRL's) airborne dual-frequency rain radar/radiometer system, some results of the airborne experiments, and considerations of system design and system parameters of the PR. It also describes data processing and analysis algorithms for the PR, and examples of PR rain measurements.

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Elizabeth A. Ritchie

Abstract

The mechanisms by which mesoscale midlevel vortices that form in the stratiform anvil regions of mesoscale convective systems develop downward in the atmosphere are explored in the context of tropical cyclone genesis. Using simple two- and three-dimensional models, a theory for the processes by which midlevel vortices may interact both with each other, and with their large-scale environment in order to develop a storm-scale vortex, is developed. It is found that absorption of the circulation of one vortex by another results in a vortex of greater horizontal and vertical extent. Embedding the vortices in an enhanced vorticity environment such as might be found in the monsoon trough results in more efficient merger and greater downward development of the circulation associated with the merged vortex.

This theory is used to interpret a real case of the development of Tropical Cyclone (TC) Oliver in the Australian region during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) experiment in 1993. High-resolution flight-level and dropwindsonde data were collected during the interaction and merger phase of two large mesoscale convective systems that were embedded in the monsoon trough. Multiple mesoscale vortices were observed to interact and merge during the development phase of TC Oliver with consequences for the downward development of the vortex, and subsequent eye development.

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Edward J. Zipser

Abstract

The “hot tower” hypothesis requires the existence of deep cumulonimbus clouds in the deep Tropics as essential agents, which accomplish the mass and energy transport essential for the maintenance of the general circulation. As the role of the deep convective clouds has been generally accepted, the popularity of referring to these deep “hot” towers as undilute towers also has gained acceptance. This paper examines the consequences of assuming that the deep convective clouds over tropical oceans consist of undilute ascent from the subcloud layer.

Using simple applications of parcel theory, it is concluded that observed properties of typical cumulonimbus updrafts in low- to midtroposphere over tropical oceans are inconsistent with the presence of undilute updrafts. Such undilute updrafts are far more consistent with observations in severe storms of midlatitudes. The observations over tropical oceans can be hypothetically explained by assuming large dilution of updrafts by entrainment below about 500 hPa, followed by freezing of condensate. This freezing and subsequent ascent along an ice adiabat reinvigorates the updrafts and permits them to reach the tropical tropopause with the necessary high values of moist static energy, as the hot tower hypothesis requires. The large difference observed between ocean and land clouds can be explained by assuming slightly smaller entrainment rates for clouds over land. These small entrainment differences have a very large effect on updrafts in the middle and upper troposphere and can presumably account for the large differences in convective vigor, ice scattering, and lightning flash rates that are observed. It follows that convective available potential energy (CAPE) is not a particularly good predictor of the behavior of deep convection.

Using the Tropical Rainfall Measuring Mission (TRMM) to map a proxy for the most intense storms on earth between 36°S and 36°N, they are found mostly outside the deep Tropics, with the notable exception of tropical Africa.

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Daniel Rosenfeld and William L. Woodley

Abstract

Spaceborne inferences of cloud microstructure and precipitation-forming processes with height have been used to investigate the effect of ingested aerosols on clouds and to integrate the findings with past cloud physics research. The inferences were made with a method that analyzes data from National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA AVHRR) and Tropical Rainfall Measuring Mission Visible and Infrared Scanner (TRMM VIRS) sensors to determine the effective radius of cloud particles with height. In addition, the TRMM Precipitation Radar (PR) made it possible to measure the rainfall simultaneously with the microphysical retrievals, which were validated by aircraft cloud physics measurements under a wide range of conditions. For example, the satellite inferences suggest that vigorous convective clouds over many portions of the globe remain supercooled to near −38°C, the point of homogeneous nucleation. These inferences were then validated in Texas and Argentina by in situ measurements using a cloud physics jet aircraft.

This unique satellite vantage point has documented enormous variability of cloud conditions in space and time and the strong susceptibility of cloud microstructure and precipitation to the ingested aerosols. This is in agreement with past cloud physics research. In particular, it has been documented that smoke and air pollution can suppress both water and ice precipitation-forming processes over large areas. Measurements in Thailand of convective clouds suggest that the suppression of coalescence can decrease areal rainfall by as much as a factor of 2. It would appear, therefore, that pollution has the potential to alter the global climate by suppressing rainfall and decreasing the net latent heating to the atmosphere and/or forcing its redistribution. In addition, it appears that intense lightning activity, as documented by the TRMM Lightning Imaging Sensor (LIS), is usually associated with microphysically highly “continental” clouds having large concentrations of ingested aerosols, great cloud-base concentrations of tiny droplets, and high cloud water contents. Conversely, strongly “maritime” clouds, having intense coalescence, early fallout of the hydrometeors, and glaciation at warm temperatures, show little lightning activity. By extension these results suggest that pollution can enhance lightning activity.

The satellite inferences suggest that the effect of pollution on clouds is greater and on a much larger scale than any that have been documented for deliberate cloud seeding. They also provide insights for cloud seeding programs. Having documented the great variability in space and time of cloud structure, it is likely that the results of many cloud seeding efforts have been mixed and inconclusive, because both suitable and unsuitable clouds have been seeded and grouped together for evaluation. This can be addressed in the future by partitioning the cases based on the microphysical structure of the cloud field at seeding and then looking for seeding effects within each partition.

This study is built on the scientific foundation laid by many past investigators and its results can be viewed as a synthesis of the new satellite methodology with their findings. Especially noteworthy in this regard is Dr. Joanne Simpson, who has spent much of her career studying and modeling cumulus clouds and specifying their crucial role in driving the hurricane and the global atmospheric circulation. She also was a pioneer in early cloud seeding research in which she emphasized cloud dynamics rather than just microphysics in her seeding hypotheses and in her development and use of numerical models. It is appropriate, therefore, that this paper is offered to acknowledge Dr. Joanne Simpson and her many colleagues who paved the way for this research effort.

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Robert F. Adler, Christian Kummerow, David Bolvin, Scott Curtis, and Chris Kidd

Abstract

Three years of Tropical Rainfall Measuring Mission (TRMM) monthly estimates of tropical surface rainfall are analyzed to document and understand the differences among the TRMM-based estimates and how these differences relate to the pre-TRMM estimates and current operational analyses. Variation among the TRMM estimates is shown to be considerably smaller than among a pre-TRMM collection of passive microwave-based products. Use of both passive and active microwave techniques in TRMM should lead to increased confidence in converged estimates.

Current TRMM estimates are shown to have a range of about 20% for the tropical ocean as a whole, with variations in heavily raining ocean areas of the Intertropical Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ) having differences over 30%. In midlatitude ocean areas the differences are smaller. Over land there is a distinct difference between the Tropics and midlatitude with a reversal between some of the products as to which tends to be relatively high or low. Comparisons of TRMM estimates with ocean atoll and land rain gauge information point to products that might have significant regional biases. The bias of the radar-based product is significantly low compared with atoll rain gauge data, while the passive microwave product is significantly high compared to rain gauge data in the deep Tropics.

The evolution of rainfall patterns during the recent change from intense El Niño to a long period of La Niña and then a gradual return to near neutral conditions is described using TRMM. The time history of integrated rainfall over the tropical oceans (and land) during this period differs among the passive and active microwave TRMM estimates.

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Thomas Wilheit

Abstract

The relationship between the microwave radiometer and the precipitation radar on the Tropical Rainfall Measuring Mission TRMM satellite is inherently complementary. Neither sensor by itself would be adequate to achieve the TRMM objectives but the match between the strengths and weaknesses of each sensor results in an extremely powerful payload. Here these strengths and weaknesses are discussed and a specific example is examined.

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