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Robert A. Houze Jr.

Abstract

A tropical squall-line system which moved over the observational network of the Global Atmospheric Research Programme's Atlantic Tropical Experiment (GATE) was investigated using rawinsonde, weather radar, satellite, surface meteorological, acoustic sounder and cloud photographic data. Combining these data led to a detailed synthesis of the three-dimensional structure, dynamics and life cycle of the disturbance.

The squall-line system consisted of a squall line forming the leading edge of the system and a trailing anvil cloud region. The squall line was made up of discrete active centers of cumulonimbus convection, referred to as line elements (LE's). New LE's formed ahead of the squall line. Old LE's weekened toward the rear of the line and blended into the trailing anvil region as they dissipated. Each LE progressed through a period of rapid growth, with echo tops penetrating the tropopause to maximum heights of 16–17 km, then decreasing to heights of 13–14 km, which corresponds to the height of the anvil cloud with which the LE's merged at the end of their lifetimes.

The squall line was located along the leading edge of a mesoscale downdraft forming and spreading out in the middle and lower troposphere below the anvil cloud. Within the squall line, individual LE's contained smaller, convective-scale downdrafts which penetrated down to the sea surface. This cold, convective-scale downdraft air spread out at low levels providing lift for the sea on the leading side of the LE. The convective-scale downdraft air also spread out in a layer 200–400 mb deep toward the rear of the system. The top of this layer of cold air was bounded by a stable layer below which enhanced turbulent mixing occurred.

Precipitation failing from the trailing anvil cloud was stratiform and accounted for 40% of the total rain from the squall-line system. Thus, much of the anvil cloud is accounted for by the successive incorporation of weakened, but precipitation-laden old LE's from the back edge of the squall line and possibly by widespread upward air motion within the upper level anvil cloud.

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Robert A. Houze Jr.

Abstract

Joanne Simpson began contributing to advances in tropical convection about half a century ago. The hot tower hypothesis jointly put forth by Joanne Simpson and Herbert Riehl postulated that deep convective clouds populating the “equatorial trough zone” were responsible for transporting heat from the boundary layer to the upper troposphere. This hypothesis was the beginning of a 50-year quest to describe and understand near-equatorial deep convection. Tropical field experiments in the 1970s [Global Atmospheric Research Program Atlantic Tropical Experiment (GATE) and the Monsoon Experiment (MONEX)] in which Joanne participated documented the mesoscale structure of the convective systems, in particular the deep, stratiform, dynamically active mesoscale clouds that are connected with the hot towers. In the 1980s these new data led to better understanding of how tropical mesoscale convective systems vertically transport heat and momentum. The role of the mesoscale stratiform circulation in this transport was quantified. Tropical field work in the 1990s [especially the Coupled Ocean–Atmosphere Response Experiment (COARE), in which Joanne again participated] showed the importance of a still larger scale of convective organization, the “supercluster.” This larger scale of organization has a middle-level inflow circulation that appears to be an important transporter of momentum. The mesoscale and supercluster scale of organization in tropical convective systems are associated with the stratiform components of the cloud systems. Joint analysis of satellite and radar data from COARE show a complex, possibly chaotic relationship between cloud-top temperature and the size of a stratiform precipitation area. The Tropical Rainfall Measuring Mission (TRMM) satellite, for which Joanne served as project scientist for nearly a decade, is now providing a global census of mesoscale and supercluster-scale organization of tropical convection. The TRMM dataset should therefore provide some closure to the question of the nature of deep convection in the equatorial trough zone.

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Robert A. Houze Jr.

Abstract

Cumulus-scale vertical transports of sensible heat and angular momentum are computed by a technique which uses detailed precipitation measurements as basic input. Data from Boston, Mass., New Orleans, La., San Juan, P.R., and Tatoosh, Wash., are included in the study, and calculations are made for January, April, July and October. The seasonal and regional variations of the computed transports are consistent with climatology. The cumulus-scale transports are compared with the mean meridional, the synoptic-scale, and the total required eddy fluxes of heat and momentum, and are found to be of the same order of magnitude as the most important components of larger scale vertical flux during at least one season at each station.

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Robert A. Houze Jr.

It was once generally thought that stratiform precipitation was something occurring primarily, if not exclusively, in middle latitudes—in baroclinic cyclones and fronts. Early radar observations in the Tropics, however, showed large radar echoes composed of convective rain alongside stratiform precipitation, with the stratiform echoes covering great areas and accounting for a large portion of the tropical rainfall. These observations seemed paradoxical, since stratiform precipitation should not have been occurring in the Tropics, where baroclinic cyclones do not occur. Instead it was falling from convection-generated clouds, generally thought to be too violent to be compatible with the layered, gently settling behavior of stratiform precipitation.

In meteorology, convection is a dynamic concept; specifically, it is the rapid, efficient, vigorous overturning of the atmosphere required to neutralize an unstable vertical distribution of moist static energy. Most clouds in the Tropics are convection-generated cumulonimbus. These cumulonimbus clouds contain an evolving pattern of newer and older precipitation. The young portions of the cumulonimbus are too violent to produce stratiform precipitation. In young, vigorous convective regions of the cumulonimbus, precipitation particles increase their mass by collection of cloud water, and the particles fall out in heavy showers, which appear on radar as vertically oriented convective “cells.” In regions of older convection, however, the vertical air motions are generally weaker, and the precipitation particles drift downward, with the particles increasing their mass by vapor diffusion. In these regions the radar echoes are stratiform, and typically these echoes occur adjacent to regions of younger convective showers. Thus, the stratiform and convective precipitation both occur within the same complex of convection-generated cumulonimbus cloud.

The feedbacks of the apparent heat source and moisture sink of tropical cumulonimbus convection to the large-scale dynamics of the atmosphere are distinctly separable by precipitation region. The part of the atmospheric response deriving from the areas of young, vigorous convective cells is two layered, with air converging into the active convection at low levels and diverging aloft. The older, weaker intermediary and stratiform precipitation areas induce a three-layered response, in which environmental air converges into the weak precipitation area at midlevels and diverges from it at lower and upper levels. If global precipitation data, such as that to be provided by the Tropical Rainfall Measuring Mission, are to be used to validate the heating patterns predicted by climate and general circulation models, algorithms must be applied to the precipitation data that will identify the two principal modes of heating, by separating the convective component of the precipitation from the remainder.

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Robert A. Houze Jr.

Abstract

Clouds within the inner regions of tropical cyclones are unlike those anywhere else in the atmosphere. Convective clouds contributing to cyclogenesis have rotational and deep intense updrafts but tend to have relatively weak downdrafts. Within the eyes of mature tropical cyclones, stratus clouds top a boundary layer capped by subsidence. An outward-sloping eyewall cloud is controlled by adjustment of the vortex toward gradient-wind balance, which is maintained by a slantwise current transporting boundary layer air upward in a nearly conditionally symmetric neutral state. This balance is intermittently upset by buoyancy arising from high-moist-static-energy air entering the base of the eyewall because of the radial influx of low-level air from the far environment, supergradient wind in the eyewall zone, and/or small-scale intense subvortices. The latter contain strong, erect updrafts. Graupel particles and large raindrops produced in the eyewall fall out relatively quickly while ice splinters left aloft surround the eyewall, and aggregates are advected radially outward and azimuthally up to 1.5 times around the cyclone before melting and falling as stratiform precipitation. Electrification of the eyewall cloud is controlled by its outward-sloping circulation. Outside the eyewall, a quasi-stationary principal rainband contains convective cells with overturning updrafts and two types of downdrafts, including a deep downdraft on the band’s inner edge. Transient secondary rainbands exhibit propagation characteristics of vortex Rossby waves. Rainbands can coalesce into a secondary eyewall separated from the primary eyewall by a moat that takes on the structure of an eye. Distant rainbands, outside the region dominated by vortex dynamics, consist of cumulonimbus clouds similar to non–tropical storm convection.

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Robert A. Houze Jr.

Abstract

Clouds within the inner regions of tropical cyclones are unlike those anywhere else in the atmosphere. Convective clouds contributing to cyclogenesis have rotational and deep intense updrafts but tend to have relatively weak downdrafts. Within the eyes of mature tropical cyclones, stratus clouds top a boundary layer capped by subsidence. An outward-sloping eyewall cloud is controlled by adjustment of the vortex toward gradient-wind balance, which is maintained by a slantwise current transporting boundary layer air upward in a nearly conditionally symmetric neutral state. This balance is intermittently upset by buoyancy arising from high-moist-static-energy air entering the base of the eyewall because of the radial influx of low-level air from the far environment, supergradient wind in the eyewall zone, and/or small-scale intense subvortices. The latter contain strong, erect updrafts. Graupel particles and large raindrops produced in the eyewall fall out relatively quickly while ice splinters left aloft surround the eyewall, and aggregates are advected radially outward and azimuthally up to 1.5 times around the cyclone before melting and falling as stratiform precipitation. Electrification of the eyewall cloud is controlled by its outward-sloping circulation. Outside the eyewall, a quasi-stationary principal rainband contains convective cells with overturning updrafts and two types of downdrafts, including a deep downdraft on the band’s inner edge. Transient secondary rainbands exhibit propagation characteristics of vortex Rossby waves. Rainbands can coalesce into a secondary eyewall separated from the primary eyewall by a moat that takes on the structure of an eye. Distant rainbands, outside the region dominated by vortex dynamics, consist of cumulonimbus clouds similar to non–tropical storm convection.

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Robert A. Houze Jr.

Abstract

When cumulonimbus clouds aggregate, developing into a single entity with precipitation covering a horizontal scale of hundreds of kilometers, they are called mesoscale convective systems (MCSs). They account for much of Earth’s precipitation, generate severe weather events and flooding, produce prodigious cirriform anvil clouds, and affect the evolution of the larger-scale circulation. Understanding the inner workings of MCSs has resulted from developments in observational technology and modeling. Time–space conversion of ordinary surface and upper-air observations provided early insight into MCSs, but deeper understanding has followed field campaigns using increasingly sophisticated radars, better aircraft instrumentation, and an ever-widening range of satellite instruments, especially satellite-borne radars. High-resolution modeling and theoretical insights have shown that aggregated cumulonimbus clouds induce a mesoscale circulation consisting of air overturning on a scale larger than the scale of individual convective up- and downdrafts. These layers can be kilometers deep and decoupled from the boundary layer in elevated MCSs. Cooling in the lower troposphere and heating aloft characterize the stratiform regions of MCSs. As a result, long-lived MCSs with large stratiform regions have a top-heavy heating profile that generates potential vorticity in midlevels, thus influencing the larger-scale circulation within which the MCSs occur. Global satellite data show MCSs varying in structure, depending on the prevailing large-scale circulation and topography. These patterns are likely to change with global warming. In addition, environmental pollution affects MCS structure and dynamics subtly. Feedbacks of MCSs therefore need to be included or parameterized in climate models.

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Robert A. Houze Jr and Rebecca Houze

Abstract

Weather mapping began in the nineteenth century when telegraphs began sending simultaneous observations of conditions at the surface of Earth to weather stations around the world. Indicating the complexity of the clouds and weather seen at a site needed a common naming system and simple symbols that were independent of language, since telegraph signals crossed international borders. The first symbols representing clouds were abstracted and stylized versions of their artistic representations in naturalistic drawings and paintings. By the end of the nineteenth century, however, photography began to replace hand-drawn illustrations. Before the advent of color film, black and white photographs were sometimes even reproduced in colored inks using new mechanical printing processes. Nineteenth-century meteorologists developed symbols for various cloud types in the form of simple lines and curves suggestive of the pictures. In contrast, weather symbols (for rain, snow, fog, hail, etc.) were drawn largely from a lexicon of nonalphabetic written symbols, such as punctuation marks. Skilled map plotters used these simple symbols, suggestive of complex weather and clouds, to transfer telegraph and Teletype codes to visually meaningful hand-produced maps. The craft of manual weather map plotting reached an apex in the 1940s–60s. With advances in digital and satellite technology and automation of surface weather observations, the symbols used in traditional weather mapping have largely disappeared from daily use.

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Robert A. Houze Jr. and Rebecca Houze
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Robert A. Houze Jr. and Colleen A. Leary

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Convective fluxes of mass and beat in tropical easterly waves were computed by a slightly modified version of the method of Austin and Houze (1973). The input data for this method are radar and raingage observations of cloud heights and precipitation amounts. The computed fluxes were compared to convective fluxes inferred from budget studies of synoptic-scale rawinsonde data for the same easterly waves. Good agreement between the two methods was obtained for mass and heat fluxes in the middle troposphere. For the lower troposphere, our calculations showed smaller transports of man and heat than were indicated by large-scale budget studies. This result arises because many shallow convective clouds do not precipitate and thus axe not included in calculations based on precipitation data. When the effects of small clouds (tops below 6 km) were removed from the calculations based on large-scale budgets, excellent agreement was obtained in the lower as well as middle troposphere. This study shows that a particularly useful approach in future studies will be a combination of methods based on both precipitation and large-scale data.

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