Robert A. Houze

Chapter 10 - Clouds and Precipitation in Tropical Cyclones☆

Abstract Tropical cyclones draw energy from warm ocean surfaces and use this energy to strengthen a cyclonic disturbance into an intense vortex, which is called a hurricane, typhoon, or cyclone, depending on where in the world it occurs. The tropical cyclone initiates from a collection of cumulonimbus clouds before the intense vortex dominates the dynamics. However, the convective clouds in the genesis stage exhibit differences from ordinary cumulonimbus, including very intense “convective bursts” and rotating convective cells called “vortical hot towers.” This chapter describes the special characteristics of the cumulonimbus observed in the tropical cyclogenesis stage. Then, the chapter details the various clouds of a fully formed tropical cyclone. Once the tropical cyclone takes shape, it is characterized by a strong vortex that achieves force balance via a vertically overturning secondary circulation, which produces an eyewall cloud that is unique to tropical cyclones. This chapter details the observed structure and dynamics of the eyewall cloud. The rising branch of the secondary circulation producing the eyewall cloud connects the energy of the warm ocean boundary layer to the intense storm vortex. Observations show that the eyewall cloud is dominated by the secondary circulation of the vortex but may contain intense embedded subvortices and intermittent occurrences of deep cumulonimbus. In intense tropical cyclones, a secondary eyewall cloud forms as the storm undergoes an eyewall replacement cycle. Outside the eyewall are rainband clouds, which have a spiral configuration distinct from the circular eyewall shape. The rainband clouds lie outside the eyewall and have a more cumulonimbus-like behavior than the eyewall.

Chapter 7 - Basic Cumulus Dynamics☆

Abstract This chapter reviews the basic dynamics of convective clouds, which include cumulus, cumulonimbus, and mesoscale convective systems. The factors underlying convective cloud dynamics are buoyancy, pressure perturbations due to buoyancy and in-cloud rotation, entrainment, and three-dimensional vorticity dynamics. This chapter reviews the fundamental aspects of each of these dynamical factors.

Nimbostratus and the Separation of Convective and Stratiform Precipitation

Abstract Nimbostratus clouds occur in connection with organized storms, mostly fronts, tropical cyclones, and mesoscale convective systems. They are produced by nearly stable air motions and are deep enough to allow precipitation particles to grow to the sizes of raindrops and snowflakes. Their depth and robust precipitation production distinguish them from the shallow stratus and stratocumulus clouds considered in Chapter 5 , which do not have sufficient vertical extent to produce much precipitation. The precipitation from nimbostratus is usually referred to as stratiform precipitation, and it is produced by widespread lifting in fronts and tropical cyclones. However, it also occurs in large quantities in mesoscale convective systems. This chapter discusses the structure of stratiform precipitation as seen by radar. It features a horizontal bright band at the melting layer that distinguishes it from vertical convective precipitation cells. This chapter details how stratiform precipitation appears and develops in both frontal and deep convective situations. In the latter, the stratiform precipitation develops from active convective cells, by dissipation and/or shearing of the active cells. When stratiform precipitation occurs in connection with convective precipitation, it is important to separate the two. This chapter discusses techniques for separation of convective and stratiform precipitation.

Chapter 12 – Clouds and Precipitation Associated with Hills and Mountains ☆

Abstract Certain types of clouds are produced when air flows over hills or mountains. Mountain wave and lee wave clouds occur when stable air is set into oscillation by passing over a terrain feature such as a hill or mountain. Fohn wall and rotor clouds occur when the air flowing over a ridge becomes supercritical, with strong downslope winds and a hydraulic jump in the lee of the ridge. A rotor cloud can form at the location of the hydraulic jump. A banner cloud forms when flow is split by a sharp peak, producing a pressure perturbation in the lee of the peak. In addition to these unique nonprecipitating orographic cloud phenomena, pre-existing precipitating cloud systems in the form of fronts or tropical cyclones undergo flow modification that strongly affects the nature of the clouds and redistributes precipitation. These flow modifications vary depending on how much of the low-level flow is blocked by the mountains as opposed to rising over the terrain. The movement of oceanic frontal systems characterized as “atmospheric rivers” is especially modified as they pass over mountains on the west coasts of continents. The movement of tropic cyclones over mountains can enhance rainband precipitation and trigger convection in the eye of the storm. Besides directly altering precipitating clouds of fronts and tropical cyclones moving over terrain, mountains can lead to patterns of airflow that affect when and where deep convective clouds form. In particular, mountainous terrain can channel low-level moist flow into preferred regions. Flow that is subsiding in the lee of mountains can produce downward motion that caps low-level moist flow and prevent immediate outbreak of convection. Flow over foothills can trigger the capped deep convection. All of these effects of mountains and hills on clouds and precipitation are reviewed in this chapter.

Chapter 2 - Atmospheric Dynamics☆

This chapter reviews the basic dynamic, thermodynamic, and water-continuity relationships required to read the literature on cloud dynamics. The force balances and instabilities that affect clouds are reviewed. The primitive equations are presented and their Boussinesq and anelastic forms are presented. Horizontal and vertical vorticity equations are presented. Absolute and potential vorticity are discussed. Linearization, perturbation forms of the equations, eddy fluxes, and turbulence terms are discussed. The Ekman layer is defined and discussed. The basics of hydrostatic, geostrophic, semigeostrophic, cyclostrophic, and gradient-wind balance are described. Thermal wind for both geostrophic and gradient-wind conditions is presented. Angular momentum is discussed. Buoyancy, conditional instability, inertial instability, potential instability, symmetric instability, and Kelvin–Helmholtz instability are defined and discussed. Gravity waves and geostrophic and gradient-wind adjustment are reviewed.

Chapter 11 - Clouds and Precipitation in Extratropical Cyclones☆

Extratropical cyclones draw energy from the horizontal variation of temperature in the atmosphere; i.e., from the baroclinic structure of the atmosphere. Clouds form in response to the vertical air motions within baroclinic waves and are usually concentrated along warm, cold, and occluded fronts. The air motions within baroclinic waves are frontogenetical, meaning that they tend to bring isotherms together into sharp zones of horizontal temperature contrast to define frontal zones. Frontogenetic zones have strong secondary circulations that keep the flow in force balance. This overturning produces nimbostratus clouds along most fronts. These clouds are unique in the atmosphere because they are produced by the dynamics of frontogenesis. However, the frontal clouds exhibit considerable variability. They break down into mesoscale precipitation features known as narrow cold frontal rainbands, wide cold frontal rainbands, occlusion bands, and surge rainbands. The narrow cold frontal rainbands are associated with gravity wave dynamics at the edge of sharp cold fronts. Wide cold and warm frontal rainbands are connected with moist conditional symmetric instability. The rainbands of extratropical cyclones often contain embedded convective elements. These observed structures and associated dynamics are all reviewed in this chapter.

Chapter 1 - Types of Clouds in Earth's Atmosphere☆

Abstract This chapter identifies and describes the different types of clouds that occur in Earth’s atmosphere. Traditional observation of clouds by a ground observer includes clouds known by the nomenclature: cumulus, cumulonimbus, fog, stratus, stratocumulus, altostratus, altocumulus, cirrus, cirrostratus, cirrocumulus, noctilucent, orographic, lenticular, wave clouds, rotor clouds, and banner clouds. Cloud types observed from space include the clouds of mesoscale convective systems, fronts, and tropical cyclones. Observations from space provide global climatologies of the basic cloud types.

Chapter 4 - Remote Sensing of Clouds and Precipitation☆

Abstract Most observations of clouds and precipitation are accomplished by remote sensing, including radar, lidar, and passive microwave sensing. The student of cloud dynamics needs to know the rudiments of the theory of these techniques, and this chapter provides that background. Topics covered include passive microwave rain retrieval, Doppler radar basics, polarimetric radar basics, rain measurement by radar, and thermodynamic and microphysical retrieval from Doppler radar analysis.

Cumulonimbus and Severe Storms

Abstract Deep cumulonimbus clouds are the special category of convective clouds that produce severe weather in the forms of tornadoes, downbursts, microbursts, gust fronts, derechos, and lightning. This chapter examines how vorticity dynamics leads to mesocyclones, funnel clouds, tornadoes, and waterspouts; how gravity current dynamics produces gust fronts and derechos; the dynamical differences between single cell, multicell, and supercell thunderstorms; the dynamical processes leading to the formation of lines of convective clouds; and how electrification occurs.