Mesoscale convective systems (MCSs) have regions of both convective and stratiform precipitation, and they develop mesoscale circulations as they mature. The upward motion takes the form of a deep‐layer ascent drawn into the MCS in response to the latent heating and cooling in the convective region. The ascending layer overturns as it rises but overall retains a coherent layer structure. A middle level layer of inflow enters the stratiform region of the MCS from a direction determined by the large‐scale flow and descends in response to diabatic cooling at middle‐to‐low levels. A middle level mesoscale convective vortex (MCV) develops in the stratiform region, prolongs the MCS, and may contribute to tropical cyclone development. The propagation of an MCS may have a discrete component but may further be influenced by waves and disturbances generated both in response to the MCS and external to the MCS. Waves of a larger scale may affect the propagation velocity by phase locking with the MCS in a cooperative mode. The horizontal scale of an MCS may be limited either by a balance between the formation rate of convective precipitation and dissipation of stratiform precipitation or by the Rossby radius of the MCV. The vertical redistribution of momentum by an MCS depends on the size of the stratiform region, while the net vertical profile of heating of the large‐scale environment depends on the amount of stratiform rain. Regional variability of the stratiform rain from MCSs affects the large‐scale circulation's response to MCS heating.
This chapter reviews the basic microphysics governing the formation and growth of the liquid drops and ice crystals that occur in clouds. The various physical processes are examined separately, including nucleation, condensation, evaporation, sublimation, deposition, stochastic collection, continuous collection, coalescence, riming, aggregation, breakup, and secondary ice-particle production. Then these individual processes are combined into water-continuity equations that govern how all the microphysical processes operate together in both warm and cold clouds. Water-continuity equations are presented in both bulk (parameterized) form in which particle-size distributions are assumed and explicit (bin) form in which particle-size distributions are computed. The latest methodologies for formulating the water-continuity equations are included.
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.
Abstract Regional climate simulations over the continental United States were conducted for the 2011 warm season using the Weather Research and Forecasting model at convection‐permitting resolution (4 km) with two commonly used microphysics parameterizations (Thompson and Morrison). Sensitivities of the simulated mesoscale convective system (MCS) properties and feedbacks to large‐scale environments are systematically examined against high‐resolution geostationary satellite and 3‐D mosaic radar observations. MCS precipitation including precipitation amount, diurnal cycle, and distribution of hourly precipitation intensity are reasonably captured by the two simulations despite significant differences in their simulated MCS properties. In general, the Thompson simulation produces better agreement with observations for MCS upper level cloud shield and precipitation area, convective feature horizontal and vertical extents, and partitioning between convective and stratiform precipitation. More importantly, Thompson simulates more stratiform rainfall, which agrees better with observations and results in top‐heavier heating profiles from robust MCSs compared to Morrison. A stronger dynamical feedback to the large‐scale environment is therefore seen in Thompson, wherein an enhanced mesoscale vortex behind the MCS strengthens the synoptic‐scale trough and promotes advection of cool and dry air into the rear of the MCS region. The latter prolongs the MCS lifetimes in the Thompson relative to the Morrison simulations. Hence, different treatment of cloud microphysics not only alters MCS convective‐scale dynamics but also has significant impacts on their macrophysical properties such as lifetime and precipitation. As long‐lived MCSs produced 2–3 times the amount of rainfall compared to short‐lived ones, cloud microphysics parameterizations have profound impact in simulating extreme precipitation and the hydrologic cycle.
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.
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.
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.
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.
This chapter examines clouds confined to relatively shallow layers of air in which the rate of cooling resulting in cloud formation is rather slight. These clouds include: fog, stratus, stratocumulus, altostratus, altocumulus, cirrus, cirrostratus, cirrocumulus, and subvisible cirrus. These clouds have water contents generally < 1 g kg− 1 and in shallow layer clouds we observe mostly mean vertical motions < 1–10 cm s− 1. These small vertical air motions contrast with the strong updrafts and downdrafts of the convective clouds considered in Chapters 7–9Chapter 7Chapter 8Chapter 9. Shallow layer clouds are generally ~ 1 km or less in vertical extent, although they can occasionally be a few kilometers in depth. This chapter examines how weak vertical air motion, radiation, and turbulent mixing interact to give shallow layer clouds their particular character.
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.
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. Föhn 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.
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.
The ocean has the largest heat capacity in the climate system and as a result the ocean plays a critical role in the climate. Changes in ocean heat content dominate the Earth’s energy storage; and the ocean’s thermal expansion has been a major contributor to sea-level rise in the twentieth century and likely to be the largest contributor in the twenty-first century. The agreement between changes in ocean heat storage over recent decades and changes in the Earth’s radiative balance, within uncertainties, provides strong support for current understanding of anthropogenic climate change. As a result of improvements in observations and modeling of sea level and components contributing to sea-level change, there is now an improved explanation for twentieth century sea-level rise. Models project a continuing sea-level rise during the twenty-first century and beyond. However, a number of uncertainties remain in our understanding of the global mean and regional distribution of sea-level rise resulting from changes in ocean circulation and changes in the Earth’s gravitational field. Ocean-ice-sheet interactions are important for quantitatively estimating future ice-sheet contributions to sea-level rise.
Carbon is a fundamental component of all life and its gaseous form, carbon dioxide (CO2), is an important atmospheric regulator of Earth’s climate. Over societally relevant timescales, diverse physical, biological, and chemical processes in the global ocean have a very important role in controlling the exchanges of CO2 with the atmosphere and climate, given that the ocean contains roughly 60 times more carbon than the atmosphere. The complex and dynamic marine carbon cycle is thus a fundamental part and regulator of life on Earth, requiring monitoring of its variability and understanding of the feedbacks to the climate system. This is particularly relevant given that emissions of human produced (anthropogenic) CO2 through fossil fuel and land use changes have led to significant perturbations in the global carbon cycle. A large fraction of emitted anthropogenic carbon (Cant) has been and will be absorbed by the oceans with implications for seawater chemistry, ocean pH levels, and the biological communities contained within. During the last few decades, quantification of the ocean sink of CO2 and its temporal evolution of the marine carbon cycle has been an important driver of marine biogeochemical research. Scientific expeditions such as the World Ocean Circulation Experiment/Joint Global Ocean Flux Study global surveys conducted during the 1990s provided the data for the first global estimates of the uptake and oceanic sink of Cant. More recently, completion of a second global survey of the marine carbon cycle (from 2000 to 2013) provided sufficient data to assess the rate of uptake and storage of Cant within the interior of the global ocean. From this effort, it has become clear that there is substantial regional and temporal variability of the storage rate of Cant and those physico-biogeochemical processes that influence the marine carbon cycle. Here we review seawater carbonate chemistry, the sampling strategies, and networks required to observe variability in the global ocean carbon cycle and the flux of carbon between the ocean and the atmosphere but with a central focus, synthesizing and summarizing available estimates of the ocean uptake and inventories of Cant. This chapter is aimed at a wider audience within the oceanographic community and points to the rich literature on marine carbon cycle research.
This chapter summarizes the history of and recent progress in observations of surface circulation of the ocean. The description is biased toward Lagrangian drifting buoys and satellite altimetry that, in the authors' opinion, have revolutionized our understanding of ocean surface currents by providing a high resolution, continuous global view of mesoscale ocean "weather." We outline the current state of the observing system and characterize geostrophic surface circulation, Ekman currents, and effects of centrifugal force and nonlinear interaction between geostrophic and Ekman currents. This general description is complemented by regional examples, detailing processes in the California Current System, off Senegal, and the Kuroshio interaction with the East and South China Seas. Applications to various tasks, in which surface currents play important role, are illustrated by model simulations of motions of marine debris, including debris generated by the 2011 tsunami in Japan. The chapter concludes with a brief discussion of the next tasks needed to improve understanding of the dynamics of the surface ocean circulation.
