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Storms and Precipitation. Professor Ke-Sheng Cheng Dept. of Bioenvironmental Systems Engineering National Taiwan University. Major storm types in Taiwan. Convective storms or thunderstorms (July – October) Tropical cyclones or typhoons (July – October)
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Storms and Precipitation Professor Ke-Sheng Cheng Dept. of Bioenvironmental Systems Engineering National Taiwan University
Major storm types in Taiwan • Convective storms or thunderstorms (July – October) • Tropical cyclones or typhoons (July – October) • Frontal rainfall systems (November – April) • Mei-Yu (May – June)
Convective storms • Thunderstorm cells are the basic organizational structure of all thunderstorms. • Each cell goes through a definite life cycle which may last from 20 minutes to one or two hours, although a cluster of cells, with new cells forming and old ones dissipating, may last for 6 hours or more. • Individual thunderstorm cells typically go through three stages of development and decay. These are the cumulus, mature, and dissipating stages.
Life cycle of a thunderstorm cell Cumulus stage Mature stage Dissipating stage 積雨雲 積雲
Cumulus Stage • The cumulus stage starts with a rising column of moist air to and above the condensation level. The lifting process is most commonly that of cellular convection characterized by strong updraft. This may originate near the surface or at some higher level. The growing cumulus cloud is visible evidence of this convective activity, which is continuous from well below the cloud base up to the visible cloud top.
The primary energy responsible for initiating the convective circulation is derived from converging air below. As the updraft pushes skyward, some of the cooler and generally drier surrounding air is entrained into it. Often one of the visible features of this entrainment is the evaporation and disappearance of external cloud features. • The updraft speed varies in strength from point-to-point and minute-to-minute. It increases from the edges to the center of the cell, and increases also with altitude and with time through this stage.
The updraft is strongest near the top of the cell, increasing in strength toward the end of the cumulus stage. • Cellular convection implies downward motion as well as updraft. In the cumulus stage, this takes the form of slow settling of the surrounding air over a much larger area than that occupied by the stronger updraft. During this stage, the cumulus cloud (積雲) grows into a cumulonimbus (積雨雲).
Cloud droplets are at first very small, but they grow to raindrop size during the cumulus stage. They are carried upward by the updraft beyond the freezing level where they remain liquid at subfreezing temperatures. At higher levels, liquid drops are mixed with ice crystals, and at the highest levels, only ice crystals or ice particles are found. • During this stage, the raindrops and ice crystals do not fall, but instead are suspended or carried upward by the updraft.
Mature stage • The start of rain from the base of the cloud marks the beginning of the mature stage. Except under arid conditions or with high-level thunderstorms, this rain reaches the ground. Raindrops and ice particles have grown to such an extent that they can no longer be supported by the updraft. This occurs roughly 10 to 15 minutes after the cell has built upward beyond the freezing level.
The convection cell reaches its maximum height in the mature stage, usually rising to 25,000 (8 km) or 35,000 feet (11 km) and occasionally breaking through the tropopause (see atmospheric layers) and reaching to 50,000 (15 km) or 60,000 feet (18 km) or higher. The visible cloud top flattens and spreads laterally into the familiar "anvil" top. A marked change in the circulation within the cell takes place.
As raindrops and ice particles fall, they drag air with them and begin changing part of the circulation from updraft to downdraft. • The mature stage is characterized by a downdraft developing in part of the cell while the updraft continues in the remainder.
Dissipating Stage • As the downdrafts continue to develop and spread vertically and horizontally, the updrafts continue to weaken. Finally, the entire thunderstorm cell becomes an area of downdrafts, and the cell enters the dissipating stage. • As the updrafts end, the source of moisture and energy for continued cell growth and activity is cut off. The amount of falling liquid water and ice particles available to accelerate the descending air is diminished. The downdraft then weakens, and rainfall becomes lighter and eventually ceases.
Mesoscale Convective Systems (MCS) • The convective system begins as a number of relatively isolated convective cells, usually during the afternoon. By late evening, the anvils of the individual cells merge, and the characteristic cold cloud shield develops toward maturity sometime after midnight, local time. Dissipation then occurs typically sometime in the morning.
Although by no means restricted to the nocturnal hours, MCSs most frequently reach maturity after sunset. • MCS circular cloud tops often mask a linear structure of the convective cells when viewed on radar. • Occasionally, MCSs can produce a persistent mesoscale circulation that can persist well after the convection dissipates. These circulations have been observed to be associated with redevelopment of another MCS, so that the system as a whole can live longer than 24 h.
Mei-Yu rainfalls are produced by surface frontal systems which advance southeastward from southern China to Taiwan from mid to late spring through early to mid summer each year. The fronts are usually accompanied by a synoptic-scale cloud band with embedded mesoscale convective systems (MCSs), extending several thousand kilometres from southern Japan to southern China with an approximately east–west orientation.
During the passage of a Mei-Yu frontal system, a few very active mesoscale convective cells may develop repeatedly, causing heavy and localized rainfall for the area. Although the synoptic-scale frontal system may last for a few days, the MCSs generally have lifetime of a few hours to 1 day only.
Not all MCSs are nearly circular. Included within the category of MCS is a linearly-organized band of cold cloud tops. Such a structure is nearly always associated with a frontal boundary.
Estimating the convective rainfall using weather satellite images • The Scofield-Oliver method was originally developed for estimating half-hourly convective rainfall by analyzing the changes in two consecutive GOES satellite images. • It estimates convective rainfall at interested locations while not estimating the rain volume of the cloud systems. • Useful for early warning of flash flood.
Rationale of the Scofield-Oliver method • Bright clouds in the visible imagery produce more rainfall than darker clouds. • Brighter clouds in the visible and clouds with cold tops in the IR imagery which are expanding in areal coverage (in early and mature stages of development) produce more rainfall than those not expanding. • Decaying clouds produce little or no rainfall.
Clouds with cold tops in IR imagery produce more rainfall than those with warm tops. • Clouds with cold tops that are becoming warmer produce little or no rainfall. • Merging of cumulonimbus (Cb) clouds increases the rainfall rate of the merging clouds. • Most of the significant rainfall occurs in the upwind (at anvil level) portion of a convective system. The highest and coldest clouds form where the thunderstorms are most vigorous and the rain heaviest.
Active clouds • Tight area of IR gradient within more uniform anvil • Overshooting tops • Bright or textured part of anvil • Slower moving anvil edge • Upwind area of anvil (200-500mb wind) • Low-level inflow • Radar echoes
Rain rate assigned based on • Rain rate assigned based on • Coldness of cloud top (colder: more rain) • Cloud growth (growing: more rain) • Getting colder • Getting bigger • Divergence aloft or low-level inflow • Takes account of speed of storm motion • Available atmospheric moisture
Example Surface obs S/O satellite estimate
Operational rainfall estimates • Since the early 1980's, the Satellite Analysis Branch (SAB) of the NationalOceanic and AtmosphericAdministration/National EnvironmentalSatellite Data and Information Service(NOAA/NESDIS) has been producing satellite rainfall estimates using the Interactive Flash Flood Analyzer (IFFA). • The IFFA uses the McIDAS system which was developed by the University of Wisconsin. Special software is used to draw lines of satellite rainfall estimates. They are saved and then added for whatever time period is needed.
The IFFA is a man-machine interactive system and is very labor intensive requiring much manual input. The Scofield Convective Technique is used by the SAB Meteorologists for the estimated amounts every half-hour. • The technique uses GOES Infrared and visible imagery. The estimates are automatically corrected for parallax (viewing angle of the satellite), and an orographic correction can be done for short periods like the past 3 to 6 hours.
Tropical cyclones • A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. • They also carry heat and energy away from the tropics and transport it toward temperate latitudes, which makes them an important part of the global atmospheric circulation mechanism. As a result, tropical cyclones help to maintain equilibrium in the Earth's troposphere, and to maintain a relatively stable and warm temperature worldwide.
All tropical cyclones are areas of low atmospheric pressure near the Earth's surface. The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth's surface at sea level. • Tropical cyclones are characterized and driven by the release of large amounts of latent heat of condensation, which occurs when moist air is carried upwards and its water vapour condenses.
This heat is distributed vertically around the center of the storm. Thus, at any given altitude (except close to the surface, where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.
Stratiform (or frontal) rainfall • There are three distinct ways that rain can occur. These methods include convective, stratiform (or frontal), and orographic rainfall. • Stratiform rainfall is caused by frontal systems. • When masses of air with different density (moisture and temperature characteristics) meet, warmer air overrides colder air, causing precipitation.
Warm fronts occur where the warm air scours out a previously lodged cold air mass. The warm air 'overrides' the cooler air and moves upward. Warm fronts are followed by extended periods of light rain and drizzle, because, after the warm air rises above the cooler air, it gradually cools due to the air's expansion while being lifted, which forms clouds and leads to precipitation.
Cold fronts occur when a mass of cooler air dislodges a mass of warm air. This type of transition is sharper, since cold air is more dense than warm air. The rain duration is less, and generally more intense, than that which occurs ahead of warm fronts. • The stability of the warm air mass determines the type of precipitation generated by a cold front.
If the warm air is stable the clouds are of stratiform form. The clouds are of the cumuliform type and precipitation convective, if the warm air is unstable. • Frontal systems in UK.
Orographic rainfall • Orographic or relief rainfall is caused when masses of air pushed by wind are forced up the side of elevated land formations, such as large mountains. • The lift of the air up the side of the mountain results in adiabatic cooling, and ultimately condensation and precipitation. In mountainous parts of the world subjected to relatively consistent winds (for example, the trade winds), a more moist climate usually prevails on the windward side of a mountain than on the leeward (downwind) side. Moisture is removed by orographic lift, leaving drier air on the descending, leeward side where a rain shadow is observed.
Spatial variability of hourly rainfall • The influence range of hourly rainfall varies with storm type. In particular, hourly rainfall of Mei-Yu has the smallest influence range of 24 km, suggesting the highest spatial variation among all storm types. The small influence range of Mei-Yu rainfall may be attributed to redevelopment of MCSs.
