Storms and Precipitation

1

Applied Hydrology

RSLAB-NTU

Lab for Remote Sensing Hydrology and Spatial Modeling

Storms and Precipitation

Professor Ke-Sheng ChengDept. of Bioenvironmental Systems Engineering

National Taiwan University

2Lab for Remote Sensing Hydrology and Spatial ModelingRSLAB-NTU

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

積雲積雨雲


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.


Cumulus cloud


Cumulonimbus


Cumulonimbus


Mature stageThe 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.


Cumulus stage


Mature stage


Dissipating stage


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 StageAs 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.


Digital enhancement curve (the Mb curve)



Active clouds

Tight area of IR gradient within more uniform anvil

Overshooting topsBright or textured part of anvilSlower moving anvil edgeUpwind area of anvil (200-500mb wind)Low-level inflowRadar echoes


Enhanced IR


Rain rate assigned based on

Rain rate assigned based onColdness of cloud top (colder: more rain)Cloud growth (growing: more rain)

Getting colderGetting bigger

Divergence aloft or low-level inflowTakes account of speed of storm motionAvailable atmospheric moisture


Example

Surface obs S/O satellite estimate


Step 1 – Finding active areas







Operational rainfall estimates

Since the early 1980's, the Satellite Analysis Branch (SAB) of the National Oceanic and Atmospheric Administration/National Environmental Satellite 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) rainfallThere are three distinct ways that ra

in 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.


Orographic rainfall


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.


Using the variogram to characterize the spatial variability of rainfall data



Atmospheric Layers

The atmosphere is divided into five layers. It is thickest near the surface and thins out with height until it eventually merges with space. The troposphere is the first layer above the surface and

contains half of the Earth's atmosphere. Weather occurs in this layer.

Many jet aircrafts fly in the stratosphere because it is very stable. Also, the ozone layer absorbs harmful rays from the Sun.

Meteors or rock fragments burn up in the mesosphere.The thermosphere is where the space shuttle orbits.The atmosphere merges into space in the extremely thin

exosphere. This is the upper limit of our atmosphere.


Ozone layer



Troposphere & Tropopause (對流層頂 )

The tropopause is the atmospheric boundary between the troposphere(對流層 ) and the stratosphere. Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry.

About 80 % of the total mass of the atmosphere is contained in troposphere. It is also the layer where the majority of our weather occurs.


The exact definition used by the World Meteorological Organization is:the lowest level at which the lapse rate

decreases to 2 °C/km or less, provided that the average lapse rate between this level and all higher levels within 2 km does not exceed 2 °C/km.

The troposphere is the lowest of the Earth's atmospheric layers and is the layer in which most weather occurs.


The troposphere begins at ground level and ranges in height from an average of 11 km (6.8 miles/36,080 feet at the International Standard Atmosphere) at the poles to 17 km (11 miles/58,080 feet) at the equator.

It is at its highest level over the equator and the lowest over the geographical north pole and south pole.


Measuring the lapse rate through the troposphere and the stratosphere identifies the location of the tropopause. In the troposphere, the lapse rate is, on average, 6.5 °C per kilometre. In the stratosphere, however, the temperature increases with altitude.


Stratosphere

This stratosphere contains about 19.9 % of the total mass found in the atmosphere.

Very little weather occurs in the stratosphere. Occasionally, the top portions of thunderstorms breach this layer.

In the first 9 kilometers of the stratosphere, temperature remains constant with height. A zone with constant temperature in the atmosphere is called an isothermal layer.


From an altitude of 20 to 50 kilometers, temperature increases with an increase in altitude.

The higher temperatures found in this region of the stratosphere occurs because of a localized concentration of ozone gas molecules. These molecules absorb ultraviolet sunlight creating heat energy that warms the stratosphere.


Ozone is primarily found in the atmosphere at varying concentrations between the altitudes of 10 to 50 kilometers. This layer of ozone is also called the ozone layer. The ozone layer is important to organisms at the Earth's surface as it protects them from the harmful effects of the sun's ultraviolet radiation. Without the ozone layer life could not exist on the Earth's surface.


Mesophere & thermosphere

In the mesosphere, the atmosphere reaches its coldest temperatures (about -90° Celsius) at a height of approximately 80 kilometers. At the top of the mesosphere is another transition zone known as the mesopause.

The thermosphere has an altitude greater than 80 kilometers. Temperatures in this layer can be as high as 1200°C. These high temperatures are generated from the absorption of intense solar radiation by oxygen molecules (O2).

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Storms and Precipitation