Lecture 4_5 PDF - Precipitation Processes

Precipitation processes The mechanics of cloud formation Lecture 5 Essential reading Tyson PD and Preston-Whyte RA. (2000). The weather and climate of southern Africa. Oxford University Press, Cape Town. Chapters 3 & 4 Lecture summary Cloud formation Dew point temperature Lapse...

Precipitation processes The mechanics of cloud formation Lecture 5 Essential reading Tyson PD and Preston-Whyte RA. (2000). The weather and climate of southern Africa. Oxford University Press, Cape Town. Chapters 3 & 4 Lecture summary Cloud formation Dew point temperature Lapse rates Atmospheric instability and uplift Turbulence Cloud condensation nucleii Clouds Cumuliform clouds Stratiform clouds Orographic clouds Introduction 60% of earth covered with cloud at any moment Most clouds do not bring rain In this lecture we will discuss: What are the necessary conditions for cloud formation? Different processes that result in uplift of air Lapse rates What factors control whether there will be condensation? Requirements of cloud formation 1. A mechanism for cooling air to dew point temperature such that water vapor may condense i.e. a) Thermal updrafts b) Physical barriers Mountains Air masses c) Turbulence due to surface friction 2. Cloud condensation nuclei 3. Sufficient water vapour Dew point temperature Condensation occurs at a temperature called dew point The dew point temperature depends on the air humidity Essentially, the more humid the air, the higher the dew point T, saturation & condensation occurs more easily than for dry air Dew point T is reached in the atmosphere through the cooling of air How can we cool air to dew point T? Clouds may form at ground surface if air is cooled in situ More commonly, air is cooled through uplift such that condensation can occur Introduction to lapse rates The air parcel approach to understanding atmospheric processes Approach assumes that rising air does not mix with surrounding air Air parcel approach An air parcel’s ascent will continue as long as it maintains positive buoyancy Has a lower density than the surrounding air Once its density and T match that of surrounding air, the parcel will cease to rise If the T of the surrounding air is greater than the parcel’s, it will begin to descend Lapse rates Whether parcel rises, falls or stays stationary depends on it’s own T and the T of the surrounding environment The observed variation with height is called the ELR Is the actual change in T in the environment, varies from place to place and from time to time In general, the T of the atmosphere decreases with height The air expands as atmospheric pressure decreases. As energy is used for decompression, T decreases. Basic concepts: Lapse rates Lapse Rate is the rate at which a parcel of air cools as it raises in the atmosphere and the rate at which it cools as it descends. The difference in temperature between the air and the ground causes the change temperature gradient creating the lapse rate. Types of lapse rates 1. Environmental Lapse Rate (ELR) 2. Dry Adiabatic Lapse Rate (DALR) 3. Saturated Adiabatic Lapse Rate (SALR) Environmental Lapse Rate (ELR) The ELR is the actual measured decrease in temperature with height above the ground It is usually a decline of 6.5 C per 1000m. This rate does variety with the local air conditions and surround topography. Other Influencing factors: 1. Height of the formation of the lapse rate 2. Lapse rates are lower in winter and during the rainy season 3. Surface, lapse rates are lower over land than sea 4. Large air Masses causes differences with lapse rate Dry Adiabatic Lapse Rate (DALR) The DALR is a theoretical lapse rate. The dry air parcel does not mix with the surrounding air and is adiabatic. As the air parcel rises, the pressure decreases and this causes the air to expand. The air parcel uses energy to expand which causes the decrease in temperature The DALR is 9.8 C per 1000m. The DALR only applies when the realtive humidity is less that 100%  The SALR takes into account the fact that when water condenses it releases energy at heat (latent heat). Once the air has cooled low enough to the condensation level it starts to rise slower. The SALR has a high range, it can range from 4-9 C per 1000m. The average SALR is about 5.4 C per 1000m. Atmospheric instability & uplift Convection Movement of particles within a liquid or gas May be free or forced Free convection Thermals of warm air move vertically Forced convection Air is forced horizontally up and over a topographical barrier or along a frontal zone Thermals of warm air move vertically Degree to which convection produces clouds depends on the ELR & humidity at the time An unstable atmosphere forms when an air parcel cools slower than the surrounding air and the parcel continues to rise. It means that the parcel of air rises higher and can lead to the production of clouds and thunderstorms. if an air parcel cools at a quicker rate such as the DALR it cause instability in the air. If an air parcel is displaced, it will continue to move away from its initial locations, free convections or convection due to buoyancy develops in an unstable atmosphere An unstable atmosphere Increasing lapse rate In Summer - Sun heats the surface creating an unstable atmosphere with up and down circulations – thunderstorms Winter - cold air moving over warm air increase lapse rate, increases instability and results in rain associated with the cold front A stable atmosphere there is very little moisture in the air and that the temperature drops along a Dry Adiabatic Lapse Rate (9.8 C per 1000m) the air is cooling faster than the Environmental Lapse Rate, If a parcel of air is displaced then it will return to its initial location. There is usually no convection in the air and the only time convection will develop is through forced convection. The stable air will only be forced to rise through the Topography and from weather fronts. Conditional instability Saturated air becomes unstable under lower temperatures compared to dry air. If the conditions have a vertical temperature gradient which means both saturated air and dry air are stable the conditions are known as absolutely stable, whereas conditions that cause instability for both saturated and dry air is known as absolutely unstable. If the concitions are stable for dry air and unstable for saturated air it is called conditionally unstable. In the lower layers of the atmosphere it is a state of conditional instability, so when water vapor reaches the saturation points it forms cumulus clouds and cumulonimbus clouds. Turbulence Slower ascent in turbulent airflow also produces clouds, and in some cases, rain Turbulence arises from: frictional interference with obstacles on the land surface Wind shear May result in significant surface eddies Mixing of surface layers of air Warm air from surface moved upwards, while cooler air transported to the surface Air Motion The pressure gradient force (PGF) sets air in motion from High toward Low pressure. High Pressure: air sinks toward the surface and flows outward from the center. When air sinks, few clouds form and clear skies prevail. Low Pressure: Air spirals in toward the center at the surface and then rises. As air rises, it cools, clouds form, and rain is possible Associated with thunderstorms, hurricanes, and midlatitude cyclones. Cloud condensation nuclei Atmosphere is predominantly gaseous Also small liquid and solid particles Held in suspension by air currents, winds & thermals Small particles provide a surface on which condensation occurs Majority of these particles are derived from natural sources Worldwide production of aerosols Cloud types & morphology Broad 3 way division based on process: 1. Clouds formed through convection Cumuliform clouds 2. Clouds produced by slower but more widespread ascent Uplift at frontal surface Layered or stratiform clouds, Cumuliform 3. Clouds produced by turbulent flow Shallow, stratiform and wave- clouds Cumuliform clouds Usually associated with convective activity May grow to the height of the tropopause, where after the stratosphere prevents positive buoyancy Often responsible for heavy rain, thunderstorms and hail Tend to tower as they require warm moist air, air from the sides is cooler & dryer, and therefore tends to erode the cloud In fair weather conditions, cloud generation is linked to a series of thermals Clouds appear to be moving in the wind by the progressive eroding on the upwind side, matched by the regeneration downwind Cumuliform development Stratiform clouds Form in 3 ways: Turbulence Fog Slow but widespread ascent along a frontal surface Clouds produced by turbulence or as fog tend to be thin and close to the surface Orographic clouds Wave clouds Develop at the crests of waves set up by the interference of horizontal air flow by a topographic barrier Conditions suitable for formation: Unstable layer below barrier Deep stable layer above Foehn effect The Foehn effect