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Lecture 3

Lecture 3 - mass balance.docx

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McMaster University
Environmental Science
Altaf Arain

Lecture 3 – Mass balances Water, carbon, nitrogen, oxygen and sulfur are important mass cycles in the Earth-atmosphere system. The hydrologic or water cycle is the most important of all because survival on earth depends on it. Another important cycle which we have recently become concerned with is the carbon cycle. Hydrologic Cycle:  Water continuously moves between oceans, the atmosphere, the cryosphere and the land.  The total amount of water on earth is constant, but it changes forms between liquid, gaseous, and solid forms.  The movement of water between ocean, atmosphere and land reservoirs is called the hydrologic cycle.  Water enters the atmosphere from land and ocean through evaporation, travels great distances in the form of water vapour and clouds and then returns to the surface through precipitation.  The amount of water moved through the hydrologic cycle each year is roughly equivalent to about 1m depth of liquid water spread uniformly over the surface of the earth.  To evaporate 1 m of water requires an average of 80 Wm of energy, which comes from the sun Water affects climate in different ways  Atmospheric water vapour absorbs solar and terrestrial radiation and accounts for about half of the greenhouse gas effects  Clouds reflect solar radiation and block the escape of longwave radiation  Evaporation from surface and vegetation cools these surfaces  Energy released during condensation in the cloud formation process drives global atmospheric circulation  Water can alter surface albedo of a surface through snow fall, enhanced soil moisture and by fostering vegetation growth The water balance: The water budget of a region may be written as: ΔS = P + D – E – Δr Where ΔS is the net storage change in soil moisture content, P is precipitation in liquid or frozen form, D is surface condensation (dewfall or frost), E is evapotranspiration and Δr is runoff. The annual average longitudinal distribution of the surface hydrological cycle is given in above figure. Note: most precipitation occurs just north of equator; most evaporation occurs just south of equator Cloud formation:  About 60% of the earth is covered with clouds, but their presence does not necessarily mean rain.  Clouds are important in climatology because they strongly influence and redistribute the surface input of water and energy (radiant energy fluxes and latent heat transport from the tropics poleward)  Cloud formation requires the following three conditions: o Sufficient moisture in the atmosphere to condense o A medium on which condensation can occur – cloud nuclei o A mechanism to produce atmospheric cooling by rising air Mechanism of Air Lifting There are different mechanisms that cause air to rise in the atmosphere Convective Lifting: parcels of hot, buoyant air from the surface rise (and cool) until the temperature in the parcel drops and the air saturates to produce convective clouds. When condensation begins, the latent heat is released which further warms the air, making it more buoyant. Topographic Movement: large-scale horizontal pressure forces push bodies of moist air mass against topography Air Convergence: warm, moist air is rapidly forced to mix with cooler, dry air in the overhead air stream. Frontal Lifting: advancing warm air fronts rise over dense cold air masses to form frontal clouds. Types of Clouds:  Cold clouds -50 to -15 °C ice particles  Mixed clouds -40 to 2 °C water and ice  Warm clouds > 0 °C water droplets Clouds are also classified according to their temperature (see above), morphology and the height of formation. Precipitation Formation Mechanisms in Clouds: Clouds contain water droplets, ice particles or both depending upon cloud type, height and location. It is very difficult for water and ice particles to reach the ground because of uplift processes in the cloud and re-evaporation of falling droplets. Apart from droplet growth by condensation on nuclei, there are other mechanisms of cloud droplet growth: Collision process: it produces progressively large cloud particles. There are three types of collisions 1. Coalescence of liquid droplets (in warm mixed clouds) 2. Aggregation of ice particles (in cold and mixed clouds) 3. Accretion of water droplets on to ice particles (in mixed clouds) Bergeron-Findeisen Process: in this process ice particles grow at the expense of water droplets due to relative difference in vapour pressure at the same temperature. Water evaporates from water droplets and deposits on ice particles, because more energy is required to release a water molecule from ice particles. This process occurs in mixed clouds. Precipitation and Dewfall  Precipitation is produced when air parcels become supersaturated with water vapour and condensation and cloud formation occurs  It is highly variable in space and time but it’s average values in a particular region is fairly constant under current climate conditio
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