EAE2111 - Topic 2 - Wk 1 Flashcards
<p><strong><span>What is the Latitudinal energy redistribution?</span></strong></p>
Net radiative budget of EA system is in deficit poleward of 40 degrees N and S<br></br>Sets up the General Circulation of the atmosphere and oceans<p style="text-align:right;"><span>EAE2111 1aa</span></p>
<p><strong><span>What is the Vertical Wind Speed Profile?</span></strong></p>
<p>4 points.</p>
<ul><li>Theoretical vertical profile is logarithmic </li><li>Winds increase with height to be near geostrophic at the top of the boundary layer </li><li>Frictional influence (i.e. drag) at the surface cause this profile </li><li>Frictional influence also causes change in direction with height</li></ul>
<p><span>EAE2111 1ab</span></p>
<p><strong><span>What is atmospheric stability?</span></strong></p>
<p><span>The ease with which some sections of the atmosphere are liable to overturn</span></p>
<div>Three broad categories of stability: Unstable, neutral, and stable</div>
<div>These are fundamentally dependent on the vertical temperature structure of the atmosphere.</div>
<div>Note that stability can be broken down into two components conditional stability and absolute stability - which is covered later.</div>
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<p><strong><span>What drives atmospheric stability?</span></strong></p>
<p>4 points.</p>
<p><span>Stability is fundamentally dependent on the vertical temperature structure of the atmosphere</span></p>
<div><strong>BUT…</strong> </div>
<ul> <li>Lower atmosphere is usually not isothermal i.e. the temperature is rarely the same throughout the layer depth.</li> <li>Typically, temperature decreases with height.</li> <li>As parcels from the surface rise, they cool due to expansion</li> <li>A non-saturated parcel of air cools at the dry adiabatic lapse rate (~1°C per 100 m)</li></ul>
<div>When thinking about atmospheric stability, we need to take into account this cooling with height as an air parcel rises… </div>
<p><span>EAE2111 1ad</span></p>
<p><strong><span>What is potential temperature (θ)?</span></strong></p>
<p><span>The temperature the air parcel would have if it were expanded or compressed from its original existing pressure and temperature to a standard pressure</span></p>
<div>(Standard pressure = 1000 hPa)</div>
<div>Temperature naturally decreases with height, so we use potential temperature to describe the temperature and different heights in relative terms i.e. so they are equivalent for comparison.</div>
<p><span>EAE2111 1ae</span></p>
<p><strong><span>What is the wind speed profile with neutral<br></br>
atmospheric stability?</span></strong></p>
<div>Temperature varies little with height (typically early-mid morning - late afternoon) </div>
<div>Windspeed increases logarithmically with height associated with surface friction </div>
<div>Turbulence is entirely mechanical (shear driven)</div>
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<p><strong><span>What is the wind speed profile with atmospheric<br></br>
stability (stable environment)?</span></strong></p>
<div>The temperature increases with height (middle of night) </div>
<div>There is suppressed vertical mixing - any eddies only bring air from a short way aloft </div>
<div>Less influence from thermal convection, z₀ moves closer to ground as frictional effects dominate </div>
<div>Suppressed thermal turbulence</div>
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<p><strong><span>What is the wind speed profile with atmospheric<br></br>
instability (unstable environment)?</span></strong></p>
<p>4 points.</p>
<ul><li>The temperature decreases rapidly with height (typically middle of the day) </li><li>Enhanced vertical mixing as heated bubbles of air move through cooler air above. </li><li>Therefore z₀ moves away from the ground because of the influence of thermal convection at surface </li><li>Enhanced thermal turbulence</li></ul>
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<p><strong><span>What is turbulent transfer?</span></strong></p>
<p>3 points.</p>
<p><span>The net transfer of an atmospheric property (e.g. heat) from vertical eddies of turbulent motion.</span></p>
<ul> <li>Upward moving air in one place is replaced by downward moving air in another place</li> <li><em>Vertical motion, and hence transport, at the surface is a combination of the mean wind field and turbulent eddies</em></li> <li>Net transfer of an atmospheric property (i.e. heat) from vertical eddies of turbulent motion can occur.</li></ul>
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<p><strong><span>How do we think about moisture in the atmosphere??</span></strong></p>
<p>4 points.</p>
<ul><li>As an atmospheric constituent i.e. water vapour </li><li>Water vapour has a different effect on the atmosphere than dry air </li><li>Moisture in the air makes it behave differently because it affects properties of the air e.g. pressure </li><li>For example, pressure in atmosphere is the sum of all the different pressures that various gases exert. These different pressures are called partial pressures.</li></ul>
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<p><strong><span>What is total atmospheric pressure?</span></strong></p>
<div>Total atmospheric pressure is the <em>arithmetic sum of all the partial pressures due to its constituent gases</em>, e.g. </div>
<div>p(total) = p(N₂)+p(O₂)+p(CO₂)+p(H₂O)…, etc</div>
<div></div>
<div>The vapour pressure of water in the atmosphere (e) is an important measurement</div>
<div></div>
<div>Total atmospheric pressure varies between ~970- 1040 hPa (hectopascals). </div>
<div>Typical vapour pressures are much smaller, in the range ~5-25 hPa, i.e. they only represent a small proportion of total pressure</div>
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<p><strong><span>What is the dew point temperature?</span></strong></p>
<p>2 points.</p>
<p><span>The temperature to which air must be cooled to become saturated with water vapor</span></p>
<div>We can bring an air parcel to its saturation point in one of two ways: </div>
<ol> <li>By increasing the amount of water vapour available until it reaches saturation, or </li> <li>By decreasing the temperature of the air parcel until it reaches the point where its ability to 'hold' the vapour will be exceeded if any further drop in temperature occurs. </li></ol>
<div>If we do the latter, the point at which saturation occurs is called the <strong>dew point temperature</strong>. Further cooling would result in vapour condensation (cloud formation).</div>
<p><span>EAE2111 1al</span></p>
<p><strong><span>What is relative humidity?</span></strong></p>
<p>4 points.</p>
<p><span>The amount of water vapour present in air expressed as a percentage of the amount needed for saturation at the same temperature.</span></p>
<div>Relative humidity calculated as the ratio RH = e/eₛ x 100</div>
<ul> <li>Relative humidity is very dependent upon temperature</li> <li>Remember: eₛ is strongly dependent on temperature and will change a lot through course of a day</li> <li>BUT actual moisture content does not change this much.</li> <li>We need better measures of moisture</li></ul>
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<p><strong><span>What is the mixing ratio?</span></strong></p>
<p>3 points.</p>
<div>Mixing ratio (r) expresses the ratio of a mass of water vapour to the mass of dry air in a given volume </div>
<div>(g.kg⁻¹, ranges from 0- 20)</div>
<div></div>
<div>r = 0.622 (e/p) where </div>
<ul> <li>0.622 is the ratio of molecular weights of water vapour and dry air</li> <li>e is vapour pressure</li> <li>p is ambient air pressure</li></ul>
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<p><strong><span>What is specific humidity?</span></strong></p>
<div>Specific humidity (q) expresses the ratio of a mass of water vapour to the mass of moist air in a given volume (g.kg-1) </div>
<div>q = (0.622e)/(p - 0.378e)</div>
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