G304 – Physical Meteorology and Climatology Chapter Energy balance and Temperature By Vu Thanh Hang, Department of Meteorology, HUS 3.1 Atmospheric influences on Insolation • The atmosphere absorbs some radiation directly and thereby gains heat • Another portion of radiation disperses as weaker rays going out in many different directions through a process called scattering • Some of the scattered radiation is directed back to space • The energy that is scattered is not absorbed by the atmosphere Æ not contribute to its heating • The remaining insolation passes through the atmosphere without modification, reaching the surface as direct radiation 3.1 Atmospheric influences on Insolation (cont.) • Absorption: - Atmospheric gases, particulates, and droplets all reduce the intensity of solar radiation (insolation) by absorption, a process in which radiation is captured by a molecule - Absorption represents an energy transfer to the absorber - This transfer has two effects: the absorber gains energy and warms, while the amount of energy delivered to the surface is reduced - The gases of the atmosphere are not equally effective at absorbing sunlight & different wavelengths of radiation are not equally subject to absorption 3.1 Atmospheric influences on Insolation (cont.) • Absorption (cont.): - If the atmosphere were able to absorb all the incoming solar energy, the sky would appear completely dark - Ultraviolet radiation is almost totally absorbed by ozone in the stratosphere - Visible radiation passes through the atmosphere with only a minimal amount of absorption - Near infrared radiation is absorbed mainly by two gases: water vapor & carbon dioxide 3.1 Atmospheric influences on Insolation (cont.) • Reflection & scattering: - The reflection of energy is a process whereby radiation making contact with some material is simply redirected away from the surface without being absorbed - All substances reflect visible light, but with vastly differing effectiveness - Objects not reflect all wavelengths equally - The percentage of visible light reflected by an object or substance is called its albedo - When light strikes a mirror, it is reflected back as a beam of equal intensity, in a manner known as specular reflection 3.1 Atmospheric influences on Insolation (cont.) • Reflection & scattering (cont.): - When a beam is reflected from an object as a larger number of weaker rays traveling in different directions, it is called diffuse reflection, or scattering - Large solid surfaces, gas molecules, particulates, and small droplets scatter radiation - The scattered energy reaching Earth’s surface is thus diffuse radiation, which is in contrast to unscattered direct radiation 3.1 Atmospheric influences on Insolation (cont.) • Reflection & scattering (cont.): - In a scattering process, the radiation is redirected but not absorbed - The characteristics of radiation scattering by the atmosphere depend on the size of the scattering agents relative to the wavelength of the incident electromagnetic energy - Three general categories of scattering exist: Rayleigh scattering, Mie scattering and nonselective scattering 3.1 Atmospheric influences on Insolation (cont.) • Reflection & scattering (cont.): + Rayleigh scattering: - Scattering agents smaller than about one-tenth the wavelength of incoming radiation disperse radiation known as Rayleigh scattering, which is performed by individual gas molecules in the atmosphere - It primarily affects shorter wavelengths - Rayleigh scattering disperses radiation both forward and backward Æ blue sky on a clear day, the blue tint of the atmosphere when viewed from space, the redness of sunsets and sunrises Fig 3-3 The sky appears blue because gases and particles in the atmosphere scatter some of the incoming solar radiation in all directions Air molecules scatter shorter wavelengths most effectively Thus, we perceive blue light, the shortest wavelength of the visible portion of the spectrum Fig 3-5 Sunrises and sunsets appear red because sunlight travels a longer path through the atmosphere This causes a high amount of scattering to remove shorter wavelengths from the incoming beam radiation The result is sunlight consisting almost entirely of longer (e.g., red) wavelengths Fig 3-16 The circulation of ocean currents Those moving warm water are depicted by red arrows, those moving cold water by blue arrows 3.6 Influences on temperature (cont.) • Local conditions: - Slope orientation and steepness can influence the temperature characteristics of an area - In NH, slopes that are south-facing receive mid-day sunlight at a more direct angle than those oriented in other directions Æ higher surface temperatures - Vegetation patterns often respond to the change in microclimate - Densely wooded areas also have different temperature regimes than areas devoid of vegetation cover 3.7 Daily and Annual temperature patterns • At solar noon the Sun achieves its greatest angle above the horizon and the surface receives its greatest input of solar radiation Æ max air temperature occur at least a couple hours afterward • Surface temperature increases as long as energy gained by the surface is greater than energy lost • The amplitute of the daily temperature pattern is depended on a lot of factors: cloud, strong wind, temperature difference between surface and air, … • The relative amounts of incoming and outgoing radiation affect energy budgets and annual temperature • Max temp is in July or Aug in NH 3.8 Measurement of temperature • When a group of molecules (microscopic) move predominantly in the same direction, the motion is called wind (macroscopic) • When they move in random directions, the motion is associated with temperature Æ higher temperatures are associated with greater average molecular speed Celsius Temperature = (oF - 32) / 1.8 Fahrenheit Temperature = (1.8 x oC) + 32 Kelvin Temperature = oC + 273 3.8 Measurement of temperature (cont.) 3.8 Measurement of temperature (cont.) • The measurement of temperature is a simple and routine procedure in which the expansion and contraction of the fluid in a thermometer is noted • The most accurate thermometers contain mercury (or dyed alcohol) • A maximum thermometer is very similar to a regular thermometer, but with two differences: - must contain mercury - in the tube just beyond the bulb is a very narrow constriction that allows the mercury to expand outward when the temperature increases but prevents it from contracting back when temperature decreases 3.8 Measurement of temperature (cont.) • A minimum thermometer is also similar to a regular thermometer • If the index is at the end of the alcohol and temperature is decreasing, surface tension pulls the index toward the bulb • Thermograph gives a continuous record of temperature • Temperature measuring instruments should always be kept in an instrument shelter 3.8 Measurement of temperature (cont.) 3.8 Measurement of temperature (cont.) • The daily mean is defined as the average of the maximum and minimum temperature for a day • The daily temperature range is obtained by subtracting the minimum temperature from the maximum • The monthly mean temperature is found by summing the daily means and dividing by the number of days in the month • The annual mean temperature is obtained by summing the monthly means for a year and dividing by 12 • The annual range is obtained as the difference between the highest and lowest monthly mean temperatures 3.8 Measurement of temperature (cont.) If low temperatures are accompanied by windy conditions, a person’s body loses heat much more rapidly than it would under calm conditions due to an increase in sensible heat loss It is common for weather reports to state both the actual temperature and how cold that temperature actually feels, the wind chill temperature index 3.9 Thermodynamic diagrams and vertical temperature profiles • x axis: T, temperatures increase from left to right • y axis: logp, decrease with height • Isobars: horizontal lines, 10mb spacing, p values from 1050mb to 200mb • Isotherms: vertical lines, 1o spacing, T values from -85oC to 40oC • Dry adiabats: ascent line of unsaturated air (constant θ, 5oC spacing) χ χ y=− where x=T and y = lg χ +1 po p lg x + lg p o + χ −1 lg θ − 3.9 Thermodynamic diagrams and vertical temperature profiles (cont.) • Saturated adiabats: ascent line of saturated air (curved) • Saturation mixing ratio lines: “point” at which air would be saturated q max = 0,622 E = const p − 0,378E 3.9 Thermodynamic diagrams and vertical temperature profiles (cont.) Thermodynamic diagrams (such as the Stuve above) 3.9 Thermodynamic diagrams and vertical temperature profiles (cont.) Thermodynamic diagrams (Skew-T log-P) 3.9 Thermodynamic diagrams and vertical temperature profiles (cont.) • • Used to examine current conditions (e.g presence of clouds or dry layers) Use to predict cloud: – LCL = level at which a parcel of moist air lifted dryadiabatically would become saturated (prediction of cloud base) – LFC = level at which a parcel of air lifted dry-adiabatically until saturated and saturation-adiabatically thereafter would first become warmer than its surroundings (assessment of cloud vertical development) – EL = height where temperature of a buoyantly rising parcel again equals environmental temperature (estimation of cloud top) [...]... gained or lost by radiation Figure created by Leland McInnes from published EPICA data Fig 3- 10 Net radiation is the end result of the absorption of insolation and the absorption and radiation of longwave radiation The surface has a net radiation surplus of 29 units, while the atmosphere has a deficit of 29 units 3. 3 Energy transfer processes between the surface and the atmosphere (cont.) • Sensible heat:... Aug in NH 3. 8 Measurement of temperature • When a group of molecules (microscopic) move predominantly in the same direction, the motion is called wind (macroscopic) • When they move in random directions, the motion is associated with temperature Æ higher temperatures are associated with greater average molecular speed Celsius Temperature = (oF - 32 ) / 1.8 Fahrenheit Temperature = (1.8 x oC) + 32 Kelvin... of the insolation available at the top of the atmosphere actually reaches the surface, of which another 5 units are reflected back to space The net solar radiation absorbed by the surface is 45 units 3. 3 Energy transfer processes between the surface and the atmosphere • Surface-atmosphere radiation exchange: - Earth’s surface and atmosphere radiate energy almost in the longwave portion of the spectrum... surface receives a portion of this radiation Æ causes surface heating Æ increases in longwave radiation emission from the surface… Æ an infinite cycle of exchange with energy transferring back and forth 3. 3 Energy transfer processes between the surface and the atmosphere (cont.) • Surface-atmosphere radiation exchange (cont.): - Water vapor, CO2, other greenhouse gases are good at absorbing most wavelengths... 3. 1 Atmospheric influences on Insolation (cont.) • Transmission: - When solar radiation travels the vacuum of outer space, there is no modification of its intensity, direction, or wavelength - When it enters the atmosphere, only some of the radiation can pass unobstructed to the surface - There is a reduction in the amount of radiation reaching the surface due to scattering process in cloudy days 3. 2... resulting from a surplus of energy receipt also depends on the mass of a substance - Sensible heat travels by conduction through the laminar boundary layer and is then dispersed upward by convection 3. 3 Energy transfer processes between the surface and the atmosphere (cont.) • Latent heat: - is the energy required to change the phase of a substance - In the case of melting ice, the energy is called... to evaporate some of the water Fig 3- 14 Both the surface and atmosphere lose exactly as much energy as they gain The surface has a surplus of 29 units of net radiation, which is offset by the transfer of sensible and latent heat to the atmosphere The atmosphere offsets its 29 units of radiation deficit by the receipt of sensible and latent heat from the surface 3. 4 The Greenhouse effect • The interactions... emitted by the surface Æ warming the lower atmosphere Æ emits radiation downward 3. 5 Global temperature distributions • One of the most immediate and obvious outcomes of radiation gain or loss is a change in the air temperature • The map depicts differences between mean temperatures in January and July through the use of isotherms 3. 5 Global temperature distributions (cont.) • Temperatures hemispheres tend... altitude above sea level 3. 6 Influences on temperature (cont.) • Atmospheric circulation patterns: - An organized pattern of mean atmospheric pressure and air flow across the globe strongly influences the movement of warm and cold air Æ directly effects on temperature - The large-scale circulation patterns also influence the development of cloud cover Æ an indirect effect on temperature 3. 6 Influences on... the mid-latitudes - Where the water temperatures are high, heat is transferred to the atmosphere and promotes higher air temperatures Fig 3- 16 The circulation of ocean currents Those moving warm water are depicted by red arrows, those moving cold water by blue arrows 3. 6 Influences on temperature (cont.) • Local conditions: - Slope orientation and steepness can influence the temperature characteristics