Thermal Processes SOEE 1400 : Lecture 10 SOEE1400 : Meteorology and Forecasting2 Radiation Processes Incoming solar radiation 342 W m 2 Reflected by clouds, aerosol & atmosphere Reflected by surface Absorbed by surface Absorbed by atmosphere 67 thermals 24 Evapo- transpiration Surface radiation Absorbed by surface reflected solar radiation 107 W m 2 back radiation emitted by atmosphere 165 Outgoing longwave radiation 235 W m 2 SOEE1400 : Meteorology and Forecasting3 Adiabatic Processes An adiabatic process is one in which no energy enters or leaves the system. Many atmospheric processes are adiabatic (or nearly so) particularly those involving the vertical movement of air. Air is a poor thermal conductor, and mixing is often slow enough for a body of air to retain its identity distinct from the surrounding air during ascent. Near-surface processes are frequently non-adiabatic. Adiabatic Processes: Ascent of dry convective plumes Large scale lifting/subsidence Non-Adiabatic Processes: Radiative heating/cooling Surface heating/cooling Loss of water through precipitation Addition of water from evaporation of precipitation falling from above Condensation/evaporation within an airmass is known as pseudoadiabatic SOEE1400 : Meteorology and Forecasting4 Lapse Rate Lapse Rate is the term given to the vertical gradient of temperature. The fall in temperature with altitude of dry air that results from the decrease in pressure is called the Dry Adiabatic Lapse Rate = -9.8C/km. 1km 9.8C Temperature Altitude Dry Adiabatic Lapse Rate SOEE1400 : Meteorology and Forecasting5 Condensation releases latent heat, thus saturated air cools less with altitude than dry air. There is no single value for the saturated adiabatic lapse rate. It increases as temperature decreases, from as low as 4C/km for very warm, tropical air, up to 9C/km at -40C. Temperature Altitude Saturated Adiabatic Lapse Rate Dry Adiabatic Lapse Rate SOEE1400 : Meteorology and Forecasting6 Pressure & Temperature A column of air has pressure levels P 1, P 2, etc. If the column is warmed, the air will expand and its density at any given level will decrease. The vertical interval between pressure levels increases, so that at any given altitude the pressure in the warmer column is greater than in the cooler. N.B. since the total mass of air in the column is constant, the pressure at the surface does not change P0P0 P1P1 P2P2 P3P3 P4P4 P5P5 z cool P0P0 P1P1 P2P2 P3P3 P4P4 P5P5 warm Thickness The thickness is the vertical spacing between two pressure levels (usually 1000 hPa and 500 hPa) It can be shown that the thickness is directly proportional to the mean temperature in the layer: z = R / g SOEE1400 : Meteorology and Forecasting7 8 H L coolwarm cold-core High weakens with height, may form a low aloft H H Warm-core High intensifies with height cool warm L L Cold-core Low intensifies with height coolwarm L H cool warm Warm-core Low weakens with height, may form a high aloft SOEE1400 : Meteorology and Forecasting9 Mid-latitude low-pressure cells have colder air to the rear. As a result, the axis of the low slopes towards the colder air L Sea-level isobars 500 mb contours Cold low Warm high SOEE1400 : Meteorology and Forecasting10 High pressure cells slope towards the warmest air aloft. The centre of the cell at 3000m may be displaced 10-15 towards the equator. Sea-level isobars 500 mb contours Warm high H Cold low SOEE1400 : Meteorology and Forecasting11 The Thermal Low (or heat low) Thermal lows result from the strong contrast in surface heating between land and sea Land heats up (solar radiation) and cools down (infra-red radiation) much more rapidly than ocean large diurnal cycle cross-coast temperature gradient N.B. A thermal low results from fine, clear, warm weather, and thus differs from the depressions associated with cloud and bad weather. SOEE1400 : Meteorology and Forecasting12 1. Start with a horizontally uniform pressure distribution. Solar radiation starts to warm land. Air near surface is warmed by land, convection mixes warm air upwards and whole boundary layer warms. 2. Air over land warms and expands. Cant expand sideways, so column expand upwards produces high pressure aloft. Thickness has increased with temperature. N.B. Surface pressure remains constant at this stage. warm cool H SOEE1400 : Meteorology and Forecasting13 3. Horizontal pressure gradient aloft drives a flow from over land to over ocean. warm cool H 4. Mass of air in column over land is reduced surface pressure falls to produce a surface low. High pressure aloft weakens, but is maintained by continued heating at surface. Surface pressure gradient drives flow from sea to land: the sea breeze. warm cool H L SOEE1400 : Meteorology and Forecasting14 Note that steps 1-4 happen almost simultaneously, and continuously, throughout the day. As the air warms, the high pressure aloft is continually removing mass from the warm column (and it is gradually replaced by air drawn into the low pressure at low levels). SOEE1400 : Meteorology and Forecasting15 H L 5. When solar heating stops, pressure driven flows act to equalize pressure, restoring conditions to the initial uniform pressure field. If land cools sufficiently at night, the reverse situation can be established. Over large land masses there may be insufficient time over night for the sea breeze to reach regions far from the coast, and a weak surface low is maintained over night. This then deepens during the following days, and a heat low may be maintained for days or weeks, until synoptic conditions change. warmcool H L warm SOEE1400 : Meteorology and Forecasting16 Sea Breeze Formation of local thermal low over land, results in the formation of a sea-breeze In-flowing cool air from the sea forms a sea-breeze front a miniature cold front Air ahead of the front is forced upward, contributing to the formation of cumulus mb 975 mb 950 mb 25 C15 C Sea breeze dynamics By day: The air accelerates down the pressure gradient Coriolis force turns it right (N. Hemisphere), over a period of hours If the weather is warm, convective turbulence imposes a strong drag force on the air The sea breeze front may be quite diffuse, due to turbulent mixing. At sunset Convective turbulence becomes weak: drag force reduces rapidly The sea breeze accelerates, and moves more rapidly inland ( km) A strong front may develop. SOEE1400 : Meteorology and Forecasting17 SOEE1400 : Meteorology and Forecasting18 Pressure as an indicator of temperature Because the depth of a layer of air increases as its temperature increases, we can use the thickness as an indicator of the mean temperature of the layer. Charts are usually produced of the thickness between 1000 and 500 mb. The thickness is usually quoted in deca-metres (10s of metres) A useful rule of thumb is that for mb layer depths less than 528 dm (5280 m) any precipitation will fall as snow rather than rain. SOEE1400 : Meteorology and Forecasting SLP (mb) & thickness : 48hr forecast valid SOEE1400 : Meteorology and Forecasting20 SOEE1400 : Meteorology and Forecasting SLP (mb) & thickness (dm) : 36hr forecast valid SOEE1400 : Meteorology and Forecasting SLP (mb) & thickness (dm) : analysis valid SOEE1400 : Meteorology and Forecasting23 12C 2C mb Temperature (2C contours), RH (%), wind (m s -1 ) : analysis valid SOEE1400 : Meteorology and Forecasting Surface temperature (2C contours) and SLP (mb)(5mb contours) : analysis valid SOEE1400 : Meteorology and Forecasting25 The Thermal Wind It is commonly observed that clouds at different altitudes move in different directions winds are in different directions. The gradient of wind velocity (speed & direction) is called the (vertical) wind shear. In the free air, away from surface (where friction effects complicate matters), the wind shear depends upon the temperature structure of the air. The thermal wind is a theoretical wind component equal to the difference between the actual wind at two different altitudes. Any two levels can be used, but unless otherwise stated the altitudes of the 1000mb and 500mb levels are usually used. Note that the 1000mb level might be below sea level, and is usually within the boundary layer and thus influenced by friction effects at the surface. SOEE1400 : Meteorology and Forecasting mb HIGH LOW V g(1000) warm cold 500mb LOW HIGH SOEE1400 : Meteorology and Forecasting HIGH LOW V G500 VTVT LOW HIGH V G mb thickness Contours of 1000 mb surface Contours of 500 mb surface SOEE1400 : Meteorology and Forecasting28 Note that cold air is to the left of the thermal wind vector (looking along wind) in the northern hemisphere, to the right in the southern hemisphere. The decrease in temperature towards the poles results in a westerly thermal wind in the upper atmosphere in both hemispheres. The largest meridional temperature gradient occurs in mid-latitudes across the polar front. The thermal wind makes up a significant component of the jet-stream, located over the upper part of the polar front.