Moist Processes ENVI1400: Lecture 7

ENVI 1400 : Meteorology and Forecasting2 Water in the Atmosphere Almost all the water in the atmosphere is contained within the troposphere. Most is in the form of water vapour, with some as cloud water or ice. Typical vapour mixing ratios are: ~10 g kg -1 (low troposphere) (can be up to ~20 g kg -1 ) ~1 g kg -1 (mid troposphere)

ENVI 1400 : Meteorology and Forecasting3 METEOSAT Water vapour image : – 1200 UTC

ENVI 1400 : Meteorology and Forecasting4 METEOSAT visible image : – 1200 UTC

ENVI 1400 : Meteorology and Forecasting5 Typical cloud water contents are: cumulus (early stage) : 0.2 – 0.5 g m -3 cumulus (later stage) : 0.5 – 1.0 g m -3 cumulonimbus : 3 g m -3 (>5 g m -3 observed in very strong updrafts) alto-cumulus : 0.2 – 0.5 g m -3 stratocumulus / stratus : 0.1 – 0.5 g m -3

ENVI 1400 : Meteorology and Forecasting6 Sources and Sinks Sources: –Evaporation from surface: requires energy to supply latent heat of evaporation – sunlight, conduction from surface (cools surface). –Evaporation of precipitation falling from above: latent heat supplied by cooling of air Sinks: –Precipitation: rain, snow, hail,… –Condensation at the surface: dew, frost N.B. Most of the water in the atmosphere above a specific location is not from local evaporation, but is advected from somewhere else.

ENVI 1400 : Meteorology and Forecasting7 Buoyancy Effects Water in the atmosphere has important effects on dynamics, primarily convective processes. –Water vapour is less dense than dry air –Latent heat released/absorbed during condensation/evaporation. molecular weight of water = 18 g mol -1 mean molecular weight of dry air ≈ 29 g mol -1  water vapour = 0.62  air  A mixture of humid air is less dense than dry (or less humid) air at the same temperature and pressure

ENVI 1400 : Meteorology and Forecasting8 Latent Heat Latent heat of evaporation of water L v ≈ 2.5 MJ kg -1 large compared with specific heat of dry air C p ≈ 1004 J kg -1 k -1 Evaporation of 1 gram of liquid water (=1 cm 3 ) into 1 cubic metre of air: latent heat used ≈ 2500 J cools air by ≈ 1.9 K. Similarly latent heat is released and air warmed when liquid water condenses out – e.g. as cloud droplets.

ENVI 1400 : Meteorology and Forecasting9 Condensation Conditions Temperature is reduced to below dew point. Two most common mechanisms for cooling are: –Contact cooling : loss of heat to a surface colder than the overlying air, e.g. following advection over a cooler surface, or due to radiative cooling of the surface at night. –Dynamic cooling : adiabatic lifting results in very efficient cooling of the air. (see below)

ENVI 1400 : Meteorology and Forecasting10 Adiabatic lifting may occur on many scales: –Large­scale ascent along a warm or cold front (100s of kilometers) –The rise of individual convective plumes to form cumulus clouds (~100m to ~1km) –Forced ascent over topographic features (hills, mountains) to form orographic cloud (~1km to >10s km). –Gravity waves above, and downwind of mountains (few km). Radiative cooling (non-adiabatic process) Direct radiative cooling of the air takes place, but is a very slow process. Once cloud has formed, radiative cooling of the cloud droplets (and cooling of surrounding air by conduction of heat to drops) is much more efficient. Radiative cooling  reduced saturation vapour pressure  more condensation  higher cloud water content.

ENVI 1400 : Meteorology and Forecasting11 Addition of water vapour, at constant temperature, raising humidity to saturation point. –Will occur over any water surface. Since temperature decreases with altitude, evaporation into unsaturated surface layer can result in saturation of the air in the upper boundary layer. –Cold air moving over warmer water can sometimes produce ‘steam fog’ : common in the arctic, and observed over rivers and streams on cold mornings.

ENVI 1400 : Meteorology and Forecasting12 Mixing of two unsaturated air masses as different temperatures such that final humidity is above saturation point The Temperature and vapour pressure resulting from mixing is are averages of the initial values in proportion to masses of each being mixed e.g. T mix = T 1 *M 1 + T 2 *M 2 M 1 +M 2 T1T1 T mix T2T2

ENVI 1400 : Meteorology and Forecasting13 Adiabatic Lifting As a parcel of air is lifted, the pressure decreases & the parcel expands and cools at the dry adiabatic lapse rate. As the parcel cools, the saturation mixing ratio decreases; when it equals the actual water vapour mixing ratio the parcel becomes saturated and condensation can occur. The level at which saturation occurs is called the lifting condensation level. Lifting condensation level Saturation mixing ratio equal to actual water vapour mixing ratio of parcel Dew point at surface

ENVI 1400 : Meteorology and Forecasting14 If the parcel continues to rise, it will cool further; the saturation mixing ratio decreases, and more water condenses out. Condensation releases latent heat; this offsets some of the cooling due to lifting so that the saturated air parcel cools at a lower rate than dry air. The saturated (or wet) adiabatic lapse rate is NOT constant, but depends upon both the temperature and pressure.

ENVI 1400 : Meteorology and Forecasting15 The high the air temperature, the greater the saturation mixing ratio, and the more water vapour can be held in a parcel of air. Because the gradient of the saturation vapour pressure with temperature increases with temperature, a given decrease in temperature below the dew point will result in more water condensing out at higher temperatures than at low, and hence more latent heat is released. Thus the wet adiabatic lapse rate decreases as the temperature increases. TT Q1Q1 TT Q2Q2

ENVI 1400 : Meteorology and Forecasting16 The Föhn Effect 0 m 100 m 200 m 300 m 400 m 500 m Lifting condensation level Unsaturated air cooling at -0.98°C per 100m Saturated air cooling at -0.5°C per 100m 10°C Unsaturated air warming at +0.98°C per 100m 9.02°C 8.04°C 7.06°C 6.08°C 5.58°C 5.08°C 6.54°C 7.52°C 8.50°C 9.48°C 10.46°C 11.44°C The different lapse rates of unsaturated and saturated air mean that air flowing down the lee side of a mountain range is frequently warmer than the air on the upwind side. In the Alps this warm dry wind is called the Föhn, in American Rockies it is known as a Chinook. The onset of such winds can result in very rapid temperature rises (22°C in 5 minutes has been recorded) and is associated with rapid melting of snow, and avalanche conditions. 4.58°C 5.56°C