Atmospheric stability

Transcription

1 Atmosheric stability The stability or instability of the atmoshere (or a layer thereof) is the state of the atmoshere with resect to the reaction of a volume or arcel of air to a vertical islacement. The stability of the atmoshere etermines the likelihoo of convective activity, clou tye (stratus or cumulus), likelihoo of atmosheric turbulence, the extent of mixing (ollutants etc.). Stability is classifie as: stable neutral unstable, or conitionally unstable (This latter classification eening on whether the air is saturate with moisture or not) To etermine stability classification we must examine the tenency of the atmoshere to resist or enhance an initial islacement. For examle, consier a ball bearing on three ifferent surfaces: Neutral Stable Unstable islacement no tenency to continue or to return islacement tenency The same thing haens in the atmoshere when we islace a arcel or bubble of air in the vertical irection. We can use a thermoynamic iagram to examine this. 1

2 When (environmental lase rate) z tenency T (ry aiabatic lase rate) Consier a arcel at a given ressure level. Imose a islacement in the vertical irection (uwar or ownwar). A islace arcel will change temerature (if aiabatic) at a rate governe by the Poisson equation (1 st law of thermoynamics) or at the aiabatic lase rate. The examle above is of an atmoshere that is stable (at least to a ry rocess with no conensation) because a arcel of air islace uwars (ownwars) will cool (warm) at the ry aiabatic lase rate, become cooler (warmer) than its surrounings an therefore enser (lighter) an will ten to return to its original osition. When, calle a sueraiabatic lase rate, the islace arcel wants to kee moving in the irection of the islacement. This is an unstable layer of atmoshere. z 2 tenency T

3 Layers of atmoshere can be classifie accoring to their stability or instability. 700 ry aiabats souning stable inversion (stable) ressure log (1000/) 800 isotherm stable 900 inversion (stable) neutral Temerature sueraiabatic (unstable) Note: some of these layers may be conitionally stable eening on whether the air is saturate with moisture. Moist aiabatic rocesses (saturation aiabats) So far we have iscusse only ry rocesses in which neither conensation nor evaoration of existing water rolets in a clou is occurring. But conensation releases large amounts of heat to the atmoshere an evaoration consumes large amounts of heat. The latent heat of evaoration (or conensation), L= J/kg. Aroximately six times as much heat is neee to evaorate 1 kg of water as is neee to raise its temerature from 0 o C to 100 o C, the boiling oint. If, uring a lifting rocess, aiabatic cooling is sufficient to bring the air to saturation an, if lifting continues, the air will continue to cool but the release of latent heat will offset the temerature ecrease to some extent. Thus, uring saturate ascent, the rate of ecrease of temerature of a arcel of air will be less than uring a ry aiabatic rocess. The ifference in lase rates between ry an saturate rocesses will een on the amount of water vaor 3

4 available for conensation. Thus, near the earth s surface (high ressure an air ensity) in warm climates, the saturation aiabatic lase rate s will be consierably less than (say, 5 o C/km comare with 9.8 o C/km). On the other han, high in the atmoshere an at low temeratures where there is little moisture content, the saturation aiabatic lase rate will aroach. Thermoynamic iagrams such as the skew T-log iagram inclue a series of lines reresenting saturation aiabatic rocesses (saturation aiabats). Check the Skew T-log iagrams for s lines Conitional instability Since s <, our analysis of the stability of a layer of atmoshere will een on whether or not the air is saturate (whether or not clou in resent) because, uner certain conitions, oosite conclusions woul be rawn with regar to stability or instability. Suose the observe lase rate is intermeiate between an s. s B C A T 4

5 Here, the souning is shown by the soli line. If the layer is ry an no conensation occurs, as before, lifting a arcel from oint A will result in ry aiabatic cooling to oint B. At this oint, the air is coler than its surrounings (given by the souning temerature at the same ressure level), enser, an woul want to sink back to its starting oint. Our conclusion woul be that the layer of atmoshere is stable. If the layer is saturate an lifting causes conensation, the arcel will follow the moist aiabatic to oint C an will be warmer, an less ense, than the surrouning air. It will want to continue in free buoyant ascent. Thu, is moist, the layer is unstable. This situation, in which the nature of the stability or instability is eenent uon the status of the moisture in the atmoshere (unsaturate or saturate), is calle conitional instability. In general the trooshere is rather stable to ry rocesses (excet in the lower when the sun heats the surface) an woul be generally inactive excet for the instability cause by the release of latent heat in clous. In general we can classify a layer of atmoshere accoring to the relationshi between the observe lase rate an the two rocess lase rate an s. Thus: > > unstable (absolutely) > s conitionally unstable < s stable (absolutely) If < 0, i.e., temerature increases with height, this is calle an inversion an is an extreme vase of stability. Pollution eisoes are commonly associate with inversions, or at least, inversions aloft which tra ollutants in the lowest layers of the atmoshere. The lifting conensation level (LCL) A arcel of air force to lift in the atmoshere will ultimately cool to the saturation oint. During the initial unsaturate art of the ascent no moisture is 5

6 conensing out an the mixing ratio (w) of the air remains constant (the amount of vaor in grams remains in constant roortion to the amount of ry air in kilograms). On the other han, other moisture variables such as ew oints T, relative humiity, an vaor ressure all change. For this reason, thermoynamics iagrams inclue a fifth set of lines reresenting constant mixing ratio. s w (mixing ratio) T T T (skewe) Knowing T an (int T on the iagram) the mixing ratio lines can be interolate to give the saturation mixing ratio - the amount of vaor the air can hol in g/kg at saturation. Knowing T an (oint T on the iagram) the mixing ratio lines can be interolate to give at the actual mixing ratio the amount of vaor actually existing in the air at that ressure level. Because mixing ratio remains constant, we can observe the change in ew oint uring a lifting rocess, the increase in relative humiity, an the aroach to saturation. On ascent T changes at the rate an T changes at a rate etermine by w = constant. This rocess scan be examine grahically: 6

7 s T T lifting conensation level (LCL) T (skewe) The lifting conensation level is the level at which T reaches T (which itself has ecreases but at a lesser rate). The air is now saturate an continue lifting causes conensation in the form of a clou. Above the LCL, a arcel of air will follow the saturation aiabat. The convective conensation level (CCL) The convective conensation level is obtaine when the surface is heate (e.g. solar heating uring the ay) an a convectively mixe layer (of constant an w) is create until saturation is achieve at the to. This assumes sufficient moisture in the air. In California summers, the air is too ry an the Pacific high ressure makes the air too stable for sufficient eveloment of a ee clou convective layer. Again, it is best to reresent this rocess grahically: 7

8 T s w convective conensation level (CCL) T (skewe) The soli line (labele ) is the original souning, say, at sunrise. Solar heating rogressively heats the surface an warms the lowest layer of air. The convective mixing generates a layer of constant otential temerature an constant mixing ratio until, eventually, the air is saturate at the to. If the layer above the CCL is conitionally unstable, the cumulus clou coul evelo substantially. Stability inices As a simle guie to the likelihoo of convective activity, a number of inices have been eveloe to inicate the general level of instability in the atmoshere. The inex use most commonly is the Lifte Inex (LI). To calculate the Lifte Inex: 1) Determine the mean mixing ratio an otential temerature in the lowest 1 km of the atmoshere. 2) Fin the LCL for this mean w an. 3) Follow a saturation aiabat from this level to 500 mb an fin T 500 ' of the lifte arcel. 8

9 4) Subtract T 500 ' from the observe 500 mb temerature T 500 to obtain As a rule-of-thumb: LI = T T 500 ' (T souning T arcel ) 500mb LI > 2 no activity execte 0 < LI < 2 RW robable, isolate T ossible -2 < LI < 0 T robable -4 < LI < -2 severe T ossible LI < -4 severe T robable, tornaoes ossible However, there is some geograhic variability. For examle, in the mountainous west, orograhic lifting can rouce thunerstorms at larger(move ositive) value of LI. In aition, a triggering mechanism is neee to initiate convective activity (frontal lifting, surface heating, orograhic lifting etc.), an there nees to be sufficient water vaor in the atmoshere. 9

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