Synoptic Meteorology Laboratory
Synoptic Meteorology Laboratory METR 4424
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Mr. Marilie Kreiger
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This 6 page Class Notes was uploaded by Mr. Marilie Kreiger on Sunday October 25, 2015. The Class Notes belongs to METR 4424 at University of Oklahoma taught by Staff in Fall. Since its upload, it has received 18 views. For similar materials see /class/229234/metr-4424-university-of-oklahoma in Meteorology at University of Oklahoma.
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Date Created: 10/25/15
The material reproduced in this multipart PDF series was written by Charles Doswell Lee Anderson and David Imy in 1991 It is reproduced as a convenience to students in The School of Meteorology at the University of Oklahoma For Educational Use Only REVERSIBLE MOIST ADIABATIC PROCESSES The opposite case of a pseudoadiabatic process is known as a reversible moist adiabatic process In this case any condensed water vapor remains with the parcel Therefore as the parcel begins to descend the liquid water condensed during its ascent revaporizes absorbing the latent heat of vaporization This partially offsets the heat of compression just as the release of latent heat partially offsets the parcel cooling by expansion during ascent The result is that the parcel if returned to its starting point has the same temperature it had when it started the process simply was reversed It turns out that the moist adiabats for a reversible adiabatic process are not quite the same as those for a pseudoadiabatic process There are two ways in which the reversibility affects the lapse rate First if the condensed water vapor falls out during pseudoadiabatic ascent it takes a small amount of the parcel mass with it This loss changes the parcel density mass per unit volume which affects the parcel s lapse rate Second there is a small change in CF for the parcel because of the presence of liquid water which has a diiTerent speci c heat than water vapor However these effects are very small amounting only to a few percent at most We can ignore the difference for most problems therefore and assume the moist adiabats for either process are the same The moist adiabats on most thermodynamic diagrams including the SkewT and the Stiive diagram are pseudo moist adiabats not reversible moist adiabats Nevertheless it is worth making this distinction for some technical reasons In the tropics the typical atmosphere is slightly unstable with respect to the pseudoadiabatic lapse rate but is nearly neutral with respect to the reversible moist adiabatic lapse rate Asubtle distinction for midlatitude continental systems becomes important in tropical oceanic systems eg tropical cyclones Additionally sometimes parcels rise and fall through air filled with liquid water If the water is in the form of small drops or cloud droplets it evaporates readily during descent so a parcel sinking in cloudy or rainy air may sink at a lapse rate which is less than dry adiabatic and may even approach moist adiabatic Of course in such cases the evaporating water is probably not the water vapor originally contained in the parcel so such processes are diabatic overall The material reproduced in this multipart PDF series was written by Charles Doswell Lee Anderson and David Imy in 1991 It is reproduced as a convenience to students in The School of Meteorology at the University of Oklahoma For Educational Use Only CONDITIONAL AND CONVECTIVE INSTABILITY In the determination of instabilities associated with moist air on the sounding curve the slope of the temperature curve should be compared with the slopes of the dry and moist adiabats As we have noted earlier for dry convection the relevant lapse rate is the dry adiabatic rate However if you lift a saturated parcel it rises along a moist adiabat which makes the parcel lapse rate less than that of the dry adiabat For environmental lapse rates less than moist adiabatic it makes no difference whether the rising parcel is dry or saturated it will always be cooler than its environment as it rises so that environment is said to be absolutely stable A superadiabatic lapse rate is absolutely unstable because an ascending parcel will always be warmer than its environment and will accelerate up ward If the environmental lapse rate is between the moist and dry adiabatic lapse rates that envi 4oo I 500 39 z 600 13539cl LCL 700 Td1oc C 39r4c 750 layer lapse rate after lilting lg Td839Cl Iii0390 4900 V 1000 7ampTOP2844K239 K9 BA E2862 K 20 10 o 10 20 30 Figure 19 Illustration of convective instability 850 ronment is said to be conditionally unstable If the parcel rises dry unsaturated the parcel will be stable if it rises saturated it will be unstable Thus the condition is whether the parcel is satu rated or unsaturated during ascent Another common term is convective or potentia instability Convective instability is said to exist when 9w or Be decreases with height Consider the 900 to 750 mb layer in Fig 19 The 9w values at the base 900 mb and top 750 mb ofthe layer are 2862 0K and 2844 0K respectively so this layer is characterized by convective instability Initially this layer is absolutely stable since its lapse rate is less than the moist adiabatic lapse rate see Fig 19 If the layer as a whole is lifted say 150 mb a remarkable thing happens During that lifting process the parcels in the lower part of the layer reach saturation before the upper layers Therefore they begin to cool at a lower rate namely the moist adiabatic rate than the parcels above them in the layer which continue to cool dry adiabatically This means that the lapse rate of the layer increases and increased instability is attained through this process Is this a realistic description in a convective situation No In most convective situations parcels rising out of the boundary layer initiate cloudiness and rise quickly through their LFCs forming deep convection We do not see whole layers rising at once and reaching saturation at their bases first except perhaps for certain special cases where the layer rises over frontal zones in extratropi cal cyclones Thus the physical process that convective instability is supposed to represent is not significant in most cases of real convection However the presence of convective instability is mostly dependent on the presence of dry air over moist because 9w or Be is more sensitive to water vapor content than to temperature The presence of dry air aloft helps to enhance downdrafts Therefore convective instability is a relevant issue in the diagnosis of severe weather potential in convective situations EXERCISES Exercise 22 Assume the dew point at 750 mb was 0 0C in Fig 19 Lift the layer 150 mb and compute the new SW at the bottom and top of the layer What is the condition of the atmosphere in this layer Why Exercise 23 a Using the data below a SkewT and Fig 20 determine Se and SW for the selected pressure levels below b Plot the variables for those selected pressure levels on the graph provided Fig 21 and answer the questions L is 25 x 105 J kg391 and cp is 1005 J 0K391 kg39l Radiosonde observation data for Exercise 23 from 0000 UTC P mb TOC Ta DC at 0w 955 277 187 850 190 177 825 180 150 800 181 80 744 133 83 709 116 10 700 107 16 600 00 39 540 63 72 531 67 113 500 105 150 469 145 161 435 173 201 400 200 308 359 250 387 325 306 367 300 350 417 c What layer between any two adjacent pressure levels eg 400 mb to 300 mb possesses the greatest convective instability Why d Use the graph you plotted and the sounding below Fig 20 What layer between any two adjacent pressure levels does not exhibit convective instability Why x T 400 500 700 850 1000 20 10 O 10 20 30 40 Figure 20 SkewT for exercise 23 VERTICAL PROFILE OF 9 AND GW 200 300 400 500 600 700 PRESSURE mb 800 900 1000 315 320 325 330 335 340 39345 350 355 360 9 AND 9 deg K Figure 21 Chart used to complete exercise 23 Plot of 06 and GW versus pressure
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