WMOD00418 - Laserfiche WebLink

Home Browse Search

1060 a MONTHLY WEATHER REVIEW VOLUME 108 10 FINAL TO INITIAL DEW POINT b J N IT I AL TEMPERATURE LEGEND 10 /:/.. /,\' ,! :.. . :: .. \ 20 ~o ~,}~~~;.. 'X" 290 ". ~ ~ , . . :.... . .' 30 " .' _I, -/-'" :.~. 't . " 40 V . : / 50 Z' 60 :~ ~" 85 100 ~o FINAL TEMPERATURE 20 a: "- 3D ~ w D:: ~ lI'l 40 lI'l W D:: "- 50 60 . ", .. ! . ~ . ,.? - -..- '. .:1: " <( 0:... III , ". ~.\ i..: .~ I ... L&....:: "t. ~ .1 .."'1 -- . . /~ .:t. ~', . .",. I' :o' ~o I , FINAL SOUNDING AT 15 HR 0 MIN TEMPERATURE leI IJ -50 -40 'IXj1 \. -30 , '. FINAL SOUNDTNG AT 15 HR 0 MIN TEMPERATURE ICI FIG. 4. (a) Thermodynamic changes due to lifting and cloud-environment interaction for 4 h simulation from llO0 to 1500 LST on 30 June 1977 at Goodland, Kansas. The initial and final temperature and dew point profiles and corresponding changes (shading) are shown on a skew T thermodynamic diagram. Mesoscale lifting of 20 cm s-' was applied from 1200 to 1500 LST. (b) As in Fig. 4a for the 24 August 1977 Goodland, Kansas case of intense mesoscale triggering, Note the relatively high instability from the surface to 50 kPa. tion regarding the potential for mesoscale triggered convective cloud development. Note that the two case study examples differ from the statistical analy- sis cases in that 4 h of development were simulated rather than the 3 h used in the 232 cases. The 4 h period was used in the case study to provide a clear example of differences between the two types of days. CASE 1 Clear skies in a ridge behind a cold front pre- vailed at GLD on 30 June 1977 as seen in Figs. 3a and 3b. The model simulations using this day's sounding (Fig. 4a) were unable to produce any sig- nificant convection even with 20 cm s-1Iifting. Three high-based cumulus congestus clouds ~2 km deep were produced (Fig. Sa) after 3 h of 20 cm S-1 lift- ing (CPI = 7.9 km), and no clouds were diagnosed in the no-lifting and 10 cm S-1 lifting simulations. The simulation with 20 cm S-1 lifting showed that, although the strong subsidence inversion was sharply eroded by 3 h of lifting (Fig. 4a), no sig- nificant clouds could develop in this suppressed environment. Note the lifting and weakening of the stable layer from 60-70 kPa to the 50-60 kPa level between the initial and final times. Note also the increase in the PBL dew point and advection of moisture upward from the moist layer between 50 and 60 kPa. CASE 2 On 24 August 1917, strong convective cloud de- velopment in the form of a mesoscale line was ob- served in satellite imagery at 2030 GMT, 150 km west ofGLD (Figs. 3c and 3d). This line later moved through western Kansas as a squall line producing heavy precipitation. The sequence of development simulated by the model in this case showed that no deep convection was possible without mesoscale lifting. After 1 h of 20 cm S-1 lifting, deep moist convection with cloud depths of 12-13 km was main- tained (Fig. 5b) yielding a CPI of 76.4 km. When no lifting was applied the CPI of this case was zero, whereas 10 cm s-1lifting resulted in a CPI of 49.4 km. It can be seen from Figs. 4 and 4b that the sound- ing on 24 August is much less stable than that on 30 June, and the moisture content is much higher. Care- ful examination of Fig. 4b shows that lifting causes complex changes in environmental temperature and moisture and that cloud-environment interaction further alters the resulting thermodynamic struc- ture. Strong cooling in excess of 3-50C occurred between 70 and 50 kPa where very stable lapse rates existed. Note the sUIface warming in the PBL between 87 and 80 kPa and the adiabatic cooling of the mid-tropospheric levels from the initial to final times which significantly decrease static stability and increase the positive area in this region, Up- ward moisture advection due to lifting also pro- "1 il