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U.S. DEPARTMENT of COMMERCE ENVIRONMENTAL SCIENCE SERVICES ADMINISTRATION RESEARCH LABORATORIES ESSA Research Laboratories Techn .. U.S. DEPARTMENT OF COMMERCE ENVIRONMENTAL SCIENCE SERVICES ADMINISTRATION RESEARCH LABORATORIES ESSA Research Laboratories Technical Memorandum-NSSL 39 THUNDERSTORM-ENVIRONMENT INTERACTIONS REVEALED BY CHAFF TRAJECTORIES IN THE MID-TROPOSPHERE James C. Fankhauser Nat-ional Center for Atmospheric Research Boulder, Colorado 80302 NATIONAL SEVERE STORMS LABORATORY TECHNICAL MEMORANDUM NO. 39 NORMAN, OKLAHOMA 73069 JUNE 1968 TABLE OF CONTENTS Page ABSTRACT iv l. INTRODUCTION 1 2. STORM EVOLUTION AND ATTENDING ATMOSPHERIC 1 CONDITIONS 3. EXPERIMENTAL AND ANALYTICAL PROCEDURES 5 4. DISCUSSION OF RELATIVE AIRFLOW 8 5. SUMMARY AND CONCLUSIONS l2 6. ACKNONLEDGMENTS l3 7. REFERENCES l3 iii ABSTRACT Some physical convective storm models suggest that large thunderstorms create effective barriers to the environmental airflow through the process of vertical momentum transport in their vigorous updrafts and downdrafts. Hydrodynamically, the relative air motion near a thunderstorm has been considered analogous to potential flow around a solid cylinder. Apprais­ als of thunderstorm water and energy budgets demonstrate, on the other hand, that potentially cold dry air must enter the storm circulation at some midcloud level aloft, in order to supply air with properties observed in the downdraft region beneath the storm. Both of these opposed conditions are supported by analy­ sis of the trajectories of chaff deposited at midcloud level on the upwind and downwind sides of a mature thunderstorm. Individual chaff targets, initially located on the upwind flanks of the storm, deviate around and accelerate relative to the ra~ar precipitation echo; however, a chaff target re­ leased immediately upstream eventually becomes absorbed by the storm echo. Circulations derived from the chaff motion are compared with independent wind measurements obtained from an airborne Doppler system and rawinsondes, and to flow characteristics demonstrated by classical hydrodynamical experiments. iv THUNDERSTORM-ENVIRONMENT INTERACTIONS REVEALED BY CHAFF TRAJECTORIES IN THE MID-TROPOSPHERE James C. Fankhauser l. INTRODUCTION A vital tenet in the development of the dynamical thunderstorm model presented by Newton and Newton (1959) is that vigorous vertical drafts within large cumulonimbus clouds enable persistent storms to behave as effective barriers to the environmental airflow. The model proposes that resistance to shearing tendencies aloft is provided by vertical transport of conserved horizontal momentum in rising or fall­ ing air parcels. Numerous recent endeavors to explain anomalous thun­ derstorm motion (Fujita, 1965; Goldman, 1966; Harrold, 1966; Fujita and Grandoso, 1966; Fankhauser, 1967) have followed Byers' (1942) sug­ gestion that the relationship of a thunderstorm's circulation to the current in which it is embedded may be analogous to hydrodynamic theory pertaining to the behavior of rotating solid cylinders in potential stream flow. Normand's (1946) important recognition that potentially cool air descending in downdrafts must, according to its conserved properties, enter the cloud circulation at some level aloft stands' in contrast to the concept of a totally rigid cloud column. His premise is substan­ tiated by Brownings's (1964) analysis of airflow near and within severe thunderstorms and by Newton's (1966) convective storm mass and water budget assessments. Comprehensive data collected in Oklahoma by the National Severe Storms Laboratory on May 28, 1967, provide a basis for examining each of the above hypotheses. A definition of the airflow around an iso­ lated thunderstorm is obtained from the motion of chaff targets, which were systematically released from an aircraft near the 500-mb level on the upwind and downwind sides of the storm. Chaff and precipitation echoes were monitored by RHI and PPI radars operating in L,S, C, and X band ranges. Circulations derived from chaff winds are compared with kinemat­ ical and thermodynamical properties based on data provided by a mesonetwork or rawinsondes and an airborne Doppler navigational system. Similarities between the observed flow and that demonstrated by classi­ cal hydrodynamical experiments are also discussed. 2. STORM EVOLUTION AND ATTENDING ATMOSPHERIC CONDITIONS The dynamic and thermodynamic characteristics of the atmosphere over Oklahoma early on May 28, 1967, were only marginally conducive to thunderstorm formation. Stability indexes on morning soundings were slightly negative, and, at the synoptic scale, differential advection indicated little chance for Significant destabilization. 1 0 .'20 3 kml I; j I 10 ft 12 /' 40 I .: ' 10/ I ! 30 ;t3 6 20 4 10 (a) ? 2680 1. ~ 0 0 (b) km 16 12 8 4 (c) Figure 1. Radar echo configurations, May 28, ]"967. (a) NSSL, vlSR-57, PPl at 1545 CST. Intensity contours atl0-dB inter­ vals. (b) NSSL, MPS-4, RIll at 1544 CST. (c) RFF, RDR-1D, RIll at 1548 CST; echo cancellation at 16-dB attenuation. 2 A stationary front lying .a few miles north and west of the NSSL network sagged southward during the late morning, however, providing sufficient low-level convergence to set off weak convective storms over the northwest portion of the observing fCiCility by midday. Events at low levels were simultaneously enhanced by daytime heating and the passage of a strongly diffluent upper short-wave trough during the early afternoon hours. As a result a few moderate thW').derstorms de­ veloped between 1200 and 1500 CST, one of which is the subject of this study. Figure la shows the PPI radar configuration of the storm specimen towards the end of its mature stage and figures lband lc are coinci­ dent RHI profiles recorded by the NSSL MPS-4 and REF airborne RDR l-D radars. The first echo appeared on the NSSL WSR-57 shortly after 1400 CST, and evolution through maturity and complete dissipation .had oc­ curred by 1630 CST. Radar reflectivity at maximum intensity approached 10~6m-3 for short intervals between 1500 and 1530 CST, and during this period the echo height conSistently exceeded 45,000 ft above ground. DiSSipation proceeded slowly from 1540 through 1620 CST as the echo top receded at an average rate of about 2 m sec-I • . .... KM 14-.~ __~~~~~~~~-/ 150 M B /'--~~~;::"-=-":="-----,I Figure 2. Three­ 12 dimensional representa­ tion of horizontal wind at 1530 CST, May 28, I 0 ~'-::::::::::;;'::::::::;"'::::::::::;;'::::::::;;:::::::::":::::-=I==-_-/ 1967, over south­ 300 MB .J..--_-':_-':;_===-;"::::::::-:::::~::::-::::;;::::::;:::::--:::?:::;...-::?~O:::::: . ::::.:::;r central Oklahoma; -'--~;;;:$=-:-----------~ . interpolated from ~---*--~~~~------~ 8 fo----+--.=----~--- ---~ serial rawinsonde ob­ .r...---:;:::.-r------~----- servations obtained _---=-------=::1------­.--=-----+-------­ at locations designated ----~.----- as dots (10 m sec-l 6~~~~-=-=-=-=~~--/ = 500MB-------1--- A---~~~~~~~~~ 20 kIn). 4 800 MB ,,/ " • FS/ LTS ..,," ." MSL~.,,_.,,~~S~p_S£-~~~.~~-',~ o 40 80 120 160 200KM 3 200 km +.25 180 +: ~/ + I:.fl 160 + TIK• -h-',+ I I NSSL , 140 ... I Norman: + + 120 I 99° , .SI7° +55 \ +55° , ,'"\ + "-=t-+.25 (a) 100 ;' +.25 i + j 80 + I I + +ePVY 0 I I \ I 60 + +,,--... _---'- ........... ,+ "t-..... "/ + I I + + ~I 40 " +/+ + + \ / 20 ~ + + " I I + +.25 +.'25 0 O~ __~ __~s'Prs~--~--~--~-:~--~--~~~ o km20 40 60 80 100 120 140 160 180 200 200r--.r--.r-~~~---.---r~-r---r--~--~ km W (fLb sec-I) 0 -5 -~5 180 5/28/67 h 1530CST + c....J.!J:'IO 97° 5° 0 + 5 (b) 100 + 5 + 80 60 40. + 10 20 + 0 SPS Okm20 40 60 80 100 120 140 160 180 200 Figure 3. PPI radar echo configuration at ~S30 CST, May 28, ~967. (a) Divergence of horizontal wind (units, 10-4 sec-l ). (b) Vertica~ motion (units, ~b sec-l ) at 500 rob (1 ~b sec-~~ -1.5 cm sec-l ). 4 I Echo velocity (247°/8 m sec-I) was remarkably constant througnout the lifetime of the storm. Direction of travel was about 20° to the right of, and 1 m sec-l less than, the mean wind in the troposphere (225°/9 m sec-I). A special network of nine rawinsonde stations obtained soundings to 100 mb at 90-minintervals between 1100 and 1700 CST, within a 240-by-240-km area surrounding the other NSSL facilities. Computer programs developed to produce synoptic portrayal of the sounding data allow for balloon drift from the point of release and take account of nonsimultaneous releases. FigUre 2 is a three-dimensional representation of the horizontal wind at 1530 CST interpolated from the serial soundings. Divergence of the horiZontal wind at 500 mb is presented in figure 3a, and the asso­ ciated vertical motion field derived fram the equation of continuity is shown in figure 3b. Precipitation echo intensity contours from the NSSL ' WSR-57 radar are superimposed on both fields. 3. EXPERIMENTAL AND ANALYTICAL PR~EDURES A meteorologically instrumented DC-6, operated by the ESSA Research Flight Facility, systematically sounded the c1oud's environment through­ out its mature and dissipating stages. Two clockwise circumnavigations flown at the 500-mb level between 1500 and 1550 CST were followed by three more at the 800-mb level during the period 1600 to 1630 CST. Air­ craft tracks and winds measured by the Doppler navigational system, positioned relative to the radar echo center, are displayed in figures 4a and 4b. 80 40 KM KM 70 35 ._./-LL_L 60 ~ 30 ir.L~~ 50 r, ,-L 25 ~ ...\..'-1 ,( V v-< ~ }' .. ·"-t{l 40 \--- \--+ 11 20 J' ~\ + yJ ~y / \i 30 , . 15 '1-<:)""'-\ ~J-j~i~~\:~::\:: 20 ~~ 10 10 (a) 5 (b) 0 0 0 10 20 30 40 50 60 70 BOKM 0 5 10 15 20 25 30 35 40KM Figure 4. (a) Aircraft track and winds at 500 mb, averaged over 1 min and pOSitioned relative to radar echo centered at cross. Outer loop flown between 1503 and 1530 CST; inner circuit, 1534-1551 CST (10 m sec-l = 5 km). (b) Same as (a) except data collected between 1559 and 1634 CST at 800 mb and averaged over 30-sec intervals (10 m sec-l = 2.5 km). 5 Seven chaff packets were ejected from the aircraft at 500 mb on the northward-bound upwind leg of the outer loop, and a few min­ utes later six more were released in the downwind cloud region (see fig.
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