Vertical velocity, horizontal divergence and turbulence associated with tropical mesoscale convective system

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1 ndian ournal of Radio & Space Physics Vol. 3. April 21. pp Vertical velocity, horizontal divergence and turbulence associated with tropical mesoscale convective system D Narayana Rao. H Randhir Singh & S Vijaya Bhaskara Rao Department of Physics. S V University. Tirupati 1 2 Received March 2; accepted 29 September 2 The MST radars have been used to measure directly the vertical motions above the radar site. Vertical velocity profiles and horizontal divergence/convergence derived from the vertical velocity using the mass continuity equation as well as eddy dissipation rate estimated under strong-to-moderate convection period are presented in the paper. Data obtained from the ndian MST radar located at Gadanki( 1 3 SN, 9. E) have been used. The height range of the study is from kn to 2 kn. The aim of the present study is to calculate the horizontal divergence from the vertical velocity and then to find the extent of the enhancement in the vertical velocity and horizontal divergence as well as turbulence distribution during the well developed tropical mesoscale disturbances. Strong enhancement is observed in the vertical velocity and the horizontal divergence/convergence. Maximum turbulence has been observed near the top of the cloud and above. Eddy dissipation rate 2 3 of the order of.m s- has also been observed. 1 ntroduction Vertical velocity of air is a key variable in meteorology. The MST radars offer the unique capability measuring the vertical velocity of air in the atmosphere. This capability has been amply demonstrated by measuring relatively larger vertical velocities in mesoscale circulation systems such as convective cells l. One of the major dynamic features of the tropical atmosphere is the tropical mesoscale convection complex (MCC) or cloud cluster. Tropical 2 MCCs comprise large area (approximately 1 km ) of clouds systems that are organized both on the synoptic and the mesoscale. t contains distinct regions of both stratiform and covection precipitation and strongly influences the large scale atmospheric circulation both by releasing latent heat and by altering the local radiation budget. Vertical circulation within the stratiform portion of these complexes consists primarily of deep but gentle updraft ( 1 1 cmls) in the cloud layer above the freezing levels2-. These updrafts are superposed over unsaturated mesoscale downdrafts.6 Somewhat different picture describes the convective portion of the complexes, where both moderate convection and the narrow region of intense updrafts and downdrafts (the hot tower first described by Riehl and Malkus ) are supported by the low level convergence. These systems have been studied extensively during GARP Atlantic Tropical Experiment (GATE) and nternational Monsoon Experiment (MONEX). t is of interest to assess the amount of divergence and the vertical velocity that are likely to occur in the monsoon systems. Knowledge of vertical motion in mesoscale convective systems (MCS) is important for understanding the physical processes within these systems as well as to understand how these features interact with large environment through transport of heat, moisture and momentum Unfortunately, the amount of divergence and the vertical velocity is so small that accurate measurement of them is very Table -ndian MST radar specifications Parameter Specification Operating frequency No. of Yagi antennas Antenna dimensions A verage power aperture product Beam width Peak power Max. duty ratio nter-pulse period(lpp} Pulse width 3 MHz m x 1 3 m 2 x 1 Wm Max. No. of range bins Max. No. of coherent integrations Max. No. of FFf points Side lobe Gain Beam positions 3 2. MW 2.% l s 1-32 s; ntegral multiples of 2 of uncoded and 1 6 & 32s coded with ls baud length db 36 db Zenith; ±2 off zenith in EW and NS directions

2 92 NDAN RADO & SPACE PHYS, APRL 21 difficult. The development of radars has solved this problem. With the help of MST radar, one can get vertical velocity directly with very good height and time resolution from which one can estimate divergence. This paper presents horizontal divergence observed in moderate to strong convection period for the first time using ndian MST radar. 2 System description and the data base Data used for the present study are obtained from the ndian MST radar located at Gadanki( l 3 SN, 9. E). The ndian MST radar is a highly sensitive phased-array radar, operating at 3 MHz. t consists of a phased array of 1 2 crossed three-element Yagi antennas occupying an area of 1 3x 1 3 m2, generating a radiation pattern with a beam of 3, a gain of 36 db and a side lobe level of -2 db. The MST radar system specifications are given in Table 1. Detailed system description of ndian MST radar system has been given by Rao et al.9 On 6 une 1 996, thunder storm occurred over MST radar site. A range time intensity plot observed on the same day is shown in Fig. 1. The data were collected and analysis was carried out for the convective event of 6 une The starting time of the event is 1 2 hrs 1ST and ending time is hrs 1ST. The time interval between each scan varies from 2 to min. 3 Methodology for horizontal divergence calculation Three largely independent methods, namely, the kinematic, adiabatic and the vorticity methods have db S c a l e. ". : :" -::=;::::: ' '. : :'" r::::i: ::::: - :;::';'::::.::=-...: " ' fl : n "... _....) -- :. 19..:«:; ] == == TME, hrs 1ST Fig. l -Range-time intensity plot observed on 6 une

3 NARAYANA RAO et al. : DYNAMCS ASSOCATED WTH TROPCAL MESOSCALE CONVECfVE SYSTEMS been used in the past to compute the vertical velocities. n this paper, kinematic method has been used with a different approach. Usually divergence is calculated by the grid method, using four meteorological stations to estimate the vertical velocities. Here, we have used vertical velocities obtained from ndian MST radar observations and calculated the divergence using the continuity equation. The continuity equation in cartesian coordinates is as follows: op + V.(pU ) = O ot... (1)... (2) op = ot op + v. op = ox oy and the conditions are : (U. (W. op (U ) _p o + p. ox oy oz oz or, Ow = O OV... (3) where, (-)ve D indicates convergence and (+)ve D indicates divergence, p is the density taken from nearest radiosonde station and w is the vertical velocity. Results and discussion.1 Spectral analysis air and the other for precipitation. By windowing, we separated the clear air echo and precipitation echo. So, in this study the vertical velocity is of air only and does not contain any hydro-meteors fall velocity..2 Vertical velocities in MCS Vertical velocity profiles obtained under the conditions of strong (6 une 1 996) convection are shown in Fig. 3. The time of observation is shown on the top of each profile. Figure 3(a) indicates the condition just before the start of the convection, Fig. 3 gives the sign of convection, and Fig. 3 [(d)-(h)] is during mature stage of convection for 6 une n mature stage the upward vertical velocity is reaching up to mls.!he downward velocity is about 3 mls with a peak near melting layer. Figure 3 [(i)-(j)] indicates dissipation stage of the MCS where velocity reduces to 1 mls. The downward motion is up to km and upward motion is from 9 km to top of the profiles in mature stage. Figure shows time-height plot of vertical velocity..3 Horizontal divergence in MCS Applying above conditions, Eq. (2) reduces to + 93 Figure 2[(a) and ] shows the spectral plot for 6 une Figure 2(a) shows the spectral plot corresponding to the clear air and Fig. 2 shows the spectra during the precipitation. n Fig. 2 two clear echoes can easily be seen-one corresponds to clear Figure [(a)-(j)] shows the vertical profiles of horizontal divergence calculated from the vertical velocity using continuity equation at different times during the convection on 6 une The time height plot of horizontal convergence and divergence for 6 une 1996 is also shown in Fig. 6[(a) and ]. Figure (a) indicates the event before the convection starts, while Fig. (j) gives sign of dissipation of MCS. Figure [-(g)] indicates the mature stage of MCS. From Fig. [(a)-(j)] it is observed that magnitude of horizontal divergence/convergence is about xl O- s- l, while the magnitude of horizontal divergence/convergence in the mature stage of MCS - is reaching up to x l O s- l. From Fig. 6[(a) and ] it can be seen that convergence is present from to 9 km and divergence is present from 9. km to the top of the plot, i.e. lower tropospheric convergence and upper tropospheric divergence persist during mature MCS. From Fig. 6[(a) and], we can estimate the exact time when the well developed tropical disturbance was present over MST radar site by observing the magnitude of horizontal divergence/convergence and vertical motion.. Turbulence due to a convective system Different methods have been used for estimating the energy dissipation rate from the STMST radar measurements. Here, we have adopted the method

4 NDAN RADO & SPACE PHYS. APRL () =.3 " -: 1.6 "-' 1. '-"" =: a. O(L DOPPu;R, Hz DOPPLER. Hz \.9, us.6 Fig. 2-Spectral plot observed on 6 une (a) during clear air and during precipitation :2(iST) 1 :2 (1ST (a) 1 l:36 1 :3 (1ST (d) (c) :QO(ST) (i) ) ) : +r-,-,-,..-rf+.r.,..,,-rri -t-n-,-,rr-r'-+-rr."...,...,.-l +r-.-,-,--ri ,..j +r-."'t't'1r-t-i+-. f-,-, l VERTCAL VELOCTY, m/s : 1 (ST) (j) B - ) l> +TTT...-T!'rh"""""'''''..-l - - Fig. 3-Vertical velocity profiles observed on 6 une during different stages of convection

5 NARAYANA RAO el al. : DYNAMCS ASSOCATED WTH TROPCAL MESOSCALE CONVECTVE SYSTEMS given by Cohn 1 to estimate the mean energy dissipation rate as follows: 1. where, () = 1 m (height resolution) y =. 1 (heat / momentum diffusion const)... () d- = Half power spectral width a= 1.6 (Kolmogorof const) Figure shows the time height vanatlon of eddy dissipation rate. From Fig. it can be seen that maximum c is near upper part of the cloud that is in connection with updraft. At 1 36 hrs 1ST very high values of the order. m2 s-3 are observed. Same approach gives, in relatively calm period, value of about 2- times lower than the one observed here. The values observed are in good agreement with the. values observed by Rao et at. mls S!!) 9 Summary and conclusions We have examined the structure of vertical velocity, horizontal divergence and turbulence associated with tropical MCS. t is observed that: TlMt hn 1ST Fig. -Time-height plot of vertical velocity 19 ' 1 ]1:9: " 1 :2:9 () ( 1 :3: 1ST 13 1 :39:3 1ST (g) (it) ) 1 9: 19:3 (i) D ( D $( -h-r-r-r--+-r--;:::::r.--r-l +-r-,-,r-r-''-r-,-,.--rl - +-r r--,-,-,--i +-r-,-,r-r-t-..--r-r--rl ----rr--.-+'--r--.--l HORZONTAL DVERGENCE ',CONVERGENCE, xl - S - 1 Fig. -Vertical profiles of horizontal divergence/convergence observed during the convection on 6 une 1 996

6 96 NDAN RADO & SPACE PHYS, APRL 21 Xl - S -1 o TME, h's 1ST :; O t 13 o TME. h's 1ST Fig. 6--T ime-height plot of horizontal (a) convergence and divergence observed on 6 une Zone of important energy dissipation is near the top of the cloud. (ii) Horizontal divergence/convergence as large as - 1 x l O s- was found on 6 une (iii) Lower tropospheric convergence and upper tropospheric divergence is present during the strong convection. (i) (iv) The upward vertical velocity is of the order - mls from to 1 6 km. (v) The downward vertical velocity is 2-3 rns from to km. This kind of study is very useful to study the fi ne scale dynamics of the upper part of the convective clouds.

7 9 NARA YANA RAO et al. : DYNAMCS ASSOCATED WTH TROPCAL MESOSCALE CONVECTVE SYSTEMS 2U E ;..... ". '..c::::.., "'" ".. ::. x 1 - m! s 6 : = 3 \.!) 1 2 :: T M E, hrs 1ST Fig. -Time-height variation o f eddy dissipation rate (t:) Acknowledgements The National MST Radar Facility (NMRF) has been set up jointly by the Council of Scientific and ndustrial Research, the Defence Research and Development Organization (DRDO), the Department of Environment (DOE) and the Department of Space (DOS) (nodal agency) of Government of ndia. The authors are thankful to UGC-SVU centre for MST Radar Applications, S V University, Tirupati, for providing necessary facility to carry out the research work. References Crochet M F Cuq. Ralph F M & Venkateswaran S V. Dyn Atmos Oceans (Netherlands), 1 ( 1 99). 2 Gramache F & Houze R A (r). Mon Weather Rev (USA), 1 1 ( 1 92) Houze R A (r). Meteorol Soc (apan). 6 ( 192) 396. Houze R A & Hobbs P V, Adv Geophys ( UK). 2 ( 1 92) 22. Zipser E, Appl Meteorol (USA). ( 1969) 99 6 Zipser E, Mon Weather Rev (USA), 1 ( 1 9) 1 6. Riehl H & Malkus S, Geophys ( UK), 6 ( 19) 3. ohnson R H & Young G S, Atmos Sci ( USA), ( 193) Rao P B, Radio Sci ( USA), 3 ( 1 99) Cohn S A. Atmos & Ocean Teclmol ( USA), 1 2 ( 1 99). 11 Narayana Rao D, Radio Sci ( USA), 32 ( 199) 1 3.