Ref.46
INTERNAL STRUCTURE OF NON-LINE MESOSCALE CONVECTIVE
SYSTEM IN SOUTHERN BRAZIL.
Sanjar Abdoulaev*, Anatoli Starostin and Olga Lenskaia
Universidade Federal de Pelotas, Rio Grande do Sul, Brazil
ABSTRACT
The internal structure of 14 Non-line Mesoscale Convective Systems (NLMCS) with
severe convection (echo intensities more than 55 dBZe in some stage of development)
occured in Southern Brazil are studied. The NLCS reflectivity field structure is
complex and strongly evolves with time, and frequenly is called as “the chaotic
arrangement of convective cells and stratiform precipitation”. For analysis of this
structure is used the method of composite reflectivity structure in lagrangean
coordinate. The reflectivity field is dislocated with system velocity (passive translation
of cells) and is integrated during life of NLCS to one composite image. In contrast to
“chaotic” distribution of intense echoes on instant radar images, the time integrated
distribution of these echoes in lagrangean coordinate system moving with mean
tropospheric wind is quit organized. The strongest echoes of NLCS are concentrated to
three mesoscale ensembles: small- (30 km), large- (100 km) and small- (250300 km). The larger ensembles consist of the smaller ensembles, that are distributed in
space more or less regularly. The intensity of NLCS also is quasi-periodic with period
of 1 and 3-4 hours.
1. INTRODUCTION
Some large meso- and small meso- convective processes don’t represented any
visible organization in contrast to Linear Mesoscale Convective System (LMCS) such
as squall lines and bands of precipitation. For example Houze et all (1990) found that in
1/3 of cases of severe mesoscale systems in Oklahoma the structure of reflectivity field
was “chaotic arrangement of convective cells and stratiform precipitation”. In this study
we want to demonstrate that meso-- scale severe convection in Southern Brazil is
more regular than can be seen from preliminary analysis of radar data.
2. DATA AND METHODOLOGY
From archive radar data obtained in southern part of Rio Grande do Sul State of Brazil
(about region and data type see Abdoulaev et alii, 1996) we select the 14 cases of Nonline Mesoscale Convective System (NLMCS) with severe convection (echo intensities
more than 55 dBZe in some stage of development) without long length convective
lines. For example on Fig. 1 is demonstrated such “chaotic” evolution of NLMCS on
24 January 1995. It can be seen that structure of reflectivity field is complex and
*
Correspondig author address. Sanjar Abdoulaev,Univ. Fed. de Pelotas, Av. Ildefonso Simões Lopes 2751, 96060-290 Pelotas, RS, Brasil, e-mail
casarin@vortrex.ufrgs.br
strongly evolves with time. For analysis of this structure we were used the method of
composite reflectivity structure in lagrangean coordinate described by Starostin
(1994a): the reflectivity field is dislocated with system velocity (passive translation of
cells, see Abdoulaev et all, 1994b) and is integrated during life of NLCS to one
composite image. To describe only strong convective elements on composite image,
we used only echoes with reflectivity more than 40 dBZe.
Figure 1. Evolution of reflectivity field of NLCS on 1407-1925 LST 24 January 1995
(PPI elevation of 2) and ground relative trajectories of some storms (at the bottom of
figure). First letter of label: O-ordinary cell, S-supercell and M-multicellular storms;
Second parts of label : A1, A2, A3 and B1 mean that storms are elements of mesoscale
features A1, A2 and etc. on Figure 2
2. RESULTS AND DISCUSSION
a) 24 January 1995 case study
On Fig. 2 is exhibited the composite picture received by integrating of echoes of
reflectivity Z > 40 dBZe during 13:58-21:01 LST on 24 January 1995 in lagrangean
coordinate moving with 13.3 ms 1 from 270. The straight line with temporal marks in
this picture is trajectory of radar (in this coordinates radar are moved from left to
right). It can be noted that radar echoes on this image are concentrated to two blocks A
(130160 km 2 ) and B (100140 km 2 ) that consist of bands A1, A2, A3 and B1, B2
correspondingly and are separated by echo free region about of 100 km. The mean
distance between bands is about 50 km. The bands in block A and B are oriented
approximately from SW-NE but northern parts of A1 and A3 are inclined to SE-NW.
The detailed analysis of ground relative motion of individual storms depict that
distorting of northern part of A1 and A3 is appeared due to evolution long-lived leftmoving (in respect to ordinary cells OA1 and OA3 that moved from W) supercells
SA1 and SA3 (Fig.2). These supercells moved approximately parallel to coast line of
Dos Patos Lake and can be associated with local lake-surface circulation (the
occurrence of coast-line supercells was observed in other days also). Majority of others
storm (MA1, MA2 and etc. ) in this day were right moving multicellular storms.
Figure 2. Time-integrating composite image of reflectivity on 24/01/95 NLCS .
Echoes of 40 dBZe were integrated from 13:58 to 2101 LST in coordinates moved
with 13.3 m/s from 270.
Is the mesoscale structure in the Fig. 2 more regular in respect to structure of
reflectivity field on Fig 1? Our own experience of analysis of radar data in East
Europe shown that regular structures (more or less similar to structure on Fig. 2) is
frequently observed on composite images. Starostin et all (1983), Starostin (1994a)
Fig. 3 The examples of composite images obtained by integration of Ze>40
dBZe in coordinates moved with velocity of passive translation.
based on radar data in region of Moldavia, demonstrated that echoes were organized
to “open cell”-like structures with strong echo clear regions about 30-40 km. In large
meso-/small- scale (Abdoulaev, 1994)
the evolution of severe convection is
concentrated to large meso- -scale (about
of 100 km in equivalent diameter and life
time about 4-5 hours) ensembles and small
meso- (equivalent diameter about 300
km and time of 7-9 hours). Generally, the
Fig. 4 Distribution of the equivalent diameters of composite images associated with region
82 “open-like” small meso--scale (small) and
in present study is more complex (fig. 3)
31 large meso--scale ensembles(large).
than their analogies in Eastern Europe,
probably because of some coast/ocean influence: the some intensification of
convection along of coast line and frequent dissipation of clusters when them move to
free ocean (about 1/2 of radar observed area is oceanic region). However, in modal
sense (fig. 4) the small and large ensembles in Southern Brazil and in Eastern Europe
are similar.
b)Life time and periodic oscillations. Also was founded that evolution is quasi-regular
not only in space but also quasi-regular in time. Really in process on Fig. 2 band A1
had Z > 40 dBZe from 1340 to 1636 LST and dissipated after this time, in A2 echoes
with such reflectivity appeared from 1358 to 1822 and in A3 from 1616 to 1858, in
other word large meso- ensemble A was active about 5 hours . After the appearance
of first echo of 40 dBZe at 1717 LST, maximal reflectivity Z > 40 dBZ lasted in B1
approximately of 5 hours (life of B2 is short 2 hours). Consequently convective
activity of small meso- cluster, composed by two large meso--scale ensembles A e
B, was 7.5 hours. On Fig. 5 is exhibited time series of 10 min. maximal reflectivity
corresponding to elements of these ensembles. It can be noted 1-hour quasi-periodic
intensification of A1- A3 and
of B1-B2. The intensity of 
cluster has also two principal
maximums in 16:10 and 20:05
LST
or
period
of
intensification associated with
evolution of A1 and B1 about
4 hours. This periodic
oscillation
is
frequently
observed in clusters on
Europe
(see for detail
Figure 5. Time series of 10 min. maximal reflectivity Abdoulaev, 1994, Starostin,
of elements of ensembles A and B.
1994b). Abdoulaev et all,
(1994a) demonstrated that the oscillations of 3-4 hours of -cluster were
accompanied by oscillations of surface convergence field and precipitation intensity
on areas more than 410 4 km 2 . The local circulation and associated mesoscale
stationary convergence zone can modify the structure and intensity of cluster, but
periodic oscillations are conserved.
3. CONCLUSION
The internal structure on Non-Line Convective System in Southern Brazil represent
chaotic structure only at the first sight. In contrast to “chaotic” distribution of intense
echoes on instant radar images, the time integrated distribution of these echoes in
lagrangean coordinate system moving with mean tropospheric wind is quit organized.
The strongest echoes of NLCS are concentrated to three mesoscale ensembles: small (30 km), large- (100 km) and small- (250-300 km). The larger ensembles consist
of the smaller ensembles, that are distributed in space more or less regularly. The
intensity of NLCS also is quasi-periodic with period of 1 and 3-4 hours.
4. REFERENCES
Abdoulaev, S., N. Arskaya and A. A. Zhelnin, 1994a: Interaction of surface wind
convergence fields with precipitation from comoulonimbus clusters. Meteorology
and Hydrology.,N 8:33-37.
Abdoulaev, S., Starostin A., Lenskaia O., Starostina T., 1994b: Determination of
characteristics of meso--scale motion using conventional radar data. Prepr. of 8th
Brazilian Met. Congress, Belo Horizonte, 18-25 October, 1994, 2, 298-300
Abdoulaev, S., 1994: Evolution and hieraquia of Cb ensembles. Part 2: Evolution of
-cluster and oscilation of intensity. Ibid. 251-254.
Houze, R. A. Jr., B. F. Smull, P. Dodge, 1990: Mesoscale organization of springtime
rainstorm in Oklahoma. Mon. Wea. Rev.,118:613-654
Starostin S., 1994a: Mesoscale structure of Cb field. Prepr. of 8th Brazilian Met.
Congress, Belo Horizonte, 18-25 October, 1994, 2, 259-262
Strarostin S.,1994b: The time variations of Cb field. Ibid. 266-268
Starostin, A., E.M. Livshits, V.S. Shvetsov, 1983: Meso-scale structure of the radar
echo fields from convective clouds in Moldavia. Meteorology and Hydrology,
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