Organization and Evolution of Mesoscale Convective Systems Using Radar Data: Objective Description. The “Dominating Thunderstorm” Conception and its Application to MCS Climatology

In spite of the fact that today nobody consider the mesoscale convective systems (MCS) as indefinite entity with intermediate time – spacial scales situated between cyclones and individual clouds / convective cells, some questions about MCS origin still remained.

In this paper we try to illustrate that one of those questions can be formulated as: “What does “mesoscale organization” means?” For understanding the problem, let's put another question: “Whether exist any plausible way to distinguish the scales of processes, e.g. whether external forcing or internal interactions govern the instant precipitation structures?” Since the earliest radar observations of 1950th when it was discovered the cellular structure of both local storm and frontal precipitation bands, this question directly connects us with origin of MCS terminology. For example, consider well-known definition of MCS as “a cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area on the order of 100 km or more in horizontal scale in at least one direction” and conceptual description of MCS life cycle [see, e.g. textbooks of Cotton and Anthes, 1989, Houze, 1993, 2004].

In general, the MCS acronym is used to describe a diversity of thunderstorm ensemble forms which, as is supposing, have interdependent evolution at least in some period of their life. For some MCS it can be deduced intuitively from visual inspection of radar images temporal sequence. Thus, on the basis of precipitation patterns, life cycle of squall line MCS can be separated in the following three distinct stages. During the first formative stage, growing and merging groups of initially isolated convective cells and small thunderstorms tend to form “continuous convective line” (extension of radar reflectivity zone Z> 40 dBZ is more than 50 km); in a second, mature stage this “solid line” is continually sustained / grown due to appearance of new cells in along line direction (i.e. parallel propagation). Finally, convective activity in “solid line” is ceased and MCS enter in a third, decaying stage characterized by reasonably slow disappearance of pos-convective stratiform precipitation.

The MCS life cycle defined there, with two small exceptions, is not very different from the description of any banded structures developing in the vicinity of pre-existing convergence lines, and embraced all scales and various origin of moving or stationary quasi-linear boundaries occurred in planetary boundary layer, such as cold fronts, dry lines, breeze-like circulations, remnant outflows from decayed MCS etc.

The exceptions are associated with two types of convective pattern that preceded to continuous convective line formation (a) and type (b) of “solid line” propagation just after their formation.

(a) Frequently observed, the initial organization of thunderstorms preceding to mature linear structure is characterized as “chaotic”, “broken areal”, “diffused”, “occluded” etc [Bluestain and Jain, 1985, Blanchard, 1990, Jirac et all, 2003]. As depicted earlier in our detailed observation of MCS life cycle [Abdoulaev, 1995, Aboulaev et. all, 2000, Abdullaev, Zhelnin, Lenskaya, 2009] both squall lines and MCSs with more complex morphology, reveal so-called MCS auto-organization. It means the quasi-periodically striking of intense dominating thunderstorms occurred with ~1 hour one after other, and 2-3 large meso- ensembles, compounded by 2-4 dominant and subdominant thunderstorms, that define entire convective activity. Commonly, severe dominating thunderstorms are linear or bowed shape and, as we show with some novel analysis techniques, its appeared along the some invisible axis translated by mean wind (i.e. cell's advection vector). Someone can speculate that MCS with this ulterior axis can be considered as squall line cousins with less profound external forcing / more evident self-organization.

(b) Overall, thunderstorms interaction / propagation result on mature MCS morphology. It was observed that life cycle of multi – super-cells is less than 2 hours, and solely, these local storms can't create long lived solid line. Consequently to maintain squall line structure hourly, at least one linear thunderstorm must to arise along the line, in the other words, in mature squall line the continuous parallel propagation more then ~10 m/s is expected. The values of line parallel and line normal propagation is the crucial aspect of extension of convective and stratiform region and their relative positions finalized in MCS asymmetry [Abdoulaev and Lenskaia, 1996, Abdullaev and Lenskaya, 1998, Parker and Johnson, 2000]. Thus, the stratiform precipitation region, trailing or leading in respect to line normal translation, will be observed in mature squall line only if the rate of mean value of line normal propagation is exceed the mean stratiform cloud dissipation by 3 m/s.

Some problems regarding the generalization of precipitation patterns practice are raised when we use the instantaneous radar data as initial and evaluating tool in severe weather nowcasting and numerical modelling. All of these problems caused by origin question of objective classification of mesoscale convective systems (MCS). As we pointed out 20 years ago, the basic difficulty to appropriate description of mesoscale convective organization is the absence of rigid definition of MCS stages. Consequently, the representative instance of MCS life cycle when morphology of all systems can be adequately compared is indeterminated.

As depict previous author's studies of MCSs evolution, the oscillating nature of convective activity is associated with quasi-periodic occurrence of “dominating thunderstorms” – the storms whose cells have major intensity in respect to other subdominant thunderstorms intensities. Evidently, using temporal sequence of dominating thunderstorms radar parameters (e.g. reflectivity maxima or radar echo tops) we can determine some interval around of the moment when dominating thunderstorm of maximal intensity is observed. That unique period of system's life, so-called maximal intensity stage, can be found in the life cycle of any mesoscale system independently its origin, scale and severity. Thus, only a radar data sample quality deputed to analysis may restrict the objective classification possibility. As the example of developed methodology, it is constructed a preliminary MCS climatology for Central Russian region, and the diversity of mesoscale organization is described.