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A Static Stability Index for Low-Topped Convection

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A Static Stability Index for Low-Topped Convection 246 WEATHER AND FORECASTING VOLUME 15 A Static Stability Index for Low-Topped Convection NORM L. HENRY Meteorological Service of New Zealand Limited, Wellington, New Zealand 3 March 1999 and 8 September 1999 ABSTRACT Static stability indexes provide a simple representation of a complex aspect of the atmosphere and are widely used in operational forecasting. However, their applicability is limited, since most are speci®cally designed to measure deep instability. In particular, they are ineffective at resolving instability that is capped by an inversion below the height of the 500-mb surface. Unstable thermal pro®les that are capped between 700 and 500 mb have been identi®ed as a signi®cant forecasting problem in New Zealand. Referred to here as low-topped instability, these situations are easily overlooked by forecasters, due to a lack of relevant guidance. In this paper, an index capable of resolving low-topped instability is presented. Denoted TQ, it is based on the temperature and dewpoint at 850 mb and the temperature at 700 mb. A dataset of 90 events, displaying varying degrees of low-topped instability, was used to correlate TQ to observed weather. Using an independent dataset for veri®- cation, TQ was found to be effective at resolving unstable events, demonstrating skill that is statistically signi®cant at the 2% level. 1. Introduction shear [e.g., Severe Weather Threat (SWEAT) Bidner (1970)]. Vertically integrated measures of instability Static stability refers to the tendency of the atmo- such as the convective available potential energy sphere to either resist or enhance small-scale vertical (CAPE; Blanchard 1998) provide a more detailed phys- displacements, due to buoyancy forces arising from tem- ical representation of the state of the atmosphere and perature differences between the ambient air and the are commonly used as stability indexes. displaced parcel. Although most commonly considered in relation to convection, stability in¯uences virtually Situated in the middle of a large data-sparse area, all weather phenomena, and its assessment is an integral Meteorological Service of New Zealand (MetService) part of operational forecasting. However, conducting a forecasters place considerable emphasis on NWP model thorough stability analysis over a broad area requires data for stability assessment, with guidance presented consideration of thermal pro®les at many locations, and either in the form of model soundings or as ®eld rep- at different time steps through the forecast period, which resentations of stability indexes. Detailed analyses of is often impractical due to time constraints. model soundings are carried out for areas of interest and To assist forecasters in this process, a number of in- may include, for example, the assessment of convective dexes have been developed that represent stability as a cloud-top temperatures, CAPE, convective temperature, simple numerical ®eld. They rely primarily on temper- etc. Combined with observational data, and an assess- ature differences between two or more levels in the ment of synoptic and mesoscale forcing, these analyses troposphere to assess the potential for convection, based form the basis of forecasts of convective weather. In on a set of empirical threshold values (refer to Peppler contrast, stability indexes provide limited information and Lamb 1989 for a comprehensive review). The sim- about the state of the atmosphere at any particular lo- plest indexes consider only the ambient temperature and cation, but offer a concise, broad-scale, representation dewpoint [e.g., total totals, Miller (1967); George's K of stability. They allow forecasters to easily follow index, (George 1960)], while more complex indexes trends in stability and relate these to tropospheric dy- take into account other effects such as parcel lift [e.g., namics, and to quickly identify those areas requiring a Showalter (1953); lifted index, Galway (1956)] or wind more detailed stability assessment. This implies a broad role for stability indexes in the operational environment: they assist forecasters in vi- sualising the state of the atmosphere on the synoptic Corresponding author address: Norm L. Henry, Meteorological scale, as well as providing a rough initial assessment of Service of New Zealand Limited, 30 Salamanca Road, P.O. Box 722, Wellington, New Zealand. the potential for convection. Historically, the develop- E-mail: [email protected] ment of indexes has been directed mainly at forecasting q 2000 American Meteorological Society Unauthenticated | Downloaded 09/26/21 06:39 AM UTC APRIL 2000 NOTES AND CORRESPONDENCE 247 FIG. 1. Tephigram for Invercargill valid 0000 UTC (1200 LST) 16 Mar 1997. thunderstorms and, with few exceptions, they are de- lems to which it is commonly applied include embedded signed to assess the potential for deep, surface-based, convection in synoptic-scale systems, surface-based convection. Within the broad role suggested above, in- convection in cool air masses, and the interaction of the dexes may be applied to situations that differ markedly synoptic ¯ow with the Southern Alps. from that for which they were designed. This is appro- However, as with most conventional indexes, TT is priate provided a reasonable physical link exists be- of limited value in assessing stability below the 500- tween the index and the phenomenon being considered. mb surface, which is of interest in many forecasting However, there are limits to the applicability of con- applications. The common occurrence of low-topped ventional indexes, since they are so narrowly focused convection in southerly ¯ows over New Zealand, typ- on the problem of deep convection. ically associated with an upper trough migrating east of In recent years, total totals (TT) has become the fa- the country, provides an illustrative example. Because vored stability index among MetService forecasters. of the long fetch over the Southern Ocean, air masses Originally developed for severe thunderstorm forecast- of Antarctic origin are highly modi®ed by the time they ing in the United States, TT is de®ned as reach New Zealand, with 850-mb wet-bulb potential temperature (u ) normally above zero degrees Celsius. TT 5 T 12T 2T (8C), w 850 d850 500 They are typically unstable with respect to surface-based convection, with the depth of the instability dictated by whereTd850 is the 850-mb dewpoint and T 850 and T 500 are the 850- and 500-mb temperatures, respectively. upper-air dynamics. Based on forecaster experience in the application of TT In the vicinity of the upper trough, the instability to warm season air masses in New Zealand, values less normally extends above the height of the 500-mb sur- than 45 generally indicate a stable strati®cation with face and is readily apparent in the TT ®eld. Behind the respect to deep convection, while values of 55 or greater trough, an upper-level inversion typically develops in indicate a high likelihood of thunderstorms. Other prob- response to upper-tropospheric subsidence or warm-air Unauthenticated | Downloaded 09/26/21 06:39 AM UTC 248 WEATHER AND FORECASTING VOLUME 15 TABLE 1. Accuracy scores for actual TQ, UKMO Uni®ed Model t 1 24 h forecast TQ, and Monte Carlo 2% signi®cance levels for the veri®cation dataset. Actual TQ Model TQ Monte Carlo 2% level CSI 0.57 0.56 0.36 POD 0.73 0.82 0.58 FAR 0.27 0.36 0.40 advection, gradually building down to lower altitude with increasing distance behind the trough. When the inversion falls below the height of the 500-mb surface the TT index will drop markedly, regardless of how unstable the atmosphere is at lower levels. This leads to a situation where signi®cant convective weather may occur with TT well below 45. For the purpose of this discussion, the term signi®cant convective weather re- fers to the occurrence of either moderate or heavy show- ers, hail, or thundershowers. For example, consider the 0000 UTC 16 March 1997 sounding for Invercargill (Fig. 1), which is located on the southern tip of the South Island (Fig. 2). At the time of this sounding, an upper trough lay to the east of New Zealand, with a west-southwest ¯ow through the depth FIG. 2. Location of upper-air observing stations in New Zealand of the troposphere over the South Island. The strati®- and geographical areas referred to in the text. cation is unstable below the inversion at 520 mb and, despite a TT index of only 38, the Invercargill airport reported heavy showers and hail within the hour fol- lowing the radiosonde release. This limitation in resolving towering cumulus (TCU) FIG. 3. Distribution of the (a) training dataset and (b) veri®cation dataset. Values in the table indicate the number of events corresponding to each TQ±stability category pair, with TQ rounded to integer values. The dashed line represents the optimum threshold for unstable events at TQ 5 12. Unauthenticated | Downloaded 09/26/21 06:39 AM UTC APRIL 2000 NOTES AND CORRESPONDENCE 249 and low-topped cumulonimbus (CB) convection is a seasonal dependence inherent in TT. To illustrate this, common feature of stability indexes, which typically note that on a tephigram or similar thermodynamic di- have a strong dependence on the 500-mb temperature agram, the wet adiabats become more steeply sloped as and lack sensitivity to thermal structure between 850 uw increases, since more latent heat is available through and 500 mb. Braun and Monteverdi (1991) have dis- condensation. This means that a saturated parcel raised cussed this in relation to so-called cold sector thunder- from 850 to 500 mb will experience a greater temper- storms, in which the maximum parcel buoyancy is often ature decrease in a cool air mass than it would in a found near the 700-mb level, and advise caution when warm air mass, which affects the relationship between applying traditional indexes to such cases.
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