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David M. Schultz, W. Edward Bracken, and Lance F. Bosart

Abstract

Motivated by outstanding issues from a previous case study of a midlatitude cold surge that affected Mexico and Central America, the climatology of Central American cold surges is examined in this paper. An independently derived listing of 177 cold-surge events is employed for which the following properties are tabulated: onset date, duration, time between cold-surge events, latitude of maximum equatorward penetration (ϕ min), and 48-h maximum surface temperature change at Merida, Mexico (ΔT). These data show that 75% of the cold surges have durations of 2–6 days, the same timescale as mobile disturbances in the westerlies. Also, there does not appear to be any relationship between ΔT and the duration of the event, although cold surges that penetrate to low latitudes (ϕ min = 7°–10°N) have a weak tendency to persist longer than those that do not penetrate to low latitudes (ϕ min = 15°–20°N). In addition, the Reding data indicate that the cold surges tend to reach their most equatorward extent where topographic features impede the progress of equatorward-moving cold air; the temperature decrease in the postsurge air (as measured by ΔT) does not appear to be related to the most equatorward extent.

To examine the planetary- and synoptic-scale patterns associated with different categories of cold surges, events with similar characteristics from this database were composited: COLD (ϕ min ≤ 10°N and ΔT ≥ 9°C), COOL (ϕ min ≤ 10°N and ΔT = 4°–5°C), and LONG (events lasting at least 8 days). COLD surges are characterized by a persistent upper-level ridge over the western United States, 200-hPa confluence over the Gulf of Mexico, and the migration of a Canadian lower-tropospheric anticyclone equatorward along the Rocky Mountains and the Sierra Madre. In contrast, COOL surges are associated with a progressive, upper-level ridge over the western United States, weak 200-hPa confluence over the Gulf of Mexico, and the migration of a North Pacific anticyclone over the intermountain west and into the southeast United States. LONG surges are associated with a slower- moving planetary-scale pattern; 200-hPa confluence over the Gulf of Mexico; the occurrence of multiple cold surges, which reinforce the anticyclone over Mexico; and the absence of low-latitude, upper-tropospheric, mobile short-wave troughs to prematurely weaken the anticyclone. Cold surges (especially COLD) can be associated with an acceleration of the trade winds over the eastern North Pacific Ocean and play a role in El Niño–Southern Oscillation. The results in this paper are compared to the results of previous studies of North American, Central American, and east Asian cold surges.

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David M. Schultz, Philip N. Schumacher, and Charles A. Doswell III

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In response to Sherwood’s comments and in an attempt to restore proper usage of terminology associated with moist instability, the early history of moist instability is reviewed. This review shows that many of Sherwood’s concerns about the terminology were understood at the time of their origination. Definitions of conditional instability include both the lapse-rate definition (i.e., the environmental lapse rate lies between the dry- and the moist-adiabatic lapse rates) and the available-energy definition (i.e., a parcel possesses positive buoyant energy; also called latent instability), neither of which can be considered an instability in the classic sense. Furthermore, the lapse-rate definition is really a statement of uncertainty about instability. The uncertainty can be resolved by including the effects of moisture through a consideration of the available-energy definition (i.e., convective available potential energy) or potential instability. It is shown that such misunderstandings about conditional instability were likely due to the simplifications resulting from the substitution of lapse rates for buoyancy in the vertical acceleration equation. Despite these valid concerns about the value of the lapse-rate definition of conditional instability, consideration of the lapse rate and moisture separately can be useful in some contexts (e.g., the ingredients-based methodology for forecasting deep, moist convection). It is argued that the release of potential (or convective) instability through layer lifting may occur in association with fronts, rather than with isolated convection, the terminology “convective” being an unfortunate modifier. The merits and demerits of slantwise convective available potential energy are discussed, with the hope of improving diagnostic methodologies for assessing slantwise convection. Finally, it is argued that, when assessing precipitation events, undue emphasis may appear to be placed on instability, rather than the forcing for ascent, which should be of primary importance.

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David M. Schultz, Timothy M. DelSole, Robert M. Rauber, and Walter A. Robinson
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David M. Schultz, Timothy M. DelSole, Robert M. Rauber, and Walter A. Robinson
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David M. Schultz, Timothy M. DelSole, Robert M. Rauber, and Walter A. Robinson
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David M. Schultz, Timothy M. DelSole, Robert M. Rauber, and Walter A. Robinson
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David M. Schultz, Timothy M. DelSole, Robert M. Rauber, and Walter A. Robinson
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David M. Schultz, Timothy M. DelSole, Robert M. Rauber, and Walter A. Robinson
Open access
David M. Schultz, Timothy M. DelSole, Robert M. Rauber, and Walter A. Robinson
Open access
David M. Schultz, Timothy M. DelSole, Robert M. Rauber, and Walter A. Robinson
Open access