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Gary M. Lackmann

Abstract

An elongated cold-frontal maximum in the lower-tropospheric potential vorticity (PV) field accompanies some midlatitude cyclones. These PV maxima are often of diabatic origin, and are hypothesized to contribute substantially to the strength of the low-level jet (LLJ) and moisture transport in the cyclone warm sector. Diagnosis of a representative cyclone event from the central United States during February 1997 is presented with the goals of (i) elucidating the mechanisms of development and propagation of the cold-frontal PV band, and (ii) clarifying the relation between this PV maximum and the LLJ.

A confluent upper trough and modest surface cyclone followed a track from the south-central United States northeastward into southern Ontario between 26 and 28 February 1997, accompanied by flooding and widespread straight-line wind damage. A LLJ, with maximum wind speeds in excess of 35 m s−1, was positioned at the western extremity of the cyclone warm sector, immediately east of an elongated PV maximum in the lower troposphere. Results of an Ertel PV budget confirm the importance of latent heat release to the development and eastward propagation of the PV band. Cancellation was observed between the vertical PV advection, which yielded negative (positive) tendencies beneath (above) the cold-frontal PV maximum, and the nonadvective PV tendency, which was positive (negative) beneath (above) the level of maximum heating. The nonadvective PV flux is directed opposite the absolute vorticity vector; therefore vertical wind shear (associated with westward-tilting absolute vorticity vectors) led to eastward nonadvective propagation of the PV maximum. Quasigeostrophic PV inversion indicates that the cold-frontal PV maximum contributed between 15% and 40% to the strength of the LLJ within the cyclone warm sector. The results of this study suggest that a complex interdependence can exist between cold-frontal rainbands, lower-tropospheric PV maxima, the LLJ, and warm-sector moisture transport. The implications of this linkage for numerical weather forecasting are discussed.

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Gary M. Lackmann

Abstract

Previous studies have documented a feedback mechanism involving the cyclonic low-level jet (LLJ), poleward moisture flux and flux convergence, and condensational heating. Increased water vapor content and potentially heavier precipitation accompanying climate warming suggest the hypothesis that this feedback could strengthen with warming, contributing to amplification of precipitation extremes beyond what the thermodynamically controlled vapor increase would provide. Here, this hypothesis is tested with numerical simulations of a severe flooding event that took place in early May 2010 in the south-central United States.

Control simulations with a mesoscale model capture the main features of the May 2010 flooding event. A pseudo–global warming approach is used to modify the current initial, surface, and boundary conditions by applying thermodynamic changes projected by an ensemble of GCMs for the A2 emission scenario. The observed synoptic pattern of the flooding event is replicated but with modified future thermodynamics, allowing isolation of thermodynamic changes on the moisture feedback. This comparison does not indicate a strengthening of the LLJ in the future simulation. Analysis of the lower-tropospheric potential vorticity evolution reveals that the southern portion of the LLJ over the Gulf of Mexico in this event was strengthened through processes involving the terrain of the Mexican Plateau; this aspect is largely insensitive to climate change. Despite the lack of LLJ strengthening, precipitation in the future simulation increased at a super Clausius–Clapeyron rate because of strengthened convective updrafts.

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Gary M. Lackmann

Abstract

To what extent did large-scale thermodynamic climate change contribute to the intensity and unusual track of Hurricane Sandy, which affected the U.S. mid-Atlantic region in late October 2012? How much of an impact would projected future climate change have on a storm such as Sandy? These questions are investigated using an ensemble of high-resolution numerical simulations in conjunction with analyzed and projected changes from a suite of general circulation models (GCMs). Simulations initialized with current analyses from the midpoint of Sandy’s life cycle, while the system was centered near the Bahamas, adequately replicate the observed intensity and track of Sandy. Initial and boundary condition data are then altered with thermodynamic change fields obtained from a five-member GCM ensemble, allowing hypothetical replication of the synoptic weather pattern that accompanied Hurricane Sandy, but for large-scale thermodynamic conditions corresponding to the 1880s and for projections to the twenty-second century. The past ensemble produces a slightly weaker storm that makes landfall south of the observed location. The future ensemble depicts a significantly more intense system that makes landfall farther north, near Long Island, New York. Within the limitations of the methods used, it is suggested that climate change to date exerted only a modest influence on the intensity and track of Sandy. The strengthening in the simulations run with projected future warming is consistent with increased condensational heating; changes in the synoptic steering flow also appear to result from diabatic processes. The questions of how climate change affected Sandy’s genesis and early life cycle, changes in the frequency of this type of synoptic pattern, and changes in impacts related to coastal development and sea level rise are not considered here.

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Gary M. Lackmann

Abstract

Although Rochester, New York (ROC), is not located in a climatogically favored region for extreme [i.e., ≥30 cm (12 in.) 24 h−1] lake-effect snow (LES), significant [i.e., ≥15 cm (6 in.) 24 h−1] LES can occur there under specific synoptic regimes. The purposes of this study are to document synoptic conditions that are associated with significant LES in ROC and to examine a specific event in which the passage of an upper disturbance combined with a lower-tropospheric trough to produce a surprise western New York snowstorm on 26–27 November 1996.

A database of 127 events in which 2-day ROC snowfall exceeded 15 cm (6 in.) was constructed for the years 1963 through 1992, inclusive. Each event was categorized as “LES” or “non-LES” on the basis of air–lake temperature difference, wind direction, and synoptic setting. Of the 127 events, 32 were classified as LES. Composites based on this 32-case sample reveal a mobile upper trough that moves from the western Great Lakes 48 h prior to the snowfall event to northern Maine 24 h after the event. All 32 cases were accompanied by either a mobile upper trough or a closed low at the 500-hPa level.

An unexpected snowstorm on 26–27 November 1996 resulted in accumulations of up to 30 cm (12 in.) in parts of western New York. Nonclassical LES structures developed in a rapidly changing synoptic environment that was characterized by the passage of an intense upper-tropospheric disturbance. Model forecasts underestimated the strength of this disturbance and also the intensity of lower-tropospheric troughing over and north of Lake Ontario. The upper trough is hypothesized to have increased the inversion altitude and relative humidity in the lower troposphere, and likely contributed to the strength of lower-tropospheric troughing near Lake Ontario. Cyclonic isobaric curvature accompanying the surface trough enhanced lower-tropospheric ascent through Ekman pumping and increased the overwater fetch for boundary layer air parcels traversing Lake Ontario. Comparison of Eta Model forecasts with analyses suggests that problems with model initialization and diabatic boundary layer processes both contributed to forecast errors.

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Gary M. Lackmann
and
Richard M. Yablonsky

Abstract

When water vapor is converted to cloud and precipitation and subsequently removed to the surface via precipitation, there is a corresponding hydrostatic pressure decrease due to the reduction of mass in the overlying column. Pressure changes resulting from the addition or removal of water vapor are currently neglected in most meteorological applications. However, in heavily precipitating systems such as tropical cyclones, where precipitation rates may exceed 250 mm day−1, the pressure equivalent of the precipitation mass sink is not negligible (∼25 hPa day−1). Pressure decreases due to this mechanism are most pronounced in the lower troposphere, particularly below the melting level. The resulting unbalanced pressure-gradient force can enhance convergence, which precludes full realization of the pressure decrease but may contribute to vorticity generation and moisture convergence.

The importance of the precipitation mass sink is investigated for the case of Hurricane Lili (2002) through the computation of mass and potential vorticity (PV) budgets and numerical sensitivity experiments. The precipitation mass reaching the surface within 100 km of the storm center is of the same order as the mass loss needed to explain the area-averaged pressure decrease during the intensification stage of Lili. The PV is altered by precipitation mass flux divergence across isentropic layers. A volume-integrated PV budget reveals that the mass sink term is small in comparison to the latent heating term, although the latter exhibits large cancellation. Comparison of a control simulation from the Eta Model to an experimental simulation in which the precipitation mass sink effect is included demonstrates that the mass sink mechanism contributes to lower pressure, stronger wind speeds, and heavier precipitation. The sea level pressure near the storm center in the mass sink simulation is generally 2–5 hPa deeper relative to the control simulation, with 10-m wind speed differences of 5 to 15 kt. The mass sink simulation exhibits a stronger cyclonic PV tower, especially above the melting level, and a stronger troposphere–deep cyclonic circulation relative to the control simulation. The analysis presented indicates that the precipitation mass sink mechanism, though not dominant, is not negligible for tropical cyclones.

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Kelly M. Mahoney
and
Gary M. Lackmann

Abstract

Analysis of a pair of three-dimensional simulations of mesoscale convective systems (MCSs) reveals a significant sensitivity of convective momentum transport (CMT), MCS motion, and the generation of severe surface winds to ambient moisture. The Weather Research and Forecasting model is used to simulate an idealized MCS, which is compared with an MCS in a drier midlevel environment. The MCS in the drier environment is smaller, moves slightly faster, and exhibits increased descent and more strongly focused areas of enhanced CMT near the surface in the trailing stratiform region relative to that in the control simulation.

A marked increase in the occurrence of severe surface winds is observed between the dry midlevel simulation and the control. It is shown that the enhanced downward motion associated with decreased midlevel relative humidity affects CMT fields and contributes to an increase in the number of grid-cell occurrences of severe surface winds. The role of a descending rear-inflow jet in producing strong surface winds at locations trailing the gust front is also analyzed, and is found to be associated with low-level CMT maxima, particularly in the drier midlevel simulation.

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Kelly M. Mahoney
and
Gary M. Lackmann

Abstract

The sensitivity of numerical model forecasts of coastal cyclogenesis and frontogenesis to the choice of model cumulus parameterization (CP) scheme is examined for the 17 February 2004 southeastern U.S. winter weather event. This event featured a complex synoptic and mesoscale environment, as the presence of cold-air damming, a developing coastal surface cyclone, and an upper-level trough combined to present a daunting winter weather forecast scenario. The operational forecast challenge was further complicated by erratic numerical model predictions. The most poignant area of disagreement between model runs was the treatment of a coastal cyclone and an associated coastal front, features that would affect the location and timing of precipitation and influence the precipitation type. At the time of the event, it was hypothesized that the Betts–Miller–Janjić (BMJ) CP scheme was dictating the location and intensity of the initial coastal cyclone center in operational Eta Model forecasts. For this reason, forecasts for this case were rerun with the workstation Eta Model using the Kain–Fritsch (KF) CP scheme to further examine the sensitivity to this parameterization choice. Results confirm that the model CP scheme played a major role in the forecast for this case, affecting the quantitative precipitation forecast as well as the strength, location, and structure of coastal cyclogenesis and coastal frontogenesis. The Eta Model forecast using the KF CP scheme produced a relatively uniform distribution of convective precipitation oriented along the axis of an inverted trough and strong coastal front. In contrast, the BMJ forecasts resulted in a weaker coastal front and the development of multiple distinct closed cyclonic circulations in association with more localized convective precipitation centers. An additional BMJ forecast in which the shallow mixing component of the scheme was disabled bore a closer semblance to the KF forecasts relative to the original BMJ forecast. Suggestions are provided to facilitate the identification of CP-driven cyclones using standard operational model output parameters.

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Kelly M. Mahoney
and
Gary M. Lackmann

Abstract

Operational forecasters in the southeast and mid-Atlantic regions of the United States have noted a positive quantitative precipitation forecast (QPF) bias in numerical weather prediction (NWP) model forecasts downstream of some organized, cold-season convective systems. Examination of cold-season cases in which model QPF guidance exhibited large errors allowed identification of two representative cases for detailed analysis. The goals of the case study analyses are to (i) identify physical mechanisms through which the upstream convection (UC) alters downstream precipitation amounts, (ii) determine why operational models are challenged to provide accurate guidance in these situations, and (iii) suggest future research directions that would improve model forecasts in these situations and allow forecasters to better anticipate such events. Two primary scenarios are identified during which downstream precipitation is altered in the presence of UC for the study region: (i) a fast-moving convective (FC) scenario in which organized convective systems oriented parallel to the lower-tropospheric flow are progressive relative to the parent synoptic system, and appear to disrupt poleward moisture transport, and (ii) a situation characterized by slower-moving convection (SC) relative to the parent system. Analysis of a representative FC case indicated that moisture consumption, stabilization via convective overturning, and modification of the low-level flow to a more westerly direction in the postconvective environment all appear to contribute to the reduction of downstream precipitation. In the FC case, operational Eta Model forecasts moved the organized UC too slowly, resulting in an overestimate of downstream moisture transport. A 4-km explicit convection model forecast from the Weather Research and Forecasting model produced a faster-moving upstream convective system and improved downstream QPF. In contrast to the FC event, latent heat release in the primary rainband is shown to enhance the low-level jet ahead of the convection in the SC case, thereby increasing moisture transport into the downstream region. A negative model QPF bias was observed in Eta Model forecasts for the SC event. Suggestions are made for precipitation forecasting in UC situations, and implications for NWP model configuration are discussed.

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Gary M. Lackmann
and
James E. Overland

Abstract

Gap winds occur in topographically restricted channels when a component of the pressure gradient is parallel to the channel axis. Aircraft flight-level data are used to examine atmospheric structure and momentum balance during an early spring gap-wind event in Shelikof Strait, Alaska. Alongshore sea level pressure ridging was observed. Vertical cross sections show that across-strait gradients of boundary-layer temperature and depth accounted for the pressure distribution. Geostrophic adjustment of the mass field to the along-strait wind component contributed to development of the observed pressure pattern. Boundary-layer structure and force balance during this event was similar to that often observed along isolated barriers. However, the Rossby radius was lager than the strait width, and atmospheric structure in the strait exit region indicates transition of the flow to open coastline conditions. Two across-strait momentum budgets show that the Coriolis force and across-strait pressure gradient were an order of magnitude larger than other terms. Largest terms in the along-strait balance were the pressure gradient force, acceleration, entrainment, and friction. Boundary-layer acceleration in the along-strait direction was 55% of the potential Emit determined by the along-strait pressure gradient. Entrainment of air into the boundary layer was the largest retarding force and contributed to the along-strait profile of boundary-layer depth. Large horizontal divergence was observed within the strait, yet boundary-layer depth increased slightly following the flow. Entrainment at the inversion and sea surface fluxes accounted for along-strait variation of boundary-layer equivalent potential temperature.

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Lance F. Bosart
and
Gary M. Lackmann

Abstract

An analysis is conducted from a potential vorticity (PV) perspective of the reintensification of Tropical Storm David over the northeastern United States in September 1979. David, a major long-lived hurricane, originated near the Cape Verde Islands in late August 1979. It made final landfall in Georgia on 4 September 1979 and weakened rapidly thereafter. The noteworthy aspect of David was its subsequent reintensification approximately 27 h after landfall as a warm-core disturbance in a weakly baroclinic environment. In this regard the redevelopment of David is unlike the classical extratropical transformation of a tropical storm in a strongly baroclinic environment that has been documented in the literature. The authors' analysis of the evolution of the dynamical tropopause subsequent to storm landfall revealed that David reintensified in response to “tropopause lifting” (upward displacement of the dynamic tropopause) ahead of a nondeepening and otherwise very weak upper-tropospheric disturbance. The “tropospheric lifting,” associated with both advective and diabatic warming poleward and eastward of David, resulted in steepening of the tropopause and compaction of the PV maximum associated with the weak upper-tropospheric disturbance. As the compacted upper-level trough accelerated north-eastward, the associated ascent and low-level horizontal convergence were rendered especially efficient in generating cyclonic vorticity by the neutral stability (relative to the moist adiabat) of the moist tropical air mass surrounding David and the presence of the preexisting low-level vorticity maximum associated with the remnant tropical storm circulation.

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