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.

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.

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.

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.

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.

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.

Abstract

Observations demonstrate that the radius of maximum winds in tropical cyclones (TCs) can vary by an order of magnitude; similar size differences are evident in other spatial measures of the wind field as well as in cloud and precipitation fields. Many TC impacts are related to storm size, yet the physical mechanisms that determine TC size are not well understood and have received limited research attention. Presented here is a hypothesis suggesting that one factor controlling TC size is the environmental relative humidity, to which the intensity and coverage of precipitation occurring outside the TC core is strongly sensitive. From a potential vorticity (PV) perspective, the lateral extent of the TC wind field is linked to the size and strength of the associated cyclonic PV anomalies. Latent heat release in outer rainbands can result in the diabatic lateral expansion of the cyclonic PV distribution and balanced wind field. Results of idealized numerical experiments are consistent with the hypothesized sensitivity of TC size to environmental humidity. Simulated TCs in dry environments exhibit reduced precipitation outside the TC core, a narrower PV distribution, and reduced lateral extension of the wind field relative to storms in more moist environments. The generation of diabatic PV in spiral bands is critical to lateral wind field expansion in the outer portion of numerically simulated tropical cyclones. Breaking vortex Rossby waves in the eyewall lead to an expansion of the eye and the weakening of inner-core PV gradients in the moist environment simulation. Feedback mechanisms involving surface fluxes and the efficiency of diabatic PV production with an expanding cyclonic wind field are discussed.

Abstract

The Weather Research and Forecasting Advanced Research Model (WRF-ARW) was used to perform idealized tropical cyclone (TC) simulations, with domains of 36-, 12-, and 4-km horizontal grid spacing. Tests were conducted to determine the sensitivity of TC intensity to the available surface layer (SL) and planetary boundary layer (PBL) parameterizations, including the Yonsei University (YSU) and Mellor–Yamada–Janjic (MYJ) schemes, and to horizontal grid spacing. Simulations were run until a quasi-steady TC intensity was attained. Differences in minimum central pressure (P _min) of up to 35 hPa and maximum 10-m wind (V _10max) differences of up to 30 m s⁻¹ were present between a convection-resolving nested domain with 4-km grid spacing and a parent domain with cumulus parameterization and 36-km grid spacing. Simulations using 4-km grid spacing are the most intense, with the maximum intensity falling close to empirical estimates of maximum TC intensity. Sensitivity to SL and PBL parameterization also exists, most notably in simulations with 4-km grid spacing, where the maximum intensity varied by up to ∼10 m s⁻¹ (V _10max) or ∼13 hPa (P _min). Values of surface latent heat flux (LHFLX) are larger in MYJ than in YSU at the same wind speeds, and the differences increase with wind speed, approaching 1000 W m⁻² at wind speeds in excess of 55 m s⁻¹. This difference was traced to a larger exchange coefficient for moisture, C_Q , in the MYJ scheme. The exchange coefficients for sensible heat (C_θ ) and momentum (C_D ) varied by <7% between the SL schemes at the same wind speeds. The ratio C_θ /C_D varied by <5% between the schemes, whereas C_Q /C_D was up to 100% larger in MYJ, and the latter is theorized to contribute to the differences in simulated maximum intensity. Differences in PBL scheme mixing also likely played a role in the model sensitivity. Observations of the exchange coefficients, published elsewhere and limited to wind speeds <30 m s⁻¹, suggest that C_Q is too large in the MYJ SL scheme, whereas YSU incorporates values more consistent with observations. The exchange coefficient for momentum increases linearly with wind speed in both schemes, whereas observations suggest that the value of C_D becomes quasi-steady beyond some critical wind speed (∼30 m s⁻¹).