Gary M. Lackmann

Influence of Environmental Humidity on Tropical Cyclone Size

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 hypothe...

Sensitivity of Simulated Tropical Cyclone Structure and Intensity to Horizontal Resolution

Abstract The Weather Research and Forecasting (WRF) model is used to test the sensitivity of simulations of Hurricane Ivan (2004) to changes in horizontal grid spacing for grid lengths from 8 to 1 km. As resolution is increased, minimum central pressure decreases significantly (by 30 hPa from 8- to 1-km grid spacing), although this increase in intensity is not uniform across similar reductions in grid spacing, even when pressure fields are interpolated to a common grid. This implies that the additional strengthening of the simulated tropical cyclone (TC) at higher resolution is not attributable to sampling, but is due to changes in the representation of physical processes important to TC intensity. The most apparent changes in simulated TC structure with resolution occur near a grid length of 4 km. At 4-km grid spacing and below, polygonal eyewall segments appear, suggestive of breaking vortex Rossby waves. With sub-4-km grid lengths, localized, intense updraft cores within the eyewall are numerous and bo...

Cold-frontal potential vorticity maxima, the low-level jet, and moisture transport in extratropical cyclones

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 maxi...

Atmospheric structure and momentum balance during a gap-wind event in Shelikof Strait, Alaska

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 w...

Analysis of Idealized Tropical Cyclone Simulations Using the Weather Research and Forecasting Model: Sensitivity to Turbulence Parameterization and Grid Spacing

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 (Pmin) of up to 35 hPa and maximum 10-m wind (V10max) 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...

The Impact of Future Climate Change on TC Intensity and Structure: A Downscaling Approach

AbstractA comprehensive analysis of tropical cyclone (TC) intensity change in a warming climate is undertaken with high-resolution (6- and 2-km grid spacing) idealized simulations using the Weather Research and Forecasting (WRF) model. With the goal of isolating the influence of thermodynamic aspects of climate change on maximum hurricane intensity, an idealized TC is placed within a quiescent, horizontally uniform tropical environment computed from averaged reanalysis data for the tropical Atlantic Ocean. The analyzed tropical environment is used for control simulations. Changes between the periods 1990–99 and 2090–99 are computed using output from 13 GCMs from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), for the A1B, A2, and B1 emissions scenarios. These changes are then added to the reanalysis-derived initial and boundary conditions used in the control simulations. Some processes known to impact TC intensity, such as environmental vertical wind shear and sea surf...

Heavy Cold-Season Precipitation in the Northwestern United States: Synoptic Climatology and an Analysis of the Flood of 17-18 January 1986

Abstract Warm, moist southwesterly airflow into the northwestern United States during the cold season can result in rapid snowmelt and flooding. The objectives of this research are to document characteristic synoptic flow patterns accompanying cold-season (November–March) flooding events, and isolate flow anomalies associated with the moisture transport during a representative event. The first objective is accomplished through a 46-case composite spanning the years 1962–88; the second objective is addressed through diagnosis of a flooding event that occurred on 17–18 January 1986. The 46-case composite is constructed for a 6-day period centered at 1200 UTC on the day of heavy precipitation onset (denoted τ0). Composite 500-hPa geopotential height anomaly fields reveal anomalous ridging over the Bering Sea preceding the precipitation event, a negative anomaly over the Gulf of Alaska throughout the composite evolution, and a positive anomaly over the southwestern Unites States and adjacent eastern Pacific O...

A Satellite-Derived Classification Scheme for Rapid Maritime Cyclogenesis

Abstract Guided by the conjecture that there exist characteristic synoptic-scale flow patterns conducive to extratropical cyclogenesis, a satellite-based classification scheme is proposed to differentiate between various types of rapid maritime cyclogenesis in the western North Atlantic region. The scheme is derived from signatures in visible and infrared satellite imagery observed prior to and during rapid deepening. Consideration of the western North Atlantic region is motivated by the presence of a relatively dense upstream observational network and by the absence of the direct influence of orography in a maritime environment; the focus on rapid development is predicated upon the assumption that the cloud signatures will be more clearly defined than in cases of slower, “ordinary” development. Examination of satellite imagery for an ensemble of 50 cyclogenesis events that occurred during the 1970s and 1980s, 46 of which satisfied the Experiment on Rapidly Intensifying Cyclones over the Atlantic criterio...

Moisture Transport Diagnosis of a Wintertime Precipitation Event in the Mackenzie River Basin

Abstract Wintertime precipitation events in the Mackenzie River basin (MRB) play an important role in the hydrology of the region because they contribute substantially to water storage prior to the spring runoff maximum. The Mesoscale Compressible Community (MC2) Model is used to simulate a representative wintertime MRB precipitation event. The MC2 simulation, gridded analyses, and raw observations are used to (i) document meteorological conditions associated with the precipitation event, (ii) assess the ability of the model to reproduce the precipitation event and antecedent large-scale moisture transport, and (iii) identify which planetary- and synoptic-scale features are responsible for the observed moisture transport using piecewise quasigeostrophic potential vorticity (QGPV) inversion. Precipitation in the MRB develops north of an intense frontal boundary as a southwesterly flow of moisture originating over the Pacific Ocean is lifted over cold, dense arctic air near the surface. A lee cyclone forms ...

The Influence of Incipient Latent Heat Release on the Precipitation Distribution of the 24–25 January 2000 U.S. East Coast Cyclone

Abstract The role of a diabatically produced lower-tropospheric potential vorticity (PV) maximum in determining the precipitation distribution of the 24–25 January 2000 U.S. East Coast cyclone is investigated. Operational numerical weather prediction (NWP) models performed poorly with this storm, even within 24 h of the event, as they were unable to properly forecast the westward extent of heavy precipitation over the Carolinas and mid-Atlantic. The development of an area of incipient precipitation (IP) around 0600 UTC 24 January over the southeastern United States prior to rapid cyclogenesis was also poorly forecasted by the operational NWP models. It is hypothesized that the lower-tropospheric diabatic PV maximum initially produced by the IP was important to subsequent inland moisture transport over the Carolinas and mid-Atlantic. A PV budget confirms that latent heat release in the midtroposphere associated with the IP led to the initial formation of a PV maximum in the lower troposphere that propagate...