Abstract

This paper examines the extratropical transition (ET) of Hurricane Floyd along the U.S. East Coast on 16–17 September 1999 using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) down to 1.33-km horizontal grid spacing. The 36-h MM5 simulation reproduced the basic features of the ET event such as the track of Floyd, the development of a deep and intense baroclinic zone along the coast and its associated precipitation evolution, and the tendency for the heavy (>30 cm) precipitation to fall in a relatively narrow (30–40 km wide) band just inland of the coast; however, the MM5 overpredicted the moderate (10–20 cm) precipitation amounts near the coast by 40%–50% as the horizontal grid spacing was reduced to 1.33 km.

The MM5 was used to diagnose the evolution of the enhanced baroclinic zone and associated heavy precipitation to the north of Floyd. A deep layer of deformation frontogenesis extended from the surface to 400 mb as a result of confluence between the southeasterlies to the northeast of Floyd at all levels and the inland northeasterlies and southwesterlies at low and midlevels, respectively. A combination of strong frontogenesis, moist symmetric instability below 800 mb, and slantwise neutrality aloft resulted in the narrow and intense band of precipitation just inland of the coast. A separate simulation without the Appalachians and coastal terrain had little effect on Floyd's wind and temperature evolution, and heavy precipitation (>30 cm) still developed just inland of the coast; therefore, terrain played a relatively minor role in the devastating flooding for this particular event over the Northeast.

Frontogenesis calculations revealed that the upper-level baroclinic zone over the northeast United States was enhanced by a horizontal gradient in midlevel latent heating between the heavy precipitation near the coast and the lighter precipitation farther inland. This was also verified by completing a simulation without latent heating, which resulted in much less baroclinicity and downstream ridging aloft. In addition, without latent heating, the central pressure of Floyd was 25 mb weaker than the full-physics (control) run, and the storm only slowly moved up the coast. Without evaporative effects from precipitation, the low-level front was 10%–20% weaker than the control, and Floyd's central pressure was about 4 mb weaker. Another simulation without surface heat fluxes resulted in a 4–5 mb weaker cyclone, and 20%–30% less precipitation shifted 100–150 km farther eastward than the control.

Abstract

This paper presents two-dimensional (2D) idealized simulations at 1-km grid spacing using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) in order to illustrate how a series of ridges along a broad windward slope can impact the precipitation distribution and simulated microphysics. The number of windward ridges for a 2000-m mountain of 50-km half-width is varied from 0 to 16 over a 150-km distance using different stratifications, freezing levels, uniform ambient flows, and ridge amplitudes.

A few (200–400 m) windward ridges can enhance the precipitation locally over each ridge crest by a factor of 2–3. Meanwhile, a series of 8–16 ridges that are 200–400 m in height can increase the net precipitation averaged over the windward slope by 10%–35%. This average precipitation enhancement is maximized when the ridge spacing is relatively small (<20 km), since there is less time for subsidence drying within the valleys and the mountain waves become more evanescent, which favors a simple upward and downward motion couplet over each ridge. In addition, small ridge spacing is shown to have a synergistic effect on precipitation over the lower windward slope, in which an upstream ridge helps increase the precipitation over the adjacent downwind ridge. There is little net precipitation enhancement by the ridges for small moist Froude numbers (Fr < 0.8), since flow blocking limits the flow up and over each ridge. For a series of narrow ridges (∼10 km wide), the largest precipitation enhancement for a 500-mb freezing level occurs over lower windward slope of the barrier through warm-rain processes. In contrast, a 1000-mb freezing level has the largest precipitation enhancement over the middle and upper portions of a barrier for a series of narrow (∼10 km wide) ridges given the horizontal advection of snow aloft.

Abstract

This paper utilizes the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) mesoscale model (MM5) in a two-dimensional (2D) configuration at 4-km horizontal grid spacing in order to better understand the relationship between orographic precipitation and the height and width of a barrier, as well as the ambient flow, uniform moist static stability, and freezing level. The focus is on how these parameters affect the orographic precipitation by changing the circulation and microphysical structures over the barrier.

As the low-level flow becomes blocked for moist nondimensional mountain heights greater than 3.0, there is a rapid upstream shift in the precipitation maximum and a reduction in precipitation over the upper windward slope. For the terrain geometries used in this study (500 to 3500 m high and 25- to 50-km half-width), the maximum precipitation is a strong function of barrier slope for relatively weak upstream flow (U = 10 m s⁻¹). For moderate wind speeds (U = 20 m s⁻¹) and a freezing level of 750 mb, melting effects lower the freezing level more along the windward slope as the mountain half-width and height increases for barrier slopes greater than 0.03. As a result, a low (1000 m) and narrow (25-km half-width) barrier has a greater surface precipitation maximum than a high (2000 m) and wide (50-km half-width) mountain of equivalent slope since the smaller barrier has more efficient warm rain processes occurring along the windward slope. For wind speeds greater than 20 m s⁻¹, a wider and higher barrier has a greater precipitation maximum since it has a more extensive orographic cloud, while a narrower barrier has more precipitation advecting into the lee.

The precipitation distribution is highly dependent on how the terrain-induced gravity wave modifies the circulation aloft. Even in the unblocked flow regime, the precipitation builds upstream of the crest for winds greater than 20 m s⁻¹, since strong flow favors a large vertical wavelength of the mountain gravity wave, and therefore a deep layer of upward motion over the lower windward slope. Both a narrower barrier and weaker stability favor less tilt to the mountain wave, resulting in a more collapsed circulation above the crest and more precipitation spillover. Reverse shear above the crest favors low-level wave amplification and a windward shift in the precipitation, while forward shear favors a weaker mountain wave over the crest and more precipitation advection into the lee. Finally, a freezing level raised from 750 to 500 mb collapses the precipitation distribution over the windward slope with less leeside spillover, therefore the windward precipitation efficiency remains high (>90%) at strong (>20 m s⁻¹) wind speeds.

Abstract

Northerly surges of cold air often move southward along the eastern side of the Rockies from southern Canada into Mexico. The strongest surges, which generally develop in midwinter, are associated with temperature decreases and pressure rises of 20°–30°C and 15–30 mb, respectively, within 24 h. Surges are usually accompanied by a meridionally elongated pressure ridge and strong low-level ageostrophic winds that parallel the terrain. The width of the pressure ridging is approximately 1000 km over the southern plains but decreases to only a few hundred kilometers when the surge enters Mexico.

This paper provides a detailed description of a northerly surge to the east of the Rocky Mountains that occurred on 12–14 November 1986. Using both observational and model data, the structural evolution of the surge is analyzed; in addition, the dynamics of the event is explored by diagnosing the momentum, thermodynamic energy, and vorticity equations. To determine the typical synoptic-scale evolution of these cold surges, a composite study is also presented. It is concluded that these cold surges result primarily from the interaction of the evolving synoptic-scale flow with the Rocky Mountains and the sloping topography of the Great Plains, and not from the generation of rotationally trapped waves such as Kelvin, shelf, and topographic Rossby waves. When an upper-level short-wave trough moves southeastward out of western Canada, northerlies, high pressure, and cold air spread southward into the northern plains at low levels. Lee troughing occurs to the east of the central and southern Rockies and, in concert with the ridging to the north, establishes an along-barrier pressure gradient that forces ageostrophic northerly flow and the meridional advection of cold air. Blocked upslope flow at the forward portion of the surge leads to large-scale damming. As the surge enters Mexico, where the topography becomes steeper and the large-scale slope is lost, the width of the damming is greatly reduced. Consistent with damming, momentum diagnostics over both the Great Plains and coastal Mexico indicate that an antitriptic balance exists parallel to the mountains, whereas a geostrophic balance exists normal to the barrier.

Abstract

A new bulk microphysical parameterization (BMP) scheme is presented that includes a diagnosed riming intensity and its impact on ice characteristics. As a result, the new scheme represents a continuous spectrum from pristine ice particles to heavily rimed particles and graupel using one prognostic variable [precipitating ice (PI)] rather than two separate variables (snow and graupel). In contrast to most existing parameterization schemes that use fixed empirical relationships to describe ice particles, general formulations are proposed to consider the influences of riming intensity and temperature on the projected area, mass, and fall velocity of PI particles. The proposed formulations are able to cover the variations of empirical coefficients found in previous observational studies. The new scheme also reduces the number of parameterized microphysical processes by ∼50% as compared to conventional six-category BMPs and thus it is more computationally efficient.

The new scheme (called SBU-YLIN) has been implemented in the Weather Research and Forecasting (WRF) model and compared with three other schemes for two events during the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) over the central Oregon Cascades. The new scheme produces surface precipitation forecasts comparable to more complicated BMPs. The new scheme reduces the snow amounts aloft as compared to other WRF schemes and compares better with observations, especially for an event with moderate riming aloft. Sensitivity tests suggest both reduced snow depositional growth rate and more efficient fallout due to the contribution of riming to the reduction of ice water content aloft in the new scheme, with a larger impact from the partially rimed snow and fallout.

Abstract

A spatial and temporal climatology of convective storms over the Northeast United States during the warm season (April–September) is presented using composite National Operational Weather radar (NOWrad) data at 2-km grid spacing from 1996 to 2007 as well as cloud-to-ground lightning from the National Lightning Data Network (NLDN) on a 10-km grid from 2001 to 2007. There are preferred regions for convective storms within New York’s Hudson River Valley, western and southeastern Pennsylvania, central New Jersey, and across the Delmarva Peninsula. A favored initiation area during the early afternoon is the immediate lee of the Appalachians, with a tendency for these convective systems to move eastward to the coast by late evening. There is a sharp gradient in convective frequency within 20 km of the coast on average as a result of the relatively stable marine boundary layer, but as the sea surface warms by midsummer this convective activity increases near the coast, with a nocturnal (0600–1200 UTC) convective maximum over the coastal waters. Convective frequency can vary by more than 40% interannually across subregions of the Northeast. There was 40%–50% more convection across southern New England and Long Island during 1998–2001 than in 2002–05, which was partially the result of more frequent and amplified trough activity in 1998–2001, which helped to trigger convective storms.

Spatial composites using the North American Regional Reanalysis (NARR) highlight some of the synoptic flow patterns associated with the enhanced convective frequencies. Convective storms tend to weaken rapidly from west to east across southern New England when there is low-level southerly flow from the relatively cool ocean. Severe convection is favored over the populated New York City and Long Island coastal regions when there is warm, moist, and unstable air extending northward along the mid-Atlantic coastal plain, with west-southwesterly flow at low levels and an approaching shortwave trough at midlevels.

Abstract

Three-dimensional idealized simulations using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) down to 6-km grid spacing were performed in order to understand how different ambient conditions (wind speed and direction, stability, and inland cold pool) and terrain characteristics impact barrier jets along the southeastern Alaskan coast. The broad inland terrain of western North America is important in Alaskan jet development, since it rotates the impinging flow cyclonically (more coast parallel) well upstream of the coast, thus favoring more low-level flow blocking while also adding momentum and width to the barrier jet. Near the steep coastal terrain, the largest wind speed enhancement factor (1.9–2.0) in the terrain-parallel direction relative to the ambient onshore-directed wind speed occurs at relatively low Froude numbers (Fr ∼ 0.3–0.4). These low Froude numbers are associated with (10–15 m s⁻¹) ambient wind speeds and wind directions orientated 30°–45° from terrain-parallel. For simulations with an inland cold pool and nearly coast-parallel flow, strong gap outflows develop through the coastal mountain gaps, shifting the largest wind speed enhancement to Fr < 0.2. The widest barrier jets occur with ambient winds oriented nearly terrain-parallel with strong static stability. The gap outflows shift the position of the jet maximum farther offshore from the coast and increase the jet width. The height of the jet maxima is typically located at the top of the shallow gap outflow (∼500 m MSL), but without strong gap outflows, the jet heights are located at the top of the boundary layer, which is higher (lower) for large (small) frictionally induced vertical wind shear and weak (strong) static stability.

Abstract

This paper highlights the observed and simulated microphysical evolution of a moderate orographic rainfall event over the central Oregon Cascade Range during 4–5 December 2001 of the Second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2). Airborne in situ measurements illustrate the spatial variations in ice crystal distributions and amounts over the windward Cascades and within some convective cells. The in situ microphysical observations, ground radars, and surface observations are compared with four bulk microphysical parameterizations (BMPs) within the Weather Research and Forecasting (WRF) model. Those WRF BMP schemes that overpredict surface precipitation along the Cascade windward slopes are shown to have too rapid graupel (rimed snow) fallout. Most BMP schemes overpredict snow in the maximum snow depositional growth region aloft, which results in excessive precipitation spillover into the immediate lee of the Cascades. Meanwhile, there is underprediction to the east of the Cascades in all BMP schemes. Those BMPs that produce more graupel than snow generate nearly twice as much precipitation over the Oregon Coast Range as the other BMPs given the cellular convection in this region. Sensitivity runs suggest that the graupel accretion of snow generates too much graupel within select WRF BMPs. Those BMPs that generate more graupel than snow have shorter cloud residence times and larger removal of available water vapor. Snow depositional growth may be overestimated by 2 times within the BMPs when a capacitance for spherical particles is used rather than for snow aggregates. Snow mass–diameter relationships also have a large impact on the snow and cloud liquid water generation. The positive definite advection scheme for moisture and hydrometeors in the WRF reduces the surface precipitation by 20%–30% over the Coast Range and improves water conservation, especially where there are convective cells.

Abstract

Organized convective structures over the northeastern United States were classified for two warm seasons (May–August) using 2-km composite radar [i.e., the National Operational Weather Radar (NOWrad)] data. Nine structures were identified: three types of cellular convection (clusters of cells, isolated cells, and broken lines), five types of linear convection (lines with no stratiform precipitation, lines with trailing stratiform precipitation, lines with parallel stratiform precipitation, lines with leading stratiform precipitation, and bow echoes), and one nonlinear system. The occurrence of all structures decreases from the western Appalachian slopes eastward to the Atlantic coast. Isolated cellular convection forms primarily during the morning to late afternoon (1200–2100 UTC) mainly over the high terrain. Clusters of cells form primarily over the Appalachians and the Atlantic coastal plain during the daytime (1200–0000 UTC). Linear convection is favored from midafternoon to early evening (1800–0000 UTC) over land areas. Nonlinear systems develop mainly from midafternoon to late evening (1800–0600 UTC) over the inland areas and over the coastal zone during the early morning (∼1200 UTC).

Composites using the North American Regional Reanalysis (NARR) highlight the ambient conditions for three main convective structures: cellular, linear, and nonlinear. Cellular convection initiates with limited quasigeostrophic forcing and moderate instability [i.e., average most unstable CAPE (MUCAPE) ∼973 J kg⁻¹]. A majority of cells develop in orographically favored upslope areas. Linear convection organizes along surface troughs, supported by 900-hPa frontogenesis and an average ambient MUCAPE of ∼1011 J kg⁻¹. Nonlinear convection organizes along warm fronts associated with larger-scale baroclinic systems, and the MUCAPE is relatively small (∼207 J kg⁻¹).

Abstract

This paper describes the leeside wind event of 12 February 1995 in which a swath of strong low-level easterly flow, with gusts to 35–40 m s⁻¹, extended downwind of a major gap in the Cascade Mountains. Over the Puget Sound lowlands, low-level easterly flow approaching the Olympics was decelerated to less than 5 m s⁻¹, resulting in enhanced (over 30 cm) snowfall along the windward side of the barrier. In addition to conventional synoptic observations, high-resolution data sources were utilized: profilers on both sides of the Cascades, a WSR-88D Doppler radar located in northern Puget Sound, and nonhydrostatic model simulations down to 1-km resolution. With the exception of underpredicting heavy snowfall to the east of the Olympics, many of the mesoscale features of this event were realistically simulated, including cold-air damming over eastern Washington, strong winds downwind of the gaps, a mountain wake downstream of Mount Rainier, and flow splitting around the Olympics.

A major objective of this study was to determine the role of the three-dimensional topography of the Cascades on the leeside windstorm and the relative importance of gap flow and mountain wave accelerations. Low-level model trajectories showed a tendency for the air to pass westward through the major gaps in the Cascades. However, additional simulations, in which the major Cascade gap was slowly removed, indicated that filling the lower passes of the gap (<1600 m) had only a small effect on the leeside winds due to the considerable depth of the cold air (∼2800 m). When the lowest terrain was increased to 2000 m, the barrier intersected the lower portion of the stable layer aloft and the leeside winds were dramatically reduced. A simulation without the Olympics showed that early in the event, upstream blocking by this barrier decelerated the winds over and to the west of central Puget Sound. As cold air pooled within the Puget Sound basin, a structure similar to a hydraulic jump was enhanced above the Cascade lee slopes, weakening the winds along the lower Cascade foothills substantially.