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

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

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

This study investigates the impact of dynamical downscaling on historical and future projections of winter extratropical cyclones over eastern North America and the western Atlantic Ocean. Six-hourly output from two global circulation models (GCMs), CCSM4 and GFDL-ESM2M, from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used to create the initial and boundary conditions for 20 historical (1986–2005) and 20 future (2080–99) winter simulations using the Weather Research and Forecasting (WRF) Model. Two sets of WRF grid spacing (1.0° and 0.2°) are examined to determine the impact of model resolution. Although the cyclone frequency in the WRF runs is largely determined by the GCM predictions, the higher-resolution WRF reduces the underprediction in cyclone intensity. There is an increase in late-twenty-first-century cyclone activity over the east coast of North America in CCSM4 and its WRF, whereas there is little change in GFDL-ESM2M and WRF given that there is a larger decrease in the temperature gradient in this region. There is a future increase in relatively deep cyclones over the East Coast in the high-resolution WRF forced by CCSM4. These storms are weaker than the historical cases early in their life cycle, but then because of latent heating they rapidly develop and become stronger than the historical events. This increase does not occur in the low-resolution WRF or the high-resolution WRF forced by GFDL since the latent heat increase is relatively small. This implies that the diabatic processes during cyclogenesis may become more important in a warmer climate, and these processes may be too weak in existing coarse-resolution GCMs.

Abstract

This study investigates the future change in precipitation associated with extratropical cyclones over eastern North America and the western Atlantic during the cool season (November–March) through the twenty-first century. A cyclone-relative approach is applied to 10 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) in order to isolate precipitation changes for different cyclone intensities and storm life cycle, as well as determine the relevant physical processes associated with these changes. The historical analysis suggests that models with better performance in predicting extratropical cyclones tend to have smaller precipitation errors, and the ensemble mean has a smaller mean absolute error than the individual models. By the late-twenty-first century, the precipitation amount associated with cyclones increases by 5%–25% over the U.S. East Coast, with about 90% of the increase from the relatively strong (<990 hPa) and moderate (990–1005 hPa) cyclones. Meanwhile, the precipitation rate increases by 15%–25% over the U.S. East Coast for the strong cyclone centers, which is larger than the moderate and weak cyclones. The relatively strong cyclones just inland of the U.S. East Coast have the largest increase (~30%) in precipitation rate, since these centers over land have the largest increase in low-level temperature (and moisture), a decrease (5%–13%) in the static stability, and an increase (~5%) in upward motion during the late-twenty-first century. This east coast region also has an increase in cyclone intensity in the future even though there is a decrease in low-level baroclinicity, which suggests that the latent heat release from heavier precipitation contributes to this storm deepening.

Abstract

This paper investigates the microphysical pathways and sensitivities within the Reisner2 bulk microphysical parameterization (BMP) of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) for a precipitation event over the central Sierra Nevada on 12 February 1986. Using a single sounding initialization, the MM5 was run two-dimensionally at 2-km horizontal grid spacing, which was needed to realistically simulate the embedded convective cells within the orographic cloud. Unlike previous modeling studies of this event, a microphysical budget over the windward slope was calculated for each experiment, in which the importance of each microphysical process was quantified relative to the water vapor loss (WVL) rate. For the control MM5, the largest microphysical processes that contribute to surface precipitation over the Sierra windward slope are condensation (63% of WVL), snow deposition (33%), riming to form graupel (35%), and melting of graupel (28%). The amount of supercooled water aloft is larger than observed and in previous modeling studies of this event using the Regional Atmospheric Modeling System (RAMS). The surface precipitation and microphysical processes over the Sierra Nevada are most sensitive to those parameters associated with the snow distribution, cloud condensation nuclei (CCN) concentrations, and snow/graupel fall speeds, while there is less sensitivity to ice initiation and autoconversions; however, all experiments overpredict the surface precipitation over the windward slope. If ice production is turned off in the cloud-ice region (above 7 km or <250 K), deposition acting on the small amount of cloud ice nucleated at warmer temperatures can still generate a similar snow cloud below 4 km and surface precipitation. The precipitation differences between the BMPs in the MM5 are greater than any single process experiment within Reisner2. The process experiments do help reveal some of the fundamental differences between BMP schemes.

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

This paper explores the structural evolution and physical processes that explain the modification of two quasi-linear convective systems (QLCSs) that encountered the densely populated New York City–Atlantic coastal region. One QLCS on 31 May 2002 traversed the Atlantic coastal boundary with little change in its intensity, producing widespread severe wind damage across New York City and Long Island. During this event, warm air advection at 925 hPa helped destabilize the layer above this level over the coastal zone, while the marine boundary layer deepened below this level. The 0–3-km line-perpendicular vertical wind shear was relatively strong, which supported ascent along the leading edge of the diabatically generated cold pool. The surface-based convective system became slightly elevated as it moved over the marine waters. In contrast, the 23 July 2002 QLCS decayed upon encountering the Atlantic coastline, despite its coincidence with a surface cold front. The most unstable CAPE values during this decaying event were 400–800 J kg⁻¹ greater than the sustaining 31 May event, though the 0–3-km vertical wind shear was approximately half. Weaker shear likely contributed to limited ascent along the leading edge of the surface based cold pool, and ultimately the demise of the convective line. Sensitivity tests highlight the importance of the relationship between the cold pool and vertical shear during these two events, and illustrate the limited role of the marine layer in modifying the evolution of these two convective systems.

Abstract

Future changes in the frequency of environmental conditions conducive for convective storm days (“CE days”) are determined for the northeastern United States (NEUS) during the warm seasons (April–September) of the twenty-first century. Statistical relationships between historical runs of seven models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) and radar-classified convective storm days are developed using linear discriminant analysis (LDA), and these relationships are then applied to analyze changes in the convective environment under the high-emissions representative concentration pathway 8.5 (RCP8.5) scenario over the period 2006–99. The 1996–2007 warm seasons are used to train the LDA thresholds using convective precipitation from two reanalysis datasets and radar data, and the 1979–95 and 2008–10 warm seasons are used to verify these thresholds. For the CMIP5 historical period (1979–2005), the frequency of warm season CE days averaged across the CMIP5 models is slightly greater than that derived using reanalysis data, although both methods indicate a slight increasing trend through the historical period. Between 2006 and 2099, warm season CE day frequency is predicted to increase substantially at an average rate of 4–5 days decade⁻¹ (50%–80% increase over the entire period). These changes are mostly attributed to a predicted 30%–40% increase in midlevel precipitable water between the historical period and the last few decades of the twenty-first century. Consistent with previous studies, there is decreasing deep-layer vertical wind shear as a result of a weakening horizontal temperature gradient, but this is outweighed by increases in instability led by the moisture increases.