Search Results

You are looking at 1 - 10 of 85 items for

  • Author or Editor: Roy Rasmussen x
  • All content x
Clear All Modify Search
István Geresdi and Roy Rasmussen

Abstract

This paper investigates how the characteristics of aerosol particles (size distribution and solubility) as well as the presence of giant nuclei affect drizzle formation in stably stratified layer clouds. A new technique was developed to simulate the evolution of water drops from wet aerosol particles and implemented into a detailed microphysical model. The detailed microphysical model was incorporated into a one-dimensional parcel model and a two-dimensional version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). Sensitivity experiments were performed with the parcel model using a constant updraft speed and with the two-dimensional model by simulating flow over a bell-shaped mountain. The results showed that 1) stably stratified clouds with weak updrafts (<10 cm s−1) can form drizzle relatively rapidly for maritime size distributions with any aerosol particle solubility, and for continental size distributions with highly insoluble particles due to the low number of activated cloud condensation nuclei (CCN) (<100 cm−3), 2) drizzle is suppressed in stably stratified clouds with weak updrafts (<10 cm s−1) for highly soluble urban and extreme urban size distributions, and 3) the presence of giant nuclei only has an effect on drizzle formation for the highly soluble continental aerosol size distributions.

Full access
Robert Tardif and Roy M. Rasmussen

Abstract

To gain insights into the poorly understood phenomenon of precipitation fog, this study assesses the evaporation of freely falling drops departing from equilibrium as a possible contributing factor to fog formation in rainy conditions. The study is based on simulations performed with a microphysical column model describing the evolution of the temperature and mass of evaporating raindrops within a Lagrangian reference frame. Equilibrium defines a state where the latent heat loss of an evaporating drop is balanced by the sensible heat flux from the ambient air, hence defining a steady-state drop temperature. Model results show that the assumption of equilibrium leads to small but significant errors in calculated precipitation evaporation rates for drops falling in continuously varying ambient near-saturated or saturated conditions. Departure from equilibrium depends on the magnitude of the vertical gradients of the ambient temperature and moisture as well as the drop-size-dependent terminal velocity. Contrasting patterns of behavior occur depending on the stratification of the atmosphere. Raindrops falling in inversion layers remain warmer than the equilibrium temperature and lead to enhanced moistening, with supersaturation achieved when evaporation proceeds in saturated inversions. Dehydration occurs in layers with temperature and water vapor increasing with height due to the vapor flux from the environment to the colder drops. These contrasts are not represented when equilibrium is assumed. The role of nonequilibrium raindrop evaporation in fog occurrences is further emphasized with simulations of a case study characterized by fog forming under light rain falling in a developing frontal inversion. Good agreement is obtained between fog water content observations and simulations representing only the effects of rainfall evaporation. This study demonstrates the need to take into account the nonequilibrium state of falling raindrops for a proper representation of an important mechanism contributing to precipitation fog occurrences.

Full access
Robert Tardif and Roy M. Rasmussen

Abstract

The character of fog in a region centered on New York City, New York, is investigated using 20 yr of historical data. Hourly surface observations are used to identify fog events at 17 locations under the influence of various physiographic features, such as land–water contrasts, land surface character (urban, suburban, and rural), and terrain. Fog events at each location are classified by fog types using an objective algorithm derived after extensive examination of fog formation processes. Events are characterized according to frequency, duration, and intensity. A quantitative assessment of the likelihood with which mechanisms leading to fog formation are occurring in various parts of the region is obtained. The spatial, seasonal, and diurnal variability of fog occurrences are examined and results are related to regional and local influences. The results show that the likelihood of fog occurrence is influenced negatively by the presence of the urban heat island of New York City, whereas it is enhanced at locations under the direct influence of the marine environment. Inland suburban and rural locations also experience a considerable amount of fog. As in other areas throughout the world, the overall fog phenomenon is a superposition of various types. Precipitation fog, which occurs predominantly in winter, is the most common type. Fog resulting from cloud-base lowering also occurs frequently across the region, with an enhanced likelihood in winter and spring. A considerable number of advection fog events occur in coastal areas, mostly during spring, whereas radiation fog occurs predominantly at suburban and rural locations during late summer and early autumn but also occurs during the warm season in the coastal plain of New Jersey as advection–radiation events.

Full access
Robert Tardif and Roy M. Rasmussen

Abstract

An analysis of the environmental conditions associated with precipitation fog events is presented using 20 yr of historical observations taken in a region centered on New York, New York. The objective is to determine the preferred weather scenarios and identify physical processes influencing the formation of fog during precipitation. Salient synoptic-scale features are identified using NCEP–NCAR reanalyses. Local environmental parameters, such as wind speed and direction, temperature, and humidity, are analyzed using surface observations, while the vertical structure of the lower atmosphere is examined using available rawinsonde data. The analysis reveals that precipitation fog mostly occurs as a result of the gradual lowering of cloud bases as continuous light rain or light drizzle is observed. Such scenarios occur under various synoptic weather patterns in areas characterized by large-scale uplift, differential temperature advection, and positive moisture advection. Precipitation fog onset typically occurs with winds from the northeast at inland locations and onshore flow at coastal locations, with flows from the south to southwest aloft. A majority of the cases showed the presence of a sharp low-level temperature inversion resulting from differential temperature advection or through the interaction of warm air flowing over a cold surface in onshore flow conditions. This suggests a common scenario of fog formation under moistening conditions resulting from precipitation evaporating into colder air near the surface. A smaller number of events formed with cooling of the near-saturated or saturated air. Evidence is also presented of the possible role of shear-induced turbulent mixing in the production of supersaturation and fog formation during precipitation.

Full access
Roy M. Rasmussen, Andrew Crook, and Cathy Kessinger

Abstract

The formation and evolution of convective rain and snow bands prior to and during the crash of Continental Airlines flight 1713 on 15 November 1987 at Denver Stapleton Airport are discussed. Convective rain occurred during the early stages of the storm in association with the approach of an upper-level trough from the west. Snow bands were observed following the passage of a shallow Canadian cold front from the north. These bands formed above the cold front and moved from southeast to northwest at 7 m s−1 with a horizontal spacing of 10–30 km. The winds within the cloud layer were southeasterly from 5 to 10 m s−1, suggesting that the bands were advected by the mean, cloud-layer flow. The most likely mechanism producing these bands was a convective instability in the shear layer above the cold front.

As the upper-level trough moved to the east, the winds in the cloud layer shifted to northerly, causing the bands to move southward with the major axis of the band oriented north–south. The high snowfall rate just prior to the takeoff of flight 1713 occurred as a result of one of these north–south–oriented bands moving over Denver Stapleton Airport from the north during the latter stages of the storm.

Full access
Roy M. Rasmussen and Andrew J. Heymsfield

Abstract

A detailed model of the melting, shedding, and wet growth of spherical graupel and hail is presented. This model is based upon recent experimental studies by Rasmussen et al. and Lesins et al. The model is presented in the form of five easy-to-use tables. Important quantities considered were the heat transfer. terminal velocity behavior, and shedding of liquid water.

Full access
Roy M. Rasmussen, Piotr Smolarkiewicz, and John Warner

Abstract

This paper presents a detailed comparison study of three-dimensional model results with an aircraft wind field mapping for the island of Hawaii. Model runs were initialized using an aircraft sounding from 1 August 1985, and detailed predictions from the model are compared with observations from that day.

The strength and location of the upwind convergence zone were well simulated, as well as the strong deflection and deceleration of the flow around the island and the geometry and location of the upstream cloud bands. The good agreement between the model results and observations supports the results of our previous study in which we show that the flow pattern and associated cloud processes around the island of Hawaii can be understood by considering the flow of a stably stratified fluid around a large three-dimensional obstacle.

Model runs with different wind directions showed that increasing northerly tradewind flow resulted in the band clouds moving closer to the shore line, and the large scale flow pattern rotating counterclockwise. Model results were also compared with various aspects of the island climatology, and good agreement was found in both the temporal and spatial distribution of precipitation on the island.

Full access
Roy M. Rasmussen and Piotr K. Smolarkiewicz

Abstract

This paper extends our earlier discussion of the flow past the island of Hawaii and the accompanying cloud band to smaller-scale effects occurring on the scale of the Hilo Bay region. The evolution of cloud bands forming upwind of the island on 1 August 1985 is studied using a high-resolution numerical model and available field observations. The current work provides further evidence in support of the view that the phenomenon of Hawaiian cloud bands is closely linked to the dynamics of strongly stratified flows past three-dimensional obstacles. In particular, results are presented that document the cloud interaction with a secondary, vertically propagating gravity wave and the formation of horizontally oriented vortices in the lower upwind flow—two characteristic features encountered in studies of idealized low Froude number flows. Quantification of the effects due to nocturnal thermal forcing is attempted, and it is shown that cooling along the volcano slope doubles the depth of the dynamically induced downslope flow as well as its maximum wind speed, whereas it has a little effect upon the position of the mesoscale convergence line and coinciding leading edge of the downslope current. Downslope surges of cold air from the volcano slope are shown to temporarily enhance the depth and strength of the downslope flow, leading to invigorated cloud development at the leading edge of the current. Analysis of the Hawaiian Rainband Project (HaRP) sounding data relates cloud bands to the theory of squall lines and suggests that the trade wind environment upstream of the island is favorable to the formation of cloud bands consisting of isolated cells advected by the local cloud-layer winds.

Full access
István Geresdi, Lulin Xue, and Roy Rasmussen

Abstract

A new version of a bin microphysical scheme implemented into the Weather Research and Forecasting (WRF) Model was used to study the effect of glaciogenic seeding on precipitation formation in orographic clouds. The tracking of silver iodide (AgI) particles inside of water drops allows the proper simulation of the immersion nucleation. The ice formations by deposition, condensational freezing, and contact nucleation of AgI particles are also simulated in the scheme. Cloud formation—both stably stratified and convective—and the spread of AgI particles were simulated by idealized flow over a two-dimensional (2D) bell-shaped mountain. The results of numerical experiments show the following: (i) Only the airborne seeding enhances precipitation in stably stratified layer clouds. Seeding can reduce or enhance precipitation in convective clouds. AgI seeding can significantly affect the spatial distribution of the surface precipitation in orographic clouds. (ii) The positive seeding effect is primarily due to additional diffusional growth of AgI-nucleated ice crystals in layer clouds. In convective clouds, seeding-induced changes of both diffusion and riming processes determine the seeding effect. (iii) The seeding effect is inversely related to the natural precipitation efficiency. (iv) Bulk seeding parameterization is adequate to simulate AgI seeding impacts on wintertime orographic clouds. More uncertainties of ground-seeding effects are found between bulk and bin simulations.

Full access
Roy M. Rasmussen and Andrew J. Heymsfield

Abstract

The 1 August severe storm during the Cooperative Convective Precipitation Experiment (CCOPE) has been analyzed making use of T-28 aircraft data, CP-2 radar data, and a particle trajectory model in conjunction with a time-averaged Doppler-derived wind field. This analysis reveals that water drops are shed from wet hailstones in this storm, and that some of them are shed in favorable locations to grow back into hailstones. A particular region of the storm is identified as the most likely source region for shed drops, and particle trajectory calculations show that hailstones grown from this region fall out in locations consistent with the reflectivity structure of the storm. Some trajectories are shown to fall back through this same region while at the same time shedding, providing new hail embryos.

Full access