Abstract

The Regional Atmospheric Modeling System was used to simulate four winter snowfall events over the Park Range of Colorado. For each event, three hygroscopic aerosol sensitivity simulations were performed with initial aerosol profiles representing clean, moderately polluted, and highly polluted scenarios. Previous work demonstrates that the addition of aerosols can produce a snowfall spillover effect, during events in which riming growth of snow is prevalent in the presence of supercooled liquid water, that is due to a modified orographic cloud containing more numerous but smaller cloud droplets. This study focuses on the detailed microphysical processes that lead to snow growth in each event and how these processes are modulated by the addition of hygroscopic aerosols. A conceptual model of hydrometeor growth processes is presented, along a vertical orographic transect, that reveals zones of vapor deposition of ice and liquid, riming growth, evaporation, sublimation, and regions in which the Wegener–Bergeron–Findeisen (WBF) snow growth process is active. While the aerosol-induced spillover effect is largely determined by the degree of reduction in ice particle riming, an enhancement in the WBF snow growth process under more polluted conditions largely offsets the loss of rime growth, thus leading to a minimal net change in the regional precipitation.

Abstract

Hygroscopic pollution aerosols have the potential to alter winter orographic snowfall totals and spatial distributions by modification of high-elevation supercooled orographic clouds and the riming process. The authors investigate the cumulative effect of varying the concentrations of hygroscopic aerosols during January–February for four recent winter snowfall seasons over the high terrain of Colorado. Version 6.0 of the Regional Atmospheric Modeling System (RAMS) is used to determine the particular mountain ranges and seasonal conditions that are most susceptible. Multiple winter seasonal simulations are run at both 3- and 1-km horizontal grid spacing with varying aerosol vertical profiles. Model-predicted snowfall accumulation trends are compared with automated snow water equivalent observations at high-elevation sites. An increase in aerosol concentration leads to reduced riming of cloud water by ice particles within supercooled, liquid orographic clouds, thus leading to lighter rimed hydrometers with slower fall speeds and longer horizontal trajectories. This effect results in a spillover of snowfall from the windward slope to the leeward slope. A snowfall spillover effect is most evident in the southern and western regions of the San Juan Range where high-moisture-laden storms are more prevalent. The effect over the Park Range is also present in each simulated season, but with lower amplitudes and slightly varying magnitudes among seasons. Seasons with greater overall snowfall exhibit a greater response in magnitude and percentage change. The smallest spillover effect occurred downwind of the primary western slope mountain barriers. Although the aerosol effect on snowfall can be locally significant in particularly wet winter seasons, the interseasonal variability in synoptic conditions can impose much larger widespread changes in snowfall accumulation.

Abstract

This paper studies the feasibility of retrieving aerosol concentrations in the planetary boundary layer (PBL) with a variational data assimilation (VDA) algorithm using dual-wavelength lidar returns and visual range simulated at multiple times. Aerosols are assumed to consist of nucleation, accumulation, and coarse modes, with each mode distributed in a gamma distribution. The VDA algorithm retrieves initial vertical profiles of aerosol concentrations of the three modes, which are predicted by a 1D PBL model. The accuracy of retrieved aerosol concentrations of the three modes is examined through a series of identical twin numerical experiments. For the VDA algorithm that uses data from a lidar wavelength pair (0.289, 11.15 μm), results show that 1) if both random and systematic errors in the observed data are less than 1.0 dB and the number densities of accumulation and coarse modes are, respectively, 0.025–0.25 and 0–1.25 × 10⁻³ times that of the nucleation mode, relative errors in the retrieved aerosol concentrations are 12%–110% for the nucleation mode, 9%–40% for the accumulation, and 3%–25% for the coarse mode; 2) the accuracy of retrieved aerosol concentrations is slightly (greatly) affected by the errors in relative humidity (RH) if RH is less (greater) than 95%, and moderately by the vertical scales of initial aerosol concentration fields; 3) systematic errors in the observed data can severely reduce the accuracy of retrieved aerosol concentrations; and 4) the VDA algorithm is more accurate than traditional methods that use single-time data.

Moreover, a method is developed to retrieve systematic errors in the observed data. Results show that if systematic errors in lidar returns are less than about 2 dB, retrieving systematic errors can increase the accuracy of retrieved aerosol concentrations, especially for the accumulation mode. The method to retrieve systematic errors in the observed data can find applications in other retrieval problems. Finally, assimilation of ceilometer data (wavelength 0.904 μm) is explored through investigating data from wavelength pairs: (0.289, 0.904 μm) and (0.904, 11.15 μm).

Experience in performing real-time mesoscale numerical prediction forecasts using the Regional Atmospheric Modeling System (RAMS) over Colorado for a winter season on high-performance workstations is summarized. Performance evaluation is done for specific case studies and, statistically, for the entire winter season. RAMS forecasts are also compared with nested grid model forecasts. In addition, RAMS precipitation forecasts with a simple “dump bucket” scheme are compared with explicit, bulk microphysics parameterization schemes. The potential applications and political/social problems of having a readily accessible, real-time mesoscale forecasting capability on low-cost, high-performance workstations is discussed.

Abstract

An unseasonal, severe downslope windstorm along the eastern foothills of the Colorado Rocky Mountains is described. The storm, which occurred on 3 July 1993, produced wind guts in Fort Collins, Colorado, over 40 m s⁻¹ and resulted in extensive tree and roof damage. The synoptic pattern preceding the wind event resembled a pattern typical of that for a Front Range late fall or wintertime wind storm, including a strong south–southwest-oriented height gradient at 700 mb and a strong west to east sea level pressure gradient across the Front Range. A particularly interesting facet of the event was that one small geographical area in and near Fort Collins experienced wind gusts nearly 40% stronger than any other location involved in the event.

The mesoscale forecast version of the Regional Atmospheric Modeling System (RAMS) with 16-km grid spacing over Colorado was run for the storm. Consistent severe winds were not predicted by the model in this configuration. Increasing resolution in postanalysis to a 4-km grid spacing along the Front Range resulted in severe downslope winds but of too strong a magnitude. The addition of explicit, bulk microphysics moderated the forecast wind strengths to observed magnitudes. That is, both a grid spacing of ∼4 km and the use of explicit bulk microphysics were required to produce an accurate representation of the downslope winds observed.

Abstract

Deep convection initiated by sea breezes over the Florida peninsula is simulated using a two-dimensional nonhydrostatic model. Reasonable agreement is obtained between model results and observations for the three types of undisturbed days classified by Blanchard and López. The convergence of the east and west coast sea breezes is the primary control on the timing and location of rapid convective development, and this is mainly determined by the low-level winds. The simulated convection is spatially concentrated and does not produce an extensive stratiform region.

Sensitivity tests are performed for a variety of wind and thermodynamic profiles, and for different soil moisture contents. During the early stages of these simulations, small convective cells develop in between the sea-breeze fronts. As the outer cells at the sea-breeze fronts deepen these smaller cells are suppressed. Typically, during the midafternoon a new cell explosively develops in between the sea-breeze fronts and the outer cells usually decay, although merger occasionally occurs. The decay of convection, subsequent to this rapid development, can generate very deep horizontally propagating gravity waves. A considerable amount of the convective available potential energy released, and associated subsidence warming is carried away from the convective region by these deep gravity-wave modes having a horizontal propagation speed much larger than the ambient winds. Model output is analyzed to examine the precipitation patterns, heat and moisture budgets, radiational heating and momentum transports.

Abstract

The development of two small mesoscale convective systems (MCSs) in northeastern Colorado is investigated via dual-Doppler radar analysis. The first system developed from several initially isolated cumulonimbi, which gradually coalesced into a minimal MCS with relatively little stratiform precipitation. The second system developed more rapidly along an axis of convection and generated a more extensive and persistent stratiform echo and MCS cloud shield. In both systems, the volumetric precipitation rate exhibited an early meso-β-scale convective cycle (a maximum and subsequent minimum), followed by reintensification into a modest mature stage. This sequence is similar to that noted previously in the developing stage of larger MCSs by McAnelly and Cotton. They speculated that the early meso-β convective cycle is a characteristic feature of development in many MCSs that is dynamically linked to a rather abrupt transition toward mature stage structure. This study presents kinematic evidence in support of this hypothesis for these cases, as derived from dual-Doppler radar analyses over several-hour periods. Mature stage MCS characteristics such as deepened low- to midlevel convergence and mesoscale descent developed fairly rapidly, about 1 h after the early meso-β convective maximum.

The dynamic linkage between the meso-β convective cycle and evolution toward mature structure is examined with a simple analytical model of the linearized atmospheric response to prescribed heating. Heating functions that approximate the temporal and spatial characteristics of the meso-β convective cycle are prescribed. The solutions show that the cycle forces a response within and near the thermally forced region that is consistent with the observed kinematic evolution in the MCSs. The initial response to an intensifying convective ensemble is a self-suppressing mechanism that partially explains the weakening after a meso-β convective maximum. A lagged response then favors reintensification and areal growth of the weakened ensemble. A conceptual model of MCS development is proposed whereby the early meso-β convective cycle and the response to it are hypothesized to act as a generalized forcing–feedback mechanism that helps explain the upscale growth of a convective ensemble into an organized MCS.

Abstract

A composite analysis technique is used to investigate the evolution of mesoscale features of mesoscale convective complexes (MCCs). The early stage of the MCC lifecycle is characterized by convergence, vertical motion and heating being centered in the lower troposphere. As the MCC matures the level of peak upward motion and heating shifts to the upper troposphere. The system achieves and maintains its maximum divergence, upward motion, and anticyclonic vorticity in the upper troposphere during the latter half of the life cycle. This is in contrast to GATE tropical clusters where the maximum divergence, upward motion, and anticyclonic vorticity occurred at the mature stage of the cluster and then weakened. This difference might be explained by an MCC being an inertially stable form of mesoscale convective system whose radius exceeds the Rossby radius of deformation.

The MCC is shown to be an efficient rain producer, exhibiting a precipitation efficiency exceeding 100% at the mature stage due to the accumulation of water substance in the stratiform anvil cloud during the earlier, predominantly convective stages of the system.

Abstract

A generalized forward-in-time upstream advective operator following the methodology of Crowley is developed and advective schemes of orders 1 through 10 are tested analytically and numerically. Flux forms of these schemes are also derived with Crowley’s methodology. It is shown that thew flux forms do not reduce to the advective form with constant velocity and grid spacing for schemes of order 3 and higher and they are not as accurate as the advective form. A new flux form is derived which does reduce to the advective form under the conditions of constant velocity and grid spacing.

The schemes were tested in two dimensions using time splitting. In the rotating cone test, the advective and new flux forms performed identically, while the other flux form had larger dissipation and dispersion errors. In the deformational flow field test, the advective forms were unstable for both time steps tested. Use flux forms were less unstable for the higher Courant number and the domain as a whole was stable for the lower Courant number.

With respect to order of the various forms, the schemes performed consistent with the linear stability analysis; errors decrease as the order of the scheme becomes greater. The sixth-order schemes appear to be the best balance between efficiency and accuracy.

Abstract

Using data collected during Colorado State University's South Park Area Cumulus Experiment in 1977, a sequence of multi-scale convective events leading to the formation of a mesoscale convective complex is described. In the first phase, surface-based cool advection in the elevated mountain basin delayed the full transition of the morning boundary layer into a deep mixed layer until well after convective instability was reached over the adjacent ridges. The second phase was earmarked by the formation of convective precipitation echoes at “hot spots” over the high mountain terrain. Two groups of cells then propagated. eastward across the mountain basin, forming a line of discrete cells which moved across the foothills toward the High Plains. The cells further intensified at the. foothills/High Plains interface and formed a still larger, north-south line of thunderstorms. In the third phase, this north–south line of thunderstorms evolved into an expanding meso-β-scale convective cluster as it continued its eastward propagation over eastern Colorado. The convective intensity of the line was apparently modulated by moisture availability over the plains, with the southern cells being most intense initially. As the northern end of the line encountered greater low-level moisture in western Kansas, the convection rapidly intensified to severe levels and produced in excess of 50 mm of precipitation over a large area. In Part II of this article it is shown that this meso-β-scale system participated in the formation of a meso-&α-scale convective complex.