Abstract

Over mountainous terrain, windward enhancement of stratiform precipitation results from a combination of warm-rain and ice-phase processes. In this study, ice-phase precipitation processes are investigated within frontal systems during the Olympic Mountains Experiment (OLYMPEX). An enhanced layer of radar reflectivity (Z _H) above the melting level bright band (i.e., a secondary Z _H maximum) is observed over both the windward slopes of the Olympic Mountains and the upstream ocean, with a higher frequency of occurrence and higher Z _H values over the windward slopes indicating an orographic enhancement of ice-phase precipitation processes. Aircraft-based in situ observations are evaluated for the 1–2 and 3 December 2015 orographically enhanced precipitation events. Above the secondary Z _H maximum, the hydrometeors are primarily horizontally oriented dendritic and branched crystals. Within the secondary Z _H maximum, there are high concentrations of large (>~2-mm diameter) dendrites, plates, and aggregates thereof, with a significant degree of riming. In both events, aggregation and riming appear to be enhanced within a turbulent layer near sheared flow at the top of a low-level jet impinging on the terrain and forced to rise above the melting level. Based on windward ground sites at low, mid-, and high elevations, secondary Z _H maxima periods during all of OLYMPEX are associated with increased rain rates and larger mass-weighted mean drop diameters compared to periods without a secondary Z _H maximum. This result suggests that precipitation originating from secondary Z _H maxima layers may contribute to enhanced windward precipitation accumulations through the formation of large, dense particles that accelerate fallout.

Abstract

A major objective of the North American Monsoon Experiment (NAME) was to quantify microphysical processes within convection occurring near the steep topography of northwestern Mexico. A previous study compared examples of isolated convection using polarimetric radar data and noted a dependence on mixed-phase processes via drop freezing and subsequent riming growth along the coastal plain and western slopes, with an even greater role of melting ice in rainfall production over the highest terrain. Despite the higher frequency of these isolated cells compared to organized convective systems, the latter were responsible for 75% of rainfall. Therefore, this study seeks to evaluate the role of mesoscale organization on microphysical processes and describes the evolution of these systems as a function of topography.

Similar to isolated convection, both warm-rain and ice-based processes played important roles in producing intense rainfall in organized convection. Although similarities existed between cell types, organized convection was typically deeper and contained greater ice mass, which melted and contributed to the development of outflow boundaries. As convection organized along the slopes, these boundaries spread over the lower terrain, converging with diurnally driven upslope flow, thus allowing for the generation of new convection and propagation toward the coast. Once over lower elevations, additional warm-cloud depth contributed to intense rainfall and allowed for continued ice production. This, along with the development of rear inflow in the trailing stratiform region, led to further development of convective outflow, similar to organized systems in the tropics and midlatitudes.

Abstract

To address questions regarding microphysical processes occurring in the core North American monsoon region, data from NCAR’s S-band polarimetric Doppler radar (S-Pol) deployed during the North American Monsoon Experiment (NAME) in the summer of 2004, were used to investigate the location, size, and type of hydrometeors in convection. A cell identification and tracking algorithm was applied to this data over 100 h of microphysical scans, characterized by increased temporal and vertical resolution, to locate and track individual convective elements. Only isolated cells over land were included for this study to investigate potential elevation-dependent trends in microphysical processes in this region.

Examples of intense, isolated convection over all elevations revealed deep cells and polarimetric signatures comparable to other studies of tropical and midlatitude convection. A case over the low terrain highlighted deep, isolated convection with precipitation-sized ice extending to 15 km. In addition, the presence of differential reflectivity Z _DR columns in these cells indicated the lofting of supercooled water above the melting level, and an enhanced linear depolarization ratio L _DR “cap” above the column implied subsequent freezing to produce graupel. Similar features were also observed in an isolated cell over the western slopes, highlighting the combined roles of collision–coalescence and melting precipitation-sized ice for producing intense rainfall over the lower elevations. Despite previous observations of weaker and shallower cells with less precipitation ice over the Sierra Madre Occidental (SMO), case studies and general statistics using polarimetric data reveal the potential for accretional processes to also play an important role in producing intense rainfall over these higher elevations. For these isolated SMO cells, reduced warm-cloud depths, increased ice mass observed just above the melting level, and a narrower distribution of drop sizes suggest a reduced role of warm-rain processes compared to intense cells over the lower terrain. A potential relationship between microphysical processes and degree of organization is also hypothesized and will be the focus of a future study.

Abstract

The diurnal cycle of the local circulation, rainfall, and heat and moisture budgets is investigated in Taiwan's heavy rain (mei-yu) season using data from the 2008 Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX). Comparisons are made between an undisturbed (UNDIST; 22–29 May) and disturbed period (DIST; 31 May–4 June). Many aspects of the diurnal evolution in surface flows and rainfall were similar during both periods. At night and during early morning hours, the low-level southwesterly flow was deflected around Taiwan's main topographic barrier, the Central Mountain Range (CMR), with rainfall focused near areas of enhanced offshore confluence created by downslope and land-breeze flows. During the day, the flow switched to onshore and upslope, rainfall shifted inland, and deep convection developed along the coastal plains and windward slopes. Atmospheric budget analysis indicates a day-to-evening transition of convective structure from shallow to deep to stratiform. Evaporation associated with the evening/nighttime stratiform precipitation likely assisted the nocturnal katabatic flow.

Though the flow impinging on Taiwan was blocked during both periods, a very moist troposphere and strengthened low-level oncoming flow during DIST resulted in more widespread and intense rainfall that was shifted to higher elevations, which resembled a more weakly blocked regime. Correspondingly, storm cores were tilted upslope during DIST, in contrast to the more erect storms characteristic of UNDIST. There were much more lofted precipitation-sized ice hydrometeors within storms during DIST, the upslope advection of which led to extensive stratiform rain regions overlying the CMR peaks, and the observed upslope shift in rainfall.

Abstract

Radar data from the 2004 North American Monsoon Experiment (NAME) enhanced observing period were used to investigate diurnal trends and vertical structure of precipitating features relative to local terrain. Two-dimensional composites of reflectivity and rain rate, created from the two Servicio Meteorológico Nacional (SMN; Mexican Weather Service) C-band Doppler radars and NCAR’s S-band polarimetric Doppler radar (S-Pol), were divided into four elevation groups: over water, 0–1000 m (MSL), 1000–2000 m, and greater than 2000 m. Analysis of precipitation frequency and average rainfall intensity using these composites reveals a strong diurnal trend in precipitation similar to that observed by the NAME Event Rain Gauge Network. Precipitation occurs most frequently during the afternoon over the Sierra Madre Occidental (SMO), with the peak frequency moving over the lower elevations by evening. Also, the precipitation events over the lower elevations are less frequent but of greater intensity (rain rate) than those over the SMO. Precipitation echoes were partitioned into convective and stratiform components to allow for examination of vertical characteristics of convection using data from S-Pol. Analyses of reflectivity profiles and echo-top heights confirm that convection over the lower terrain is more intense and vertically developed than convection over the SMO. Warm-cloud depths, estimated from the Colorado State University–NAME upper-air and surface gridded analyses are, on average, 2 times as deep over the lower terrain as compared with over the SMO. Using a simplified stochastic model for drop growth, it is shown that these differences in warm-cloud depths could possibly explain the observed elevation-dependent trends in precipitation intensity.

Abstract

Subtropical South America (SSA) east of the Andes Mountains is a global hotspot for mesoscale convective systems (MCSs). Wide convective cores (WCCs) are typically embedded within mature MCSs, contribute over 40% of SSA’s warm-season rainfall, and are often associated with severe weather. Prior analysis of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data identified WCCs in SSA and associated synoptic conditions during austral summer. As WCCs also occur during the austral spring, this study uses the 16-yr TRMM PR and ERA5 datasets to compare anomalies in environmental conditions between austral spring (SON) and summer (DJF) for the largest and smallest WCCs in SSA. During both seasons, large WCCs are associated with an anomalous midlevel trough that slowly crosses the Andes Mountains and a northerly South American low-level jet (SALLJ) over SSA, though the SON trough and SALLJ anomalies are stronger and located farther northeastward than in DJF. A synoptic pattern evolution resembling large WCC environments is illustrated through a multiday case during the RELAMPAGO field campaign (10–13 November 2018). Unique high-temporal-resolution soundings showed strong midlevel vertical wind shear associated with this event, induced by the juxtaposition of the northerly SALLJ and southerly near-surface flow. It is hypothesized that the Andes help create a quasi-stationary trough–ridge pattern such that favorable synoptic conditions for deep convection persist for multiple days. For the smallest WCCs, anomalously weaker synoptic-scale forcing was present compared to the largest events, especially for DJF, pointing to future work exploring MCS formation under weaker synoptic conditions.

Abstract

The 2014–15 Observations and Modeling of the Green Ocean Amazon (GOAmazon) field campaign over the central Amazon near Manaus, Brazil, occurred in coordination with the larger Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud-Resolving Modeling and to the Global Precipitation Measurement (CHUVA) project across Brazil. These programs provide observations of convection over the central Amazon on diurnal to annual time scales. In this study, we address the question of how Kelvin waves, observed in satellite observations of deep cloud cover over the GOAmazon region during the 2014–15 time period, modulate the growth, type, and organization of convection over the central Amazon. The answer to this question has implications for improved predictability of organized systems over the region and representation of convection and its growth on local to synoptic scales in global models. Our results demonstrate that Kelvin waves are strong modulators of synoptic-scale low- to midlevel free-tropospheric moisture, integrated moisture convergence, and surface heat fluxes. These regional modifications of the environment impact the local diurnal cycle of convection, favoring the development of mesoscale convective systems. As a result, localized rainfall is also strongly modulated, with the majority of rainfall in the GOAmazon region occurring during the passage of these systems.

Abstract

The Madden–Julian oscillation (MJO) dominates the intraseasonal variability of cloud populations of the tropical Indian and Pacific Oceans. Suppressed MJO periods consist primarily of shallow and isolated deep convection. During the transition to an active MJO, the shallow and isolated deep clouds grow upscale into the overnight hours. During active MJO periods, mesoscale convective systems occur mostly during 2–4-day bursts of rainfall activity with a statistically significant early morning peak. Yet when these rain events are separated into individual active periods, some periods do not follow the mean pattern, with the November events in particular exhibiting an afternoon peak. The radar-observed microphysical processes producing the precipitation during the major rain events of active MJO periods evolve in connection with synoptic-scale wave passages with varying influences of diurnal forcing. MJO studies that do not account for the intermittency of rainfall during active MJO phases through averaging over multiple events can lead to the misimpression that the primary rain-producing clouds of the MJO are modulated solely by the diurnal cycle.

Abstract

El Niño–Southern Oscillation (ENSO) is known to have teleconnections to atmospheric circulations and weather patterns around the world. Previous studies have examined connections between ENSO and rainfall in tropical South America, but little work has been done connecting ENSO phases with convection in subtropical South America. The Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) has provided novel observations of convection in this region, including that convection in the lee of the Andes Mountains is among the deepest and most intense in the world with frequent upscale growth into mesoscale convective systems. A 16-yr dataset from the TRMM PR is used to analyze deep and wide convection in combination with ERA-Interim reanalysis storm composites. Results from the study show that deep and wide convection occurs in all phases of ENSO, with only some modest variations in frequency between ENSO phases. However, the most statistically significant differences between ENSO phases occur in the three-dimensional storm structure. Deep and wide convection during El Niño tends to be taller and contain stronger convection, while La Niña storms contain stronger stratiform echoes. The synoptic and thermodynamic conditions supporting the deeper storms during El Niño is related to increased convective available potential energy, a strengthening of the South American low-level jet (SALLJ), and a stronger upper-level jet stream, often with the equatorward-entrance region of the jet stream directly over the convective storm locations. These enhanced synoptic and thermodynamic conditions provide insight into how the structure of some of the most intense convection on Earth varies with phases of ENSO.

Abstract

The Remote sensing of Electrification, Lightning, And Mesoscale/microscale Processes with Adaptive Ground Observations (RELAMPAGO) campaign produced unparalleled observations of the South American low-level jet (SALLJ) in central Argentina with high temporal observations located in the path of the jet and upstream of rapidly growing convection. The vertical and temporal structure of the jet is characterized using 3-hourly soundings launched at two fixed sites near the Sierras de Córdoba (SDC), along with high-resolution reanalysis data. Objective SALLJ identification criteria are applied to each sounding to determine the presence, timing, and vertical characteristics of the jet. The observations largely confirm prior results showing that SALLJs most frequently come from the north, occur overnight, and peak in the low levels, though SALLJs notably peaked higher near the end of longer-duration events during RELAMPAGO. This study categorizes SALLJs into shorter-duration events with jet cores peaking overnight in the low levels and longer 5–6-day events with elevated jets near the end of the period that lack a clear diurnal cycle. Evidence of both boundary layer processes and large-scale forcing were observed during shorter-duration events, whereas synoptic forcing dominated the longer 5–6-day events. The highest amounts of moisture and larger convective coverage east of the SDC occurred near the end of the 5–6-day SALLJ events.