Abstract

The Monsoon Himalayan Precipitation Experiment (MOHPREX) occurred during June 2001 along the south slopes of the Himalayas in central Nepal. Radiosondes were launched around the clock from two sites, one in the Marsyandi River basin on the eastern footslopes of the Annapurna range, and one farther to the southwest near the border with India. The flights supported rainfall and other hydrometeorological observations (including surface winds) from the Marsyandi network that has been operated in this region since the spring of 1999. The thermodynamic profiles obtained from the soundings support the observed nocturnal maximum in rainfall during the monsoon, with total column moisture and instability maximized just before rainfall peaks. Coinciding with the appearance of a monsoon depression over central India, the onset of the monsoon in this region was characterized by a weeklong weakening of the upper-level westerlies, and an increase in moisture and convective instability. The vertical structure of convection during the project was intense at times, and frequent thunder and lightning were observed. This is suggestive of monsoon break convection, which is expected to be predominant since the monsoon had not fully matured by the end of the month. Comparisons of the MOHPREX data with the NCEP–NCAR reanalysis data reveal that upper-level winds are characterized relatively well by the reanalysis, taking into account the coarse model topography. However, moisture is severely underestimated, leading to significant underestimation of rainfall by the reanalysis. The interaction of the ambient monsoon flow with the south slopes of the Himalayas, modulated by the diurnal variability of atmospheric state, is suggested as the primary cause of the nocturnal peak in rainfall.

Abstract

A framework for the statistical analysis of large radar and lightning datasets is described and implemented in order to analyze two research questions in atmospheric electricity: storms dominated by positive cloud-to-ground (+CG) lightning and estimating the probability of lightning in convection. The framework—a collection of computer programs running in series—is fully modular, allowing the analysis of a variety of datasets based on a study’s objectives, including radar observations, lightning data, observations of meteorological environments, and other data. The framework has been applied to over 2 months of observations of 28 463 cells. The results suggest that +CG-dominated cells contain midlevel positive charge (−10° to −30°C), in contrast to cells dominated by −CG lightning, which typically had positive charge at upper (near −40°C) and lower levels (0° to −10°C). The +CG cells also were larger and more intense, and were associated with environments that were more convectively favorable—in terms of increased moisture, shear, and especially instability—when compared to −CG cells. The framework was also used to examine the probability of lightning occurrence for a spectrum of radar structures. The existence of 30-dBZ echo above the freezing altitude is a “necessary” condition (in ~90% of cases) for lightning occurrence. A “sufficient” condition (in ~90% of cases) is 40-dBZ echo breaching the freezing altitude. Altitude or volume of 40-dBZ echo was the superior estimator for the occurrence of lightning, while 30 dBZ was better for inferring the lack of lightning.

Abstract

Combined multiparameter radar, dual-Doppler, thermodynamic sounding, and lightning observations of 11 thunderstorms (6 from the midlatitudes, 5 from the Tropics) are examined. The thunderstorms span a wide spectrum of intensities, from weak monsoontype to severe tornadic, and include both unicellular and multicellular convection. In general, the kinematically strongest storms featured lower production of negative cloud-to-ground lightning (typically <1 min⁻¹ flash rates for large portions of the storms' lifetimes) when compared with more moderate convection, in accord with an elevated charge mechanism. The only significant differences between intense storms that produced predominately positive cloud-to-ground (CG) lightning for a significant portion of their lifetimes (PPCG storms) and intense storms that produced little CG lightning of any polarity (low-CG storms) was that PPCG storms featured much larger volumes of significant updrafts (both >10 and >20 m s⁻¹) and produced greater amounts of precipitation (both rain and hail). Otherwise, peak updrafts and vertical airmass fluxes were very similar between the two types of storms, and both types were linked by anomalously low production of negative CG lightning. PPCG effects in storms may result from an elevated region of negative charge (reducing negative CG flash rates) combined with enhanced net positive charge regions created by the larger volume of significant updrafts.

Abstract

The Marsyandi River basin in the central Nepalese Himalayas is a topographically complex region, with strong spatial gradients of precipitation over various timescales. A meteorological network consisting of 20 stations was installed at a variety of elevations (528–4435 m) in this region, and measurements of rainfall were made during the 1999 and 2000 summer monsoons. The onsets of the 1999 and 2000 monsoons in central Nepal were examined at different spatial scales by using a combination of rain gauge, Meteosat-5, Tropical Rainfall Measuring Mission (TRMM), ECMWF analysis, and Indian radiosonde data. At the network, the onsets manifested themselves as multiday rain events, which included a mixture of stratiform and convective precipitation. Moist and unstable upslope flow was associated with the occurrence of heavy rainfall. During each onset, 2-day rainfall reached as high as 462 mm, corresponding to 10%–20% of the monsoon rainfall. Differences among rain gauges were up to a factor of 8, reflecting the role of small-scale terrain features in modulating rainfall amounts. At the larger scale, the onsets were associated with monsoon depressions from the Bay of Bengal that moved close enough to the Himalayas to cause the observed upslope flow from the winds on their eastern flank. During the 1999 onset, convection in this eastern flank collided with the mountains in the vicinity of the network. In 2000 no major collision occurred, and 33%–50% less rain than 1999 fell. Analysis of observations for a 5-yr period (1997–2001) suggests that the interannual variability of the monsoon onset along the Himalayan range is linked to the trajectories and strength of these depressions.

Abstract

A pyrocumulus is a convective cloud that can develop over a wildfire. Under certain conditions, pyrocumulus clouds become vertically developed enough to produce lightning. NEXRAD dual-polarization weather radar and upgraded National Lightning Detection Network (NLDN) data were used to analyze 10 case studies of ash plumes and pyrocumulus clouds from 2013 that either did or did not produce detected lightning. Past research has shown that pyrocumulus cases are most likely to produce lightning when there is a decrease in differential reflectivity (toward 0 dB) and an increase in the correlation coefficient (to >0.8), as measured by polarimetric radar, due to the transition from pure smoke/ash to frozen hydrometeors. All pyrocumulus cases that produced lightning in this study displayed the polarimetric characteristics of rimed ice within their respective clouds. Time series analysis of radar-inferred ash and rimed ice volumes within ash plumes and pyrocumulus clouds showed that NLDN-detected lightning occurred only after the cloud contained significant amounts of precipitation-sized rimed ice. The results suggest that the recently dual-pol-enabled NEXRADs and the more sensitive NLDN network can be used to explore ash plume and pyrocumulus microphysical structure and lightning production.

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

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

On a few occasions during the summer and fall of 2002, and again in the fall of 2003, the Colorado State University (CSU)–University of Chicago–Illinois State Water Survey (CHILL) S-band polarimetric Doppler radar observed dumbbell-shaped radar echo patterns in precipitation-free air returns. Dumbbell shaped refers to two distinct and quasi-symmetrical regions of echo surrounding the radar. These were horizontally widespread (thousands of square kilometers) layers, with the highest reflectivity factors (sometimes >20 dBZ) arranged approximately perpendicular to the direction of the mean wind. The echoes coincided with strongly positive differential reflectivity (Z _DR) measurements (often >4 dB). Most interestingly, the echoes were elevated near the top of the boundary layer in the 2–3-km-AGL vertical range. Assuming a horizontally uniform layer of scatterers, the observations suggest that targets aloft are quasi prolate in shape and aligned horizontally along the direction of the mean wind. The echoes tended to occur on days when nocturnal inversions persisted into the following day, and solenoidal-like circulations (easterly upslope near the surface, and westerly flow aloft) existed. In some cases, the echoes exhibited diurnal behavior, with dumbbell-shaped echoes only occurring during the day and a more azimuthally uniform echo at night. On occasion, the echoes were coincident with the occurrence of widespread smoke from nearby forest fires. It is suggested that these echoes, which are rare for the CSU–CHILL coverage region, were caused by insects flying in a preferred direction, with the trigger for the migration being either the forest fires or oncoming winter. The local meteorological conditions likely affected the structure of these echoes.

Abstract

The spatial and temporal variability of convection during the North American Monsoon Experiment (NAME) was examined via analysis of three-dimensional polarimetric radar data. Terrain bands were defined as the Gulf of California (over water) and elevations of 0–500 m above mean sea level (MSL; coastal plain), 500–1500 m MSL, and >1500 m MSL. Convective rainfall over the Gulf typically featured the smallest values of median volume diameter (D ₀) regardless of rain rate. Gulf convection also contained reduced precipitation-sized ice water mass but proportionally more liquid water mass compared to convection over land. These maritime characteristics were magnified during disturbed meteorological regimes, which typically featured increased precipitation over the Gulf and adjacent coastal plain. Overall, the results suggest increased reliance on warm-rain collision and coalescence at the expense of ice-based precipitation growth processes for convective rainfall over the Gulf, relative to the land. Over land D ₀, ice, and liquid water mass all increased with decreasing terrain elevation, suggesting intensification of convection as it moved off the Sierra Madre Occidental. The results are consistent with the hypothesis that both warm-rain and ice-based rainfall processes play important roles in precipitation formation over land. Coastal-plain convection underwent microphysical modifications during disturbed meteorological regimes that were similar to Gulf convection, but the changes were less dramatic. High-terrain convection experienced little microphysical variability regardless of meteorological regime.

Abstract

Tropical convection regimes range from deep organized to shallow convective systems. Mesoscale processes such as cold pools within tropical convective systems can play a significant role in the evolution of convection over land and open ocean. Although cold pools are widely observed, their diurnal properties are not well understood over tropical oceans and land. The oceanic cold pool identification metric applied herein uses the gradient feature (GF) technique and is compared with diurnally resolved buoy-identified thermal cold pools. This study provides a first-ever diurnal climatology of GF number, area, and attributed TRMM 3B42 precipitation using a spaceborne scatterometer (RapidSCAT). Buoy data over the Pacific, Atlantic, and Indian Oceans have been used to validate and examine the RapidSCAT-identified diurnal cycle of GF number and precipitation. Buoy-observed cold pool duration, precipitation, temperature, and wind speed is analyzed to understand the in situ cold pool properties over tropical oceans. GF- and buoy-observed cold pool number and precipitation exhibits a similar bimodal diurnal variability with morning and afternoon maxima, thus establishing confidence in using GF as a proxy to observe cold pools over tropical oceans. The morning peak is attributed to cold pools associated with deep moist convection while the afternoon peak is related to shallower clouds in relatively drier environments resulting in smaller cold pools over global tropical oceans.