During the second week of September 2013, a seasonally uncharacteristic weather pattern stalled over the Rocky Mountain Front Range region of northern Colorado bringing with it copious amounts of moisture from the Gulf of Mexico, Caribbean Sea, and the tropical eastern Pacific Ocean. This feed of moisture was funneled toward the east-facing mountain slopes through a series of mesoscale circulation features, resulting in several days of unusually widespread heavy rainfall over steep mountainous terrain. Catastrophic flooding ensued within several Front Range river systems that washed away highways, destroyed towns, isolated communities, necessitated days of airborne evacuations, and resulted in eight fatalities. The impacts from heavy rainfall and flooding were felt over a broad region of northern Colorado leading to 18 counties being designated as federal disaster areas and resulting in damages exceeding $2 billion (U.S. dollars). This study explores the meteorological and hydrological ingredients that led to this extreme event. After providing a basic timeline of events, synoptic and mesoscale circulation features of the event are discussed. Particular focus is placed on documenting how circulation features, embedded within the larger synoptic flow, served to funnel moist inflow into the mountain front driving several days of sustained orographic precipitation. Operational and research networks of polarimetric radar and surface instrumentation were used to evaluate the cloud structures and dominant hydrometeor characteristics. The performance of several quantitative precipitation estimates, quantitative precipitation forecasts, and hydrological forecast products are also analyzed with the intention of identifying what monitoring and prediction tools worked and where further improvements are needed.
Herein the authors introduce the Snowflake Video Imager (SVI), which is a new instrument for characterizing frozen precipitation. An SVI utilizes a video camera with sufficient frame rate, pixels, and shutter speed to record thousands of snowflake images. The camera housing and lighting produce little airflow distortion, so SVI data are quite representative of natural conditions, which is important for volumetric data products such as snowflake size distributions. Long-duration, unattended operation of an SVI is feasible because datalogging software provides data compression and the hardware can operate for months in harsh winter conditions. Details of SVI hardware and field operation are given. Snowflake size distributions (SSDs) from a storm near Boulder, Colorado, are computed. An SVI is an imaging system, so SVI data can be utilized to compute diverse data products for various applications. In this paper, the authors present visualizations of frozen particles (i.e., snowflake aggregates as well as individual crystals), which provide insight into the weather conditions such as temperature, humidity, and winds.
The growth of long-term space-based precipitation datasets enables cross-disciplinary discoveries about hydrological and land processes, climate, atmospheric composition, and ocean freshwater budget and provides vital help in addressing societal issues.
[1] During the July 2002 Cirrus Regional Study of Tropical Anvils and Cirrus LayersFlorida Area Cirrus Experiment (CRYSTAL-FACE), flights of a WB-57F aircraft revealed mixing ratios of nitric oxide 10-50 times background over distances of 25-175 km in the anvils of thunderstorms and in clear air downwind of storm systems due to lightning activity and possible transport from the boundary layer. Estimates of the total mass of NO x injected into the middle and upper troposphere differed considerably for a moderately versus highly electrically active storm system as expected. However, assuming that the total mass is dominated by lightning production, rough estimates of the production per average lightning flash for a moderately and a highly active storm also yielded quite different ranges of (0.33-0.66) Â 10 26 and (1.7-2.3) Â 10 26 molecules NO/flash, respectively. If the common assumption is made that intracloud flashes have 1/10th the NO production efficiency of cloud-to-ground (CG) flashes, the ranges of production for the moderately and highly active storms were (0.88-1.8) Â 10 26 and (4.5-6.1) Â 10 26 molecules NO/CG flash, respectively. The observed CG flash accumulations and NO x mass production estimate for the month of July 2002 over the Florida area are compared with results from the MOZART-2 global chemistry-transport model that uses a common lightning flash parameterization. Reasonable agreement was found after a correction to the lightning parameterization was made. Finally, broad-scale median mixing ratios of NO within anvils over Florida were significantly larger than found in storms previously investigated over Colorado and New Mexico.
Radar rainfall measurements over the equatorial western Pacific warm pool were collected by two shipboard Doppler radars as part of the Tropical Oceans Global Atmosphere Coupled Ocean-Atmosphere Response Experiment during the intensive observing period (November 1992-February 1993). A comprehensive dataset of gridded rainfall fields, convective/stratiform identification maps, and vertical structure products has been produced, covering an area approximately 400 km (E-W) by 300 km (N-S) within the Intensive Flux Array (IFA), centered near 2°S, 156°E. The radar rainfall product, which was used as validation for the Third Algorithm Intercomparison Project of the Global Precipitation Climatology Project, indicates an overall average of 4.8 mm day-1 ; however, correction for range dependence increases the total to 5.4 mm day-1. Rainfall patterns varied considerably during the experiment with isolated convection dominating periods of light winds, while squall lines and organized mesoscale systems were abundant during two westerly wind bursts. An area-average rainfall of 9.9 mm day-1 was observed during the active 2-week period at the end of December, while 0.4 mm day-1 was observed during the quiescent week of 2-8 February. The eastern portion of the IFA received the most rainfall with localized maxima exceeding 16 mm day-1 for the most active 3-week period. Comparison of daily radar rainfall totals with those observed by an optical rain gauge (ORG) on the 2°S, 156°E buoy shows ORG totals to be systematically higher, by a factor of 2.5. The discrepancy results from a higher average rainfall rate, when raining, as reported by the buoy ORG. However, rainfall rate statistics from the ORGs on the research vessel Xiang Yang Hong #5 and from its radar are in excellent agreement under the following conditions: 1) the ship is drifting, and 2) the radar data are in the near vicinity of the ship (3-7 km).
The challenges of monitoring and forecasting flash-flood-producing storm events in data-sparse and arid regions are explored using the Weather Research and Forecasting (WRF) Model (version 3.5) in conjunction with a range of available satellite, in situ, and reanalysis data. Here, we focus on characterizing the initial synoptic features and examining the impact of model parameterization and resolution on the reproduction of a number of floodproducing rainfall events that occurred over the western Saudi Arabian city of Jeddah. Analysis from the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) data suggests that mesoscale convective systems associated with strong moisture convergence ahead of a trough were the major initial features for the occurrence of these intense rain events. The WRF Model was able to simulate the heavy rainfall, with driving convective processes well characterized by a high-resolution cloud-resolving model. The use of higher (1 km vs 5 km) resolution along the Jeddah coastline favors the simulation of local convective systems and adds value to the simulation of heavy rainfall, especially for deep-convection-related extreme values. At the 5-km resolution, corresponding to an intermediate study domain, simulation without a cumulus scheme led to the formation of deeper convective systems and enhanced rainfall around Jeddah, illustrating the need for careful model scheme selection in this transition resolution. In analysis of multiple nested WRF simulations (25, 5, and 1 km), localized volume and intensity of heavy rainfall together with the duration of rainstorms within the Jeddah catchment area were captured reasonably well, although there was evidence of some displacements of rainstorm events.
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