Jeffrey L. Stith

Rain in shallow cumulus over the ocean: the RICO Campaign

Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow ...

Observations and Parameterizations of Particle Size Distributions in Deep Tropical Cirrus and Stratiform Precipitating Clouds: Results from In Situ Observations in TRMM Field Campaigns

Abstract This study reports on the evolution of particle size distributions (PSDs) and habits as measured during slow, Lagrangian-type spiral descents through deep subtropical and tropical cloud layers in Florida, Brazil, and Kwajalein, Marshall Islands, most of which were precipitating. The objective of the flight patterns was to learn more about how the PSDs evolved in the vertical and to obtain information of the vertical structure of microphysical properties. New instrumentation yielding better information on the concentrations of particles in the size (D) range between 0.2 and 2 cm, as well as improved particle imagery, produced more comprehensive observations for tropical stratiform precipitation regions and anvils than have been available previously. Collocated radar observations provided additional information on the vertical structure of the cloud layers sampled. Most of the spirals began at cloud top, with temperatures (T) as low as −50°C, and ended at cloud base or below the melting layer (ML)....

Microphysical observations of tropical clouds

Abstract The results from airborne in situ sampling of convective tropical storms in the Amazon and Kwajalein are presented. Three cases from the Amazon and two from Kwajalein are compared and provide examples of the much larger dataset that was obtained from field campaigns in these two regions during 1999. The strength of the updraft was a major factor in determining the microphysical characteristics of hydrometeors. Weak updrafts exhibited a well-developed warm rain process by the time droplets had reached the freezing level. Stronger updrafts (>5 m s−1) contained smaller droplets or ice particles at cloud midlevels than regions with the weaker updrafts. Significant supercooled liquid water was found only at temperatures warmer than −12°C, although traces of liquid water were observed at temperatures as cold as −18°C. In deep stratiform anvil regions, aggregation was observed to be a major growth mechanism. These clouds did not contain appreciable amounts of supercooled water. Clouds with similar updra...

A cloud‐scale model study of lightning‐generated NO x in an individual thunderstorm during STERAO‐A

Understanding lightning NOx (NO 1 NO2) production on the cloud scale is key for developing better parameterizations of lightning NOx for use in regional and global chemical transport models. This paper attempts to further the understanding of lightning NOx production on the cloud scale using a cloud model simulation of an observed thunderstorm. Objectives are (1) to infer from the model simulations and in situ measurements the relative production rates of NOx by cloud-to-ground (CG) and intracloud (IC) lightning for the storm; (2) to assess the relative contributions in the storm anvil of convective transport of NOx from the boundary layer and NOx production by lightning; and (3) to simulate the effects of the lightning-generated NOx on subsequent photochemical ozone production. We use a two-dimensional cloud model that includes a parameterized source of lightning-generated NOx to study the production and advection of NOx associated with a developing northeast Colorado thunderstorm observed on July 12, 1996, during the Stratosphere-Troposphere Experiment—Radiation, Aerosols, Ozone (STERAO-A) field campaign. Model results are compared with the sum of NO measurements taken by aircraft and photostationary state estimates of NO2 in and around the anvil of the thunderstorm. The results show that IC lightning was the dominant source of NOx in this thunderstorm. We estimate from our simulations that the NOx production per CG flash (PCG) was of the order of 200 to 500 mol flash 21 .N O x production per IC flash (PIC) appeared to be half or more of that for a CG flash, a higher ratio of P IC/PCG than is commonly assumed. The results also indicate that the majority of NOx (greater than 80%) in the anvil region of this storm resulted from lightning as opposed to transport from the boundary layer. The effect of the lightning NOx on subsequent photochemical ozone production was assessed using a column chemical model initialized with values of NOx ,O 3, and hydrocarbons taken from a horizontally averaged vertical profile through the anvil of the simulated storm. The lightning NOx increased simulated ozone production rates by a maximum of over 7 ppbv d 21 in the upper troposphere downwind of this storm.

An overview of the Stratospheric‐Tropospheric Experiment: Radiation, Aerosols, and Ozone (STERAO)‐Deep Convection experiment with results for the July 10, 1996 storm

The Stratospheric-Tropospheric Experiment: Radiation, Aerosols and Ozone (STERAO)-Deep Convection Field Project with closely coordinated chemical, dynamical, electrical, and microphysical observations was conducted in northeastern Colorado during June and July of 1996 to investigate the production of NOx by lightning, the transport and redistribution of chemical species in the troposphere by thunderstorms, and the temporal evolution of intracloud and cloud-to-ground lightning for evolving storms on the Colorado high plains. Major observations were airborne chemical measurements in the boundary layer, middle and upper troposphere, and thunderstorm anvils; airborne and ground-based Doppler radar measurements; measurement of both intracloud (IC) and cloud-to-ground (CG) lightning flash rates and locations; and multiparameter radar and in situ observations of microphysical structure. Cloud and mesoscale models are being used to synthesize and extend the observations. Herein we present an overview of the project and selected results for an isolated, severe storm that occurred on July 10. Time histories of reflectivity structure, IC and CG lightning flash rates, and chemical measurements in the boundary layer and in the anvil are presented showing large spatial and temporal variations. The observations for one period of time suggest that limited mixing of environmental air into the updraft core occurred during transport from cloud base to the anvil adjacent to the storm core. We deduce that the most likely contribution of lightning to the total NOx observed in the anvil is 60–90% with a minimum of 45%. For the July 10 storm the NOx produced by lightning was almost exclusively from IC flashes with a ratio of IC to total flashes >0.95 throughout most of the storms lifetime. It is argued that in this storm and probably others, IC flashes can be major contributors to NOx production. Superposition of VHF lightning source locations on Doppler retrieved air motion fields for one 5 min time period shows that lightning activity occurred primarily in moderate updrafts and weak downdrafts with little excursion into the main downdraft. This may have important implications for the vertical redistribution of NOx resulting from lightning production, if found to be true at other times and in other storms.

Particle emissions and the production of ozone and nitrogen oxides from the burning of forest slash

Airborne measurements in the plumes from three prescribed burns of conifer slash showed the number concentration-size spectra to be bimodal with peaks at ~0.1 and ~0.5 μm. The mass distribution was sharply peaked at ~0.3 μm where over 80% of the mass of the particles in the plume resided. Most of the particles in the plumes were of primary rather than secondary origin. The burns emitted particulate mass into the atmosphere at rates of 0.1–15 kg s−1 during the active burning phase. The average density of the particles ranged from 0.75 to 1.34 g cm−3. Estimates of particle emission factors for the burns ranged from 0.2 to 2%, higher values being associated with higher fuel consumption rates. The burns were prolific sources of cloud condensation nuclei (CCN), producing ~1010−1011 CCN per gram of wood consumed. The CCN resulted in anomalously high concentrations of water droplets < 10 μm diameter in the cumulus clouds produced by the burns. Ozone concentrations near the tops of the plumes reached values as high as 44 ppb above ambient values, the higher values generally being associated with high u.v. intensities. Peak concentrations of NO2 and NO in the plumes were ~60 ppb; the ratio of NOxNO ranged from 1 to 3. SO2 was not found in the plumes.

Airborne studies of particles and gases from forest fires

Airborne measurements have been made in the emissions from several prescribed (deliberate) forest fires in Western Washington State. Particle measurements in the size range of 0.01 to 60 fim diameter show a multimodal character with the majority of the mass contained in particles between 0.1 to 1 pm in diameter. A particle emission factor (mass of particles produced per unit mass of forest fuel consumed) of about 4% was deduced for one of the fires. In addition to expected emissions of nitrogen dioxide, some of the fires appeared to be substantial sources of ozone. The fires were negligible sources of nitric oxide and sulfur dioxide.

Ice Initiation by Aerosol Particles: Measured and Predicted Ice Nuclei Concentrations versus Measured Ice Crystal Concentrations in an Orographic Wave Cloud

The initiation of ice in an isolated orographic wave cloud was compared with expectations based on ice nucleating aerosol concentrations and with predictions from new ice nucleation parameterizations applied in a cloud parcel model. Measurements of ice crystal number concentrations were found to be in good agreement both with measured number concentrations of ice nuclei feeding the clouds and with ice nuclei number concentrations determined from the residual nuclei of cloud particles collected by a counterflow virtual impactor. Using lognormal distributions fitted to measured aerosol size distributions and measured aerosol chemical compositions, ice nuclei and ice crystal concentrations in the wave cloud were reasonably well predicted in a 1D parcel model framework. Two different empirical parameterizations were used in the parcel model: a parameterization based on aerosol chemical type and surface area and a parameterization that links ice nuclei number concentrations to the number concentrations of particles with diameters larger than 0.5 μm. This study shows that aerosol size distribution and composition measurements can be used to constrain ice initiation by primary nucleation in models. The data and model results also suggest the likelihood that the dust particle mode of the aerosol size distribution controls the number concentrations of the heterogeneous ice nuclei, at least for the lower temperatures examined in this case.

NO signatures from lightning flashes

In situ measurements of cloud properties, NO, and other trace gases were made in active thunderstorms by two research aircraft. Concurrent measurements from a three-dimensional (3-D) VHF interferometer and the 2-D National Lightning Detection Network were used to determine lightning frequency and location. The CHILL Doppler radar and the NOAA-WP-3D Orion X band Doppler radar were also used to measure storm characteristics. Two case studies from the (STERAO) Stratosphere-Troposphere Experiments: Radiation, Aerosols, and Ozone project in northeastern Colorado during the summer of 1996 are presented. Narrow spikes (0.11–0.96 km across), containing up to 19 ppbv of NO, were observed in the storms. Most were located in or downwind of electrically active regions where the NO produced by lightning would be expected. However, it was difficult to correlate individual flashes with NO spikes. A simple model of the plume of NO from lightning is used to estimate NO production from the mean mixing ratio measured in these spikes. The estimates range from 2.0×1020 to 1.0×1022 molecules of NO per meter of flash length.

Numerical simulations of the July 10 Stratospheric-Tropospheric Experiment: Radiation, Aerosols, and Ozone/Deep Convection Experiment convective system: Kinematics and transport

The observed July 10, 1996, Stratospheric-Tropospheric Experiment: Radiation, Aerosols, and Ozone (STERAO) convective system is broadly reproduced in a nonhydrostatic cloud model simulation using an idealized horizontally homogeneous sounding and no terrain. System evolution from a multicellular line to a supercell, along with line orientation, anvil structure, horizontal wind fields, depth of convection, and derived radar reflectivity, compares well with observations. Simulated passive tracer transport of CO and ozone generally agrees with aircraft measurements and shows a small amount of entrainment of environmental air in the updrafts, and a small amount of dilution occurring with transport downwind in the anvil; the entrainment and dilution are less pronounced in the supercell stage. The horizontally integrated vertical flux divergence for CO in the simulation shows a net gain at almost all levels above 8 km mean sea level (msl). The rate of increase of CO mass above 8 km varies significantly in time, with a peak at early times, followed by a decline and minimum as the system transitions to a supercell and a steady increase as the supercell matures. Trajectory analyses show that updrafts in the simulation are ingesting air from a layer spanning from 2 km to 3.5 km msl (from 0.5 to 2km above the surface). The residence times for parcels in the updraft varies from just under 10 min to more than 20 min, with most parcels taking approximately 10 min to ascend to the anvil.