Eric J. Jensen

On the formation and persistence of subvisible cirrus clouds near the tropical tropopause

We have used a detailed cirrus cloud model to evaluate the physical processes responsible for the formation and persistence of subvisible cirrus near the tropical tropopause and the apparent absence of these clouds at midlatitudes. We find that two distinct formation mechanisms are viable. Energetic tropical cumulonimbus clouds transport large amounts of ice water to the upper troposphere and generate extensive cirrus outflow anvils. Ice crystals with radii larger than 10 – 20 μm should precipitate out of these anvils within a few hours, leaving behind an optically thin layer of small ice crystals (τvis ≃ 0.01 – 0.2, depending upon the initial ice crystal size distribution). Given the long lifetimes of the clouds, wind shear is probably responsible for the observed cloud thickness ≤1 km. Ice crystals can also be generated in situ by slow, synoptic scale uplift of a humid layer. Given the very low temperatures at the tropical tropopause (≃−85°C), synoptic-scale uplift can generate the moderate ice supersaturations (less than 10%) required for homogeneous freezing of sulfuric acid aerosols. In addition, simulations suggest that relatively large ice crystal number densities should be generated (more than 0.5 cm−3). The numerous crystals cannot grow larger than about 10–20 μm given the available vapor, and their low fall velocities will allow them to remain in the narrow supersaturated region for at least a day. The absorption of infrared radiation in the thin cirrus results in heating rates on the order of a few K per day. If this energy drives local parcel temperature change, the cirrus will dissipate within several hours. However, if the absorbed radiative energy drives lifting of the cloud layer, the vertical wind speed will be about 0.2 cm-s−1, and the cloud may persist for days with very little change in optical or microphysical properties. The fact that these clouds form most frequently over the tropical western Pacific is probably related (through the nucleation physics) to the very low tropopause temperatures in this region. Simulations using midlatitude tropopause temperatures near −65°C suggest that at the higher temperatures, fewer ice crystals nucleate, resulting in more rapid crystal growth and cloud dissipation by precipitation. Hence, the lifetime of thin cirrus formed near the midlatitude tropopause should be limited to a few hours after the synoptic-scale system that initiated cloud formation has passed.

Modeling coagulation among particles of different composition and size

Abstract We present a technique for simulating coagulation among any number of aerosol types, each with a different composition. The semi-implicit solution mechanism solves the coagulation equations over size ranges divided into any number of discrete bins. The scheme conserves particle volume, requires no iterations, and is numerically stable, regardless of the time-step. We compared the accuracy of the solution to both analytical and time-series numerical solutions. Practical use of the scheme demonstrates that it is computationally fast in a multiple grid-cell model.

Aircraft observations of thin cirrus clouds near the tropical tropopause

This work describes aircraft-based lidar observations of thin cirrus clouds at the tropical tropopause in the central Pacific obtained during the Tropical Ozone Transport Experiment/Vortex Ozone Transport Experiment (TOTE/VOTE) in December 1995 and February 1996. Thin cirrus clouds were found at the tropopause on each of the four flights which penetrated within 15° of the equator at 200–210 east longitude. South of 15°N, thin cirrus were detected above the aircraft about 65% of the time that data were available. The altitudes of these clouds exceeded 18 km at times. The cirrus observations could be divided into two basic types: thin quasi-laminar wisps and thicker, more textured structures. On the basis of trajectory analyses and temperature histories, these two types were usually formed respectively by (1) in situ cooling on both a synoptic scale and mesoscale and (2) recent (a few days) outflow from convection. There is evidence from one case that the thicker clouds can also be formed by in situ cooling. The actual presence or absence of thin cirrus clouds was also consistent with the temperature and convective histories derived from back trajectory calculations. Notably, at any given time, only a relatively small portion (at most 25%) of the west central tropical Pacific has been influenced by convection within the previous 10 days. The structures of some of the thin cirrus clouds formed in situ strongly resembled long-wavelength (500–1000 km) gravity waves observed nearly simultaneously by the ER-2 on one of the flights. Comparison with in situ water vapor profiles made by the NASA ER-2 aircraft provide some observational support for the hypothesis that thin cirrus clouds play an important role in dehydrating tropospheric air as it enters the stratosphere.

Dehydration of the Upper Troposphere and Lower Stratosphere by Subvisible Cirrus Clouds Near the Tropical Tropopause

The extreme dryness of the lower stratosphere is believed to be caused by freeze-drying of air as it enters the stratosphere through the cold tropical tropopause. Previous investigations have been focused on dehydration occurring at the tops of deep convective cloud systems. However, recent observations of a ubiquitous stratiform cirrus cloud layer near the tropical tropopause suggest the possibility of dehydration as air is slowly lifted by large-scale motions. In this study, we have evaluated this possibility using a detailed ice cloud model. Simulations of ice cloud formation in the temperature minima of gravity waves (wave periods of 1–2 hours) indicate that large numbers of ice crystals will likely form due to the low temperatures and rapid cooling. As a result, the crystals do not grow larger than about 10 µm, fallspeeds are no greater than a few cm-s−1, and little or no precipitation or dehydration occurs. However, ice clouds formed by large-scale vertical motions (with lifetimes of a day or more) should have fewer crystals and more time for crystal sedimentation to occur, resulting in water vapor depletions as large as 1 ppmv near the tropopause. We suggest that gradual lifting near the tropical tropopause, accompanied by formation of thin cirrus, may account for the dehydration.

Ice nucleation processes in upper tropospheric wave‐clouds observed during SUCCESS

We have compared in situ measurements near the leading-edges of wave-clouds observed during the SUCCESS experiment with numerical simulations. Observations of high supersaturations with respect to ice (>50%) near the leading edge of a very cold wave cloud (T <−60°C) are approximately consistent with recent theoretical and laboratory studies suggesting that large supersaturations are required to homogeneously freeze sulfate aerosols. Also, the peak ice crystal number densities observed in this cloud (about 4 cm−3) are consistent with the number densities calculated in our model. In the warmer wave-cloud (T ≃−37°C) relatively large ice number densities were observed (20–40 cm−3). Our model calculations suggest that these large number densities are probably caused by activation of sulfate aerosols into liquid droplets followed by subsequent homogeneous freezing. If moderate numbers of effective heterogeneous freezing nuclei (0.5–1 cm−3) had been present in either of these clouds, then the number densities of ice crystals and the peak relative humidities should have been lower than the observed values.

The potential impact of soot particles from aircraft exhaust on cirrus clouds

Homogeneous freezing of sulfate aerosols may dominate ice nucleation in cirrus, implying that large supersaturations are required for cirrus cloud initiation at low temperatures. However, insoluble particles from the surface or soot particles injected directly into the upper troposphere by jet aircraft may act as heterogeneous ice nuclei. If the soot particles are sufficiently effective ice nuclei, then they will allow ice nucleation at lower supersaturations than those required for homogeneous freezing, resulting in an increase in the areal coverage of cirrus clouds. Simulations using a detailed ice cloud model indicate that cirrus driven by slow, steady lifting (a few cm-s−1) will be transient, precipitating clouds if only pure sulfate haze aerosols are present. However, if effective heterogeneous nuclei are present, then extensive, persistent, diffuse cirrus should form. In addition, heterogeneous ice nucleation on insoluble particles may modify the number of ice crystals nucleated in cirrus, resulting in alterations of the cloud evolution and radiative properties. Heterogeneous freezing on relatively few insoluble particles (Ninv <0.1 cm−3) should result in fewer ice crystals nucleating than if homogeneous freezing were to occur. However, if large numbers of insoluble particles are present, the ice crystal number density may be increased.

Microphysical modeling of cirrus: 1. Comparison with 1986 FIRE IFO measurements

We have used a one-dimensional model of cirrus formation to study the development of cirrus clouds during the 1986 First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE) intensive field observations (IFO). The cirrus model includes microphysical, dynamical, and radiative processes. Sulfate aerosols, solution drops, ice crystals, and water vapor are all treated as interactive elements in the model. Ice crystal size distributions are fully resolved based on calculations of homogeneous freezing nucleation, growth by water vapor deposition, evaporation, coagulation, and vertical transport. We have focused on the cirrus observed on November 1, 1986. Vertical wind speed for the one-dimensional simulation is taken from a mesoscale model simulation for the appropriate time period. The mesoscale model simulation suggested that strong upward motions over Wyoming and subsequent horizontal transport of upper level moisture were responsible for the cirrus observed over Wisconsin on this date. We assumed that our one-dimensional model could be used to represent a vertical column moving from Wyoming to Wisconsin over a period of several hours. Ice crystal nucleation occurs in our model in the 8 to 10-km region as a result of the strong updrafts (and cooling) early in the simulation. Growth, coagulation, and sedimentation of these ice crystals result in a broad cloud region (5–10 km thick) with an optical depth of 1–2 after a few hours, in agreement with the FIRE measurements. Comparison with aircraft microphysical measurements made over Wisconsin indicates that the simulation generated reasonable ice water content, but the predicted ice number densities are too low, especially for radii less than about 50 μm. Sensitivity tests suggest that better agreement between simulated and observed microphysical properties is achieved if the nucleation rate is higher or stronger vertical mixing (perhaps associated with multidimensional motions) is present.

Prevalence of ice-supersaturated regions in the upper troposphere: Implications for optically thin ice cloud formation

In situ measurements of water vapor and temperature from recent aircraft campaigns have provided evidence that the upper troposphere is frequently supersaturated with respect to ice. The peak relative humidities with respect to ice (RHI) occasionally approached water saturation at temperatures ranging from −40°C to −70°C in each of the campaigns. The occurrence frequency of ice supersaturation ranged from about 20% to 45%. Even on flight segments when no ice crystals were detected, ice supersaturation was measured about 5–20% of the time. A numerical cloud model is used to simulate the formation of optically thin, low ice number density cirrus clouds in these supersaturated regions. The potential for scavenging of ice nuclei (IN) by these clouds is evaluated. The simulations suggest that if less than about 5 × 10-3 to 2 × 10-2 cm-3 ice nuclei are present when these supersaturations are generated, then the cirrus formed should be subvisible. These low ice number density clouds scavenge the IN from the supersaturated layer, but the crystals sediment out too rapidly to prevent buildup of high supersaturations. If higher numbers of ice nuclei are present, then the clouds that form are visible and deposition growth of the ice crystals reduces the RHI down to near 100%. Even if no ice clouds form, increasing the RHI from 100% to 150% between 10 and 10.5 km results in a decrease in outgoing longwave radiative flux at the top of the atmosphere of about 8 W m-2. If 0.02–0.1 cm-3 IN are present, the resulting cloud radiative forcing reduces the net radiative flux several watts per square meter further. Given the high frequency of supersaturated regions without optically thick clouds in the upper troposphere, there is a potential for a climatically important class of optically thin cirrus with relatively low ice crystal number densities. The optical properties of these clouds will depend very strongly on the abundance of ice nuclei in the upper troposphere.

A conceptual model of the dehydration of air due to freeze-drying by optically thin, laminar cirrus rising slowly across the tropical tropopause

In this study, we use a cloud model to simulate dehydration which occurs due to formation of optically thin, laminar cirrus as air rises slowly across the tropopause. The slow ascent and adiabatic cooling, which balances the radiative heating near the tropopause, drives nucleation of a very small number of ice crystals (<1 L−1). These crystals grow rapidly and sediment out within a few hours. The clouds never become optically thick enough to be visible from the ground. The ice crystal nucleation and growth prevents the relative humidity with respect to ice (RHI) from rising more than a few percent above the threshold for ice nucleation (RHInuc ≃ 110–160%, depending upon the aerosol composition); hence, laminar cirrus can limit the mixing ratio of water vapor entering the stratosphere. However, the ice number densities are too low and their sedimentation is too rapid to allow dehydration of the air from RHInuc down to saturation (RHI = 100%). The net result is that air crosses the tropopause with water vapor mixing ratios about 1.1 to 1.6 times the ice saturation mixing ratio corresponding to the tropopause temperature, depending on the threshold of ice nucleation on aerosols in the tropopause region. If the cross-tropopause ascent rate is larger than that calculated to balance radiative heating (0.2 cm s−1), then larger ice crystal number densities are generated, and more effective dehydration is possible (assuming a fixed temperature). The water vapor mixing ratio entering the stratosphere decreases with increasing ascent rate (approaching the tropopause ice saturation mixing ratio) until the vertical wind speed exceeds the ice crystal terminal velocity (about 10 cm s−1). More effective dehydration can also be provided by temperature oscillations associated with wave motions. The water vapor mixing ratio entering the stratosphere is essentially controlled by the tropopause temperature at the coldest point in the wave. Hence, the efficiency of dehydration at the tropopause depends upon both the effectiveness of upper tropospheric aerosols as ice nuclei and the occurrence of wave motions in the tropopause region. In situ humidity observations from tropical aircraft campaigns and balloon launches over the past several years have provided a few examples of ice-supersaturated air near the tropopause. However, given the scarcity of data and the uncertainties in water vapor measurements, we lack definitive evidence that air entering the stratosphere is supersaturated with respect to ice.

The life cycle of stratospheric aerosol particles

This paper describes the life cycle of the background (nonvolcanic) stratospheric sulfate aerosol. The authors assume the particles are formed by homogeneous nucleation near the tropical tropopause and are carried aloft into the stratosphere. The particles remain in the Tropics for most of their life, and during this period of time a size distribution is developed by a combination of coagulation, growth by heteromolecular condensation, and mixing with air parcels containing preexisting sulfate particles. The aerosol eventually migrates to higher latitudes and descends across isentropic surfaces to the lower stratosphere. The aerosol is removed from the stratosphere primarily at mid- and high latitudes through various processes, mainly by isentropic transport across the tropopause from the stratosphere into the troposphere.