Abstract

Following the discovery of anomalously high values of lidar integrated attenuated backscatter near the top center layers of mesoscale convective systems (MCSs) observed by the NASA Lidar In-Space Technology Experiment (LITE), a search of Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) data on board the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) platform revealed the same phenomena in a sample of eight MCSs investigated. The backscatter depolarization ratio also showed changes concurrent with the high integrated backscatter and either increased or decreased concurrently with the anomalous backscatter. Simultaneous CloudSat data in the A-Train formation showed a cloud-top altitude similar to that measured by CALIOP, indicating fairly large ice crystals were reaching cloud top. Based on previous work, the CALIOP and CloudSat returns were likely due to a mix of small ice droxtals or frozen drops extending in a continuous spectrum to large crystals composed of well-formed hexagonal columns, thick hexagonal plates, spheroids, and irregular particles. The CALIOP lidar would detect the whole spectrum whereas CloudSat would detect ice crystals greater than ∼30 μm in effective radius; there were apparently enough of such crystals to allow CloudSat to detect a cloud-top height similar to that found by CALIOP. Using such a model, it was estimated that the measured backscatter phase function in the most active part of the cloud could be reconciled approximately with theoretical values of the various crystal habits. However, it was harder to reconcile the changes in depolarization ratio given the absence of values of this parameter for small droxtal crystals.

Abstract

Several cloud optical quantities were measured for the first time in midlevel, mixed-phase clouds. These included cloud infrared emittance and absorption coefficient (10–12 μm), effective backscatter-to-extinction ratio, and lidar depolarization ratio. Contrary to expectations, the supercooled water clouds were not always optically thick and therefore had measurable infrared absorption coefficients. At times, the water clouds had quite low emittances, whereas ice clouds had emittances that sometimes approached unity. On average, the cloud emittances were greater than those measured previously at lower temperatures in cirrus, but with considerable variability. At higher temperatures, the emittance values were skewed toward unity. The infrared absorption coefficients, for the semitransparent cases, showed a similar trend. The effective isotropic backscatter-to-extinction ratio was also measured. When separated into temperature intervals, the ratio was surprisingly constant, with mean values lying between 0.42 and 0.43, but with considerable variation. These ratios were most variable (0.15–0.8) in the −20° to −10°C temperature range where various ice crystal habits can occur. When multiple scattering effects were allowed for, values of backscatter-to-extinction ratio in the supercooled water clouds agreed well with theory. Multiple scattering factors based on previously obtained theoretical values were used and, thus, validated.

Characteristic and well-known patterns of lidar backscatter coefficient and depolarization ratio were used to separate out the incidence of supercooled water and ice layers and to identify layers of horizontal planar hexagonal crystals. This approach allowed the most detailed examination yet of such incidence by ground-based remote sensing. Water was detected for 92% of the time for the temperature interval of −5° to 0°C. Between −20° and −5°C, percentages varied between 33% and 56%, dropping to 21% between −25° and −20°C and to zero below −25°C. Oriented hexagonal plate crystals were present for 20% of the total time in ice layers between −20° and −10°C, the region of their maximum growth. The depolarization ratio varied significantly among different ice fall streaks, indicating considerable variation in ice crystal habit. Although the dependence of depolarization ratio on optical depth had been predicted theoretically, the first experimental validation in terms of IR emittance was obtained in this study.

Abstract

During the Maritime Continent Thunderstorm Experiment (MCTEX), several decaying storm anvils were observed. The anvil clouds exhibited typical patterns of fallout and decay over a number of hours of observation. The anvil bases were initially very attenuating to lidar pulses, and continued that way until anvil breakup commenced. During that time, the anvil base reached some characteristic altitude (∼7 km) below which the cloud particles had evaporated fully. Some typical “tongues” of fallout below such levels also occurred. Millimeter radar showed the storm anvil cloud tops to be much higher than detected by lidar until the anvil was well dissipated.

The infrared properties of the anvils were calculated. In three of the four anvils studied, the calculated emittance never exceeded 0.8–0.85. In the remaining case, the cloud emittance approached unity only in the period before the anvil had descended appreciably. Radiative transfer calculations showed that the infrared emission originated mostly from the layer between cloud base and the height at which complete attenuation of the lidar pulse occurred. However, the correct blackbody emission at cloud base could only be obtained by assuming the existence of an additional layer, situated above the first, 1.8 km deep and with a specific backscatter coefficient. The depressed values of emittance were interpreted as a cooling (below those temperatures measured by radiosonde) for some distance above anvil cloud base due to evaporation of the cloud. Typically, this cooling amounted to about 10°C, depending on the layer thickness above cloud base at which cooling was occurring. A reexamination of data taken in 1981 at Darwin, Northern Territory, Australia, indicated a similar depression in emittance in all cases of attenuating storm anvils. A simple model of ice-mass evaporation saturating the ambient air was used to approximate the observed cooling in one anvil. Millimeter radar reflectivity measurements, which also yielded ice water content at cloud base, were also used to find equivalent cooling rates. By varying the mean volume diameter in the calculation, cooling rates similar to those found from the radiometric method could be obtained. The values of mean volume diameter agreed, within uncertainties, with those obtained by the lidar–radar method. Estimated cooling to over 1 km above cloud base confirms earlier work on anvil mammata. Values of backscatter-to-extinction ratio at the base of the anvils showed some consistent variations, indicating a change of ice-crystal habit, or size, with time.

Abstract

This paper presents further results on the optical properties of tropical and equatorial cirrus using the light detecting and ranging (lidar) radiometer (LIRAD) method. The results were obtained from observations in the Maritime Continent Thunderstorm Experiment (MCTEX). Values were obtained of cirrus cloud backscatter coefficient, infrared (IR) emittance, optical depth and absorption coefficient, cloud height and depth, and backscatter-to-extinction ratio. The values agree well with previous results obtained on equatorial cirrus in the Pilot Radiation Observation Experiment (PROBE) and extend those results to lower temperatures. Observations made of lidar linear depolarization ratio show similar trends to those observed in PROBE, extending those results to lower temperatures.

Regressions of cloud IR emittance and absorption coefficients are performed as a preliminary tropical dataset for both cloud-resolving and climate models. These regressions are compared with previous regressions on midlatitude and tropical synoptic cirrus clouds. The IR absorption coefficients in tropical and equatorial cirrus appear to be larger than in midlatitude cirrus for temperatures less than −40°C, with the difference increasing toward low temperatures. Thus, a significantly different relationship may be appropriate for tropical cirrus compared to midlatitude cirrus clouds.

Effective diameters of small particles in the colder tropical clouds are also measured using the ratio of visible extinction to infrared absorption. A new treatment of multiple scattering is used to correct the ratios. Effective diameters range from 6 to 9.3 μm at the colder temperatures.

Abstract

Two case studies of rainband evolution off the windward coast of the Big Island of Hawaii are presented along with an overview of the complete radar and satellite dataset from the Hawaiian Rainband Project conducted in the summer of 1990. These studies reveal that radar-observed rainbands and cells offshore of the windward coastline are nearly always embedded within larger-scale stratocumulus cloud patches and/or cloud lines moving in from the northeastern Pacific Ocean in the easterly trade winds. These cloud patches develop in the trade-wind flow downstream of the large shallow stratocumulus cloud mass that covers much of the northeastern Pacific Ocean between Hawaii and California. Tropical cyclones originating in the intertropical convergence zone sometimes move north through this stratocumulus cloud mass, producing, in their wake, cloud-free regions that then advect toward the Hawaiian Islands. These regions contribute to the variability of the organization of cloud patterns and rainband occurrence offshore of Hawaii.

Previous studies have employed data analysis and modeling to explain dynamical and thermodynamical processes that lead to the development of a flow separation line upwind of Hawaii and cloud formation along the line. This study shows that most rainbands occurring near the flow separation line first form upstream within cloud patches and/or cloud lines approaching the island and stretch within the deformation flow as the trade winds deflect around the island. Analyses of thermodynamic soundings suggest that lifting associated with convergence at the flow separation line is frequently sufficient to produce clouds but insufficient to trigger free convection and rainbands.

A diurnal oscillation in rainband frequency upwind of the island is documented. This diurnal behavior in rainband frequency may be related to the strong diurnal radiational forcing that occurs at the top of the trade-wind marine layer.

Abstract

The optical properties of equatorial cirrus were studied during a three-week period of the ARM Pilot Radiation and Observation Experiment at Kavieng, Papua New Guinea, in January and February 1993. The experiment consisted of vertical lidar (532 nm) and passive infrared filter radiometer (10.84 μm) observations of cirrus clouds. The observations gave values of cloud height, depth, structure, infrared emittance, infrared absorption, and visible optical depth and linear depolarization ratio. A standard lidar–radiometer analysis, with some improvements, was used to calculate these quantities. The cirrus was found to vary in altitude from a maximum cloud top of 17.6 km to a minimum cloud base of 6 km with equivalent temperatures of −82°C to −7°C respectively. The cirrus also varied widely in depth (0.7 to 7.5 km). The mean emittance (for each temperature interval) of the cooler clouds was found to be higher than that observed previously at tropical and midlatitude sites and at equivalent temperatures. The mean infrared absorption coefficients were similar to those of midlatitude clouds, except at the extreme temperature ranges, but were higher than those observed in tropical synoptic clouds over Darwin. Infrared optical depths varied from 0.01 to 2.4 and visible optical depths from 0.01 to 8.6.

Plots of integrated attenuated backscatter versus infrared emittance, for various ranges of cloud temperature, showed characteristic behavior. Values of the measured quantity k/2η, where k is the visible backscatter to extinction ratio and η a multiple scattering factor, were found to increase with temperature from 0.14 at −70°C to 0.30 at −20°C.

Values of the quantity 2αη, where α is the ratio of visible extinction to infrared absorption coefficient, varied from about 1.7 to 3.8, depending somewhat on the cloud temperature. Deduced values of α were as high as 5.3 at the lower temperature ranges, indicating smaller particles.

The lidar integrated attenuated depolarization ratio Δ decreased with temperature, as found previously in midlatitude cirrus. Values of Δ varied from 0.42 at −70°C to 0.18 at −10°C. Data obtained from the NOAA/ETL microwave radiometer gave values of water path, varying from 4 to 6 cm precipitable water. A value of the water vapor continuum absorption coefficient at 10.84 μm equal to 9.0 ± 0.5 g⁻¹ cm² atm⁻¹ was obtained in agreement with previous observations.

Abstract

Sixty-day simulations of the subinertial continental shelf circulation off Oregon are performed for a hindcast study of summer 1999. Model results are compared with in situ currents, high-frequency radar–derived surface currents, and hydrographic measurements obtained from an array of moored instruments and field surveys. The correlations between observed and modeled alongshore currents and temperatures in water depths of 50 m are in excess of 0.8. A study designed to test the model's sensitivity to different initial stratification, surface forcing, domain size, and river forcing demonstrates that surface heating is important, and that the model results are sensitive to initial stratification. An objective criterion for assessing the skill of a model simulation relative to a control simulation is outlined, providing an objective means for identifying the best model simulation. The model–data comparisons demonstrate that temperature fluctuations off Newport are primarily in response to surface heating and that subsurface density fluctuations are controlled by the wind-forced circulation through salinity. Experiments with river forcing indicate that, in the vicinity of Newport, the Columbia River plume is typically greater than 15 km from the coast and is confined to the top few meters of the water column. Additionally, the model–data comparisons suggest that the strongest upwelling occurs to the north of Newport where the continental shelf is relatively narrow and uniform in the alongshore direction. Part II of this study investigates the modeled three-dimensional circulation and dynamical balances.

Accurate and reliable predictions and an understanding of future changes in the stratosphere are major aspects of the subject of climate change. Simulating the interaction between chemistry and climate is of particular importance, because continued increases in greenhouse gases and a slow decrease in halogen loading are expected. These both influence the abundance of stratospheric ozone. In recent years a number of coupled chemistry–climate models (CCMs) with different levels of complexity have been developed. They produce a wide range of results concerning the timing and extent of ozone-layer recovery. Interest in reducing this range has created a need to address how the main dynamical, chemical, and physical processes that determine the long-term behavior of ozone are represented in the models and to validate these model processes through comparisons with observations and other models. A set of core validation processes structured around four major topics (transport, dynamics, radiation, and stratospheric chemistry and microphysics) has been developed. Each process is associated with one or more model diagnostics and with relevant datasets that can be used for validation. This approach provides a coherent framework for validating CCMs and can be used as a basis for future assessments. Similar efforts may benefit other modeling communities with a focus on earth science research as their models increase in complexity.

Abstract

The response of stratospheric climate and circulation to increasing amounts of greenhouse gases (GHGs) and ozone recovery in the twenty-first century is analyzed in simulations of 11 chemistry–climate models using near-identical forcings and experimental setup. In addition to an overall global cooling of the stratosphere in the simulations (0.59 ± 0.07 K decade⁻¹ at 10 hPa), ozone recovery causes a warming of the Southern Hemisphere polar lower stratosphere in summer with enhanced cooling above. The rate of warming correlates with the rate of ozone recovery projected by the models and, on average, changes from 0.8 to 0.48 K decade⁻¹ at 100 hPa as the rate of recovery declines from the first to the second half of the century. In the winter northern polar lower stratosphere the increased radiative cooling from the growing abundance of GHGs is, in most models, balanced by adiabatic warming from stronger polar downwelling. In the Antarctic lower stratosphere the models simulate an increase in low temperature extremes required for polar stratospheric cloud (PSC) formation, but the positive trend is decreasing over the twenty-first century in all models. In the Arctic, none of the models simulates a statistically significant increase in Arctic PSCs throughout the twenty-first century. The subtropical jets accelerate in response to climate change and the ozone recovery produces a westward acceleration of the lower-stratospheric wind over the Antarctic during summer, though this response is sensitive to the rate of recovery projected by the models. There is a strengthening of the Brewer–Dobson circulation throughout the depth of the stratosphere, which reduces the mean age of air nearly everywhere at a rate of about 0.05 yr decade⁻¹ in those models with this diagnostic. On average, the annual mean tropical upwelling in the lower stratosphere (∼70 hPa) increases by almost 2% decade⁻¹, with 59% of this trend forced by the parameterized orographic gravity wave drag in the models. This is a consequence of the eastward acceleration of the subtropical jets, which increases the upward flux of (parameterized) momentum reaching the lower stratosphere in these latitudes.

Many of the clouds important to the Earth's energy balance, from the Tropics to the Arctic, contain small amounts of liquid water. Longwave and shortwave radiative fluxes are very sensitive to small perturbations of the cloud liquid water path (LWP), when the LWP is small (i.e., < 100 g m⁻²; clouds with LWP less than this threshold will be referred to as “thin”). Thus, the radiative properties of these thin liquid water clouds must be well understood to capture them correctly in climate models. We review the importance of these thin clouds to the Earth's energy balance, and explain the difficulties in observing them. In particular, because these clouds are thin, potentially mixed phase, and often broken (i.e., have large 3D variability), it is challenging to retrieve their microphysical properties accurately. We describe a retrieval algorithm intercomparison that was conducted to evaluate the issues involved. The intercomparison used data collected at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site and included 18 different algorithms to evaluate their retrieved LWP, optical depth, and effective radii. Surprisingly, evaluation of the simplest case, a single-layer overcast stratocumulus, revealed that huge discrepancies exist among the various techniques, even among different algorithms that are in the same general classification. This suggests that, despite considerable advances that have occurred in the field, much more work must be done, and we discuss potential avenues for future research.)