Central Equatorial Pacific Experiment Research Articles

A new method to correct the electrochemical concentration cell (ECC) ozonesonde time response and its implications for “background current” and pump efficiency

Abstract. The electrochemical concentration cell (ECC) ozonesonde has been the main instrument for in situ profiling of ozone worldwide; yet, some details of its operation, which contribute to the ozone uncertainty budget, are not well understood. Here, we investigate the time response of the chemical reactions inside the ECC and how corrections can be used to remove some systematic biases. The analysis is based on the understanding that two reaction pathways involving ozone occur inside the ECC that generate electrical currents on two very different timescales. The main fast-reaction pathway with a time constant of about 20 s is due the conversion of iodide to molecular iodine and the generation of two free electrons per ozone molecule. A secondary slow-reaction pathway involving the buffer generates an excess current of about 2 %–10 % with a time constant of about 25 min. This excess current can be interpreted as what has conventionally been considered the “background current”. This contribution can be calculated and removed from the measured current instead of the background current. Here we provide an algorithm to calculate and remove the contribution of the slow-reaction pathway and to correct for the time lag of the fast-reaction pathway. This processing algorithm has been applied to ozonesonde profiles at Costa Rica and during the Central Equatorial Pacific Experiment (CEPEX) as well as to laboratory experiments evaluating the performance of ECC ozonesondes. At Costa Rica, where a 1 % KI, 1/10th buffer solution is used, there is no change in the derived total ozone column; however, in the upper troposphere and lower stratosphere, average reported ozone concentrations increase by up to 7 % and above 30 km decrease by up to 7 %. During CEPEX, where a 1 % KI, full-buffer solution was used, ozone concentrations are increased mostly in the upper troposphere, with no change near the top of the profile. In the laboratory measurements, the processing algorithms have been applied to measurements using the majority of current sensing solutions and using only the stronger pump efficiency correction reported by Johnson et al. (2002). This improves the accuracy of the ECC sonde ozone profiles, especially for low ozone concentrations or large ozone gradients and removes systematic biases relative to the reference instruments. In the surface layer, operational procedures prior to launch, in particular the use of filters, influence how typical gradients above the surface are detected. The correction algorithm may report gradients that are steeper than originally reported, but their uncertainty is strongly influenced by the prelaunch procedures.

Water Vapor and Ozone Variations in the Tropical Tropopause Layer during the Central Equatorial Pacific Experiment Campaign

Water Vapor and Ozone Variations in the Tropical Tropopause Layer during the Central Equatorial Pacific Experiment Campaign

Seasonal to decadal variations of water vapor in the tropical lower stratosphere observed with balloon‐borne cryogenic frost point hygrometers

We investigated water vapor variations in the tropical lower stratosphere on seasonal, quasi‐biennial oscillation (QBO), and decadal time scales using balloon‐borne cryogenic frost point hygrometer data taken between 1993 and 2009 during various campaigns including the Central Equatorial Pacific Experiment (March 1993), campaigns once or twice annually during the Soundings of Ozone and Water in the Equatorial Region (SOWER) project in the eastern Pacific (1998–2003) and in the western Pacific and Southeast Asia (2001–2009), and the Ticosonde campaigns and regular sounding at Costa Rica (2005–2009). Quasi‐regular sounding data taken at Costa Rica clearly show the tape recorder signal. The observed ascent rates agree well with the ones from the Halogen Occultation Experiment (HALOE) satellite sensor. Average profiles from the recent five SOWER campaigns in the equatorial western Pacific in northern winter and from the three Ticosonde campaigns at Costa Rica (10°N) in northern summer clearly show two effects of the QBO. One is the vertical displacement of water vapor profiles associated with the QBO meridional circulation anomalies, and the other is the concentration variations associated with the QBO tropopause temperature variations. Time series of cryogenic frost point hygrometer data averaged in a lower stratospheric layer together with HALOE and Aura Microwave Limb Sounder data show the existence of decadal variations: The mixing ratios were higher and increasing in the 1990s, lower in the early 2000s, and probably slightly higher again or recovering after 2004. Thus linear trend analysis is not appropriate to investigate the behavior of the tropical lower stratospheric water vapor.

Ozone sonde cell current measurements and implications for observations of near-zero ozone concentrations in the tropical upper troposphere

Abstract. Laboratory measurements of the Electrochemical Concentration Cell (ECC) ozone sonde cell current using ozone free air as well as defined amounts of ozone reveal that background current measurements during sonde preparation are neither constant as a function of time, nor constant as a function of ozone concentration. Using a background current, measured at a defined timed after exposure to high ozone may often overestimate the real background, leading to artificially low ozone concentrations in the upper tropical troposphere, and may frequently lead to operator dependent uncertainties. Based on these laboratory measurements an improved cell current to partial pressure conversion is proposed, which removes operator dependent variability in the background reading and possible artifacts in this measurement. Data from the Central Equatorial Pacific Experiment (CEPEX) have been reprocessed using the improved background treatment based on these laboratory measurements. In the reprocessed data set near-zero ozone events no longer occur. At Samoa, Fiji, Tahiti, and San Cristóbal, nearly all near-zero ozone concentrations occur in soundings with larger background currents. To a large extent, these events are no longer observed in the reprocessed data set using the improved background treatment.

Mixing Layer Formation near the Tropopause Due to Gravity Wave–Critical Level Interactions in a Cloud-Resolving Model

Abstract A plausible mechanism for the formation of mixing layers in the lower stratosphere above regions of tropical convection is demonstrated numerically using high-resolution, two-dimensional (2D), anelastic, nonlinear, cloud-resolving simulations. One noteworthy point is that the mixing layer simulated in this study is free of anvil clouds and well above the cloud anvil top located in the upper troposphere. Hence, the present mechanism is complementary to the well-known process by which overshooting cloud turrets causes mixing within stratospheric anvil clouds. The paper is organized as a case study verifying the proposed mechanism using atmospheric soundings obtained during the Central Equatorial Pacific Experiment (CEPEX), when several such mixing layers, devoid of anvil clouds, had been observed. The basic dynamical ingredient of the present mechanism is (quasi stationary) gravity wave–critical level interactions, occurring in association with a reversal of stratospheric westerlies to easterlies below the tropopause region. The robustness of the results is shown through simulations at different resolutions. The insensitivity of the qualitative results to the details of the subgrid scheme is also evinced through further simulations with and without subgrid mixing terms. From Lagrangian reconstruction of (passive) ozone fields, it is shown that the mixing layer is formed kinematically through advection by the resolved-scale (nonlinear) velocity field.

Reply to comment by John H. Helsdon Jr. on “On the roles of deep convective clouds in tropospheric chemistry”

Reply to comment by John H. Helsdon Jr. on “On the roles of deep convective clouds in tropospheric chemistry”

A New Parameterization of Single Scattering Solar Radiative Properties for Tropical Anvils Using Observed Ice Crystal Size and Shape Distributions

Parameterizations of single scattering properties currently used in cloud resolving and general circulation models are somewhat limited in that they typically assume the presence of single particle habits, do not adequately account for the numbers of ice crystals with diameters smaller than 100 mm, and contain no information about the variance of parameterization coefficients. Here, new parameterizations of mean single scattering properties (e.g., single scatter albedo, asymmetry parameter, and extinction efficiency) for distributions of ice crystals in tropical anvils are developed. Using information about the size and shape of ice crystals acquired by a twodimensional cloud probe during the Central Equatorial Pacific Experiment (CEPEX), a self-organized neural network defines shape based on simulations of how the particle maximum dimension and area ratio (ratio of projected area to that of circumscribed circle with maximum dimension) vary for random orientations of different idealized shapes (i.e., columns, bullet rosettes, rough aggregates, and particles represented by Chebyshev polynomials). The size distributions for ice crystals smaller than 100 mm are based on parameterizations developed using representative samples of 11 633 crystals imaged by a video ice particle sampler (VIPS). The meanscattering properties for distributions of ice crystals are then determined by weighting the single scattering properties of individual ice crystals, determined using an improved geometric ray-tracing method, according to number concentration and scattering cross section. The featureless nature of the calculated phase function, averaged over all observed sizes and shapes of ice crystals, is similar to that obtained using other schemes designed to account for variations in sizes and shapes of ice crystals. The new parameterizations of single scatter albedo, asymmetry parameter, and extinction efficiency are then determined by functional fits in terms of cloud particle effective radius; there was no statistically significant dependence on either ice water content or temperature. Uncertainty estimates incorporated into the parameterization coefficients are based upon a Monte Carlo approach. Comparisons with previously used parameterizations and with parameterizations developed using single crystal habits are made to show that the determination of representative crystal habits is still a major unknown in the development of parameterization schemes.

Effects of observed horizontal inhomogeneities within cirrus clouds on solar radiative transfer

[1] In situ microphysical and combined radar and radiometer measurements of 11 cirrus clouds from Central Equatorial Pacific Experiment (CEPEX), European Cloud and Radiation Experiment (EUCREX), investigation of Clouds by Ground-Based and Airborne Radar and Lidar (CARL), and First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) are used to investigate effects of horizontal cloud inhomogeneities on solar radiative transfer. A three-dimensional ray-tracing model (GRIMALDI), based on the Monte Carlo method, is used to calculate upward and downward flux densities and absorption for the spectral range from 0.38 to 4.0 μm. Radiative flux densities are calculated using the inhomogeneous clouds derived from the observations and for horizontally and vertically averaged homogeneous clouds. Horizontally averaged values of radiative flux densities and absorption for heterogeneous clouds can differ by up to 30% from those calculated for the homogeneous clouds for convectively induced tropical cirrus clouds. The midlatitude cases examined tended to be more homogeneous, and hence differences between radiative properties for the homogeneous and heterogeneous clouds did not exceed 10%. For cirrus clouds with mean optical thicknesses smaller than 5 and with relative variances of optical thickness smaller than 0.2, errors caused by the homogeneous assumption are smaller than ±10%.

Tropical water vapor correction for remotely sensed sea surface temperature: Results using narrowband window radiance profiles from TOGA COARE

This paper describes sea surface skin temperatures (SST) derived from unique aircraft‐based measurements and a physical multispectral retrieval technique. The aircraft data were acquired under moist atmospheric conditions over the western and eastern tropical Pacific Ocean during the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment and the Central Equatorial Pacific Experiment. Radiometric profiles were obtained in four contiguous channels over the spectral window region 800–1000 cm−1 during aircraft ascents from 0.03 through 4.6 km altitude. These bands subdivide the “split window” of present satellite radiometers used for measurements of SST. The use of four channels in the measurement approach was motivated by theoretical studies that suggest the need for three independent pieces of information to separate atmospheric and surface components of upwelling radiance in moist atmospheres. The analyses were unusual in that the absorption path length differed for each multispectral set of nadir observations made at discrete altitudes. The mean accuracy for retrievals at all altitudes was ≲1 K and was relatively insensitive to the guess profiles used for the simple forward radiance model. Simulated four‐channel retrievals demonstrate an rms accuracy of ≃0.3 K with negligible bias for global maritime profile conditions, which include extreme moisture‐laden samples. The results demonstrate that the combination of high‐precision measurements with a robust retrieval method can provide SST (instantaneous, global, day, or night) that meet the World Climate Research Programme accuracy objectives.

Heat Balance in the Pacific Warm Pool Atmosphere during TOGA COARE and CEPEX

The atmosphere above the western equatorial Pacific warm pool (WP) is an important source for the dynamic and thermodynamic forcing of the atmospheric general circulation. This study uses a high-resolution reanalysis and several observational datasets including Global Precipitation Climatology Project precipitation, Tropical Ocean Global Atmosphere (TOGA) Tropical Atmosphere Ocean moored buoys, and Earth Radiation Budget Experiment, TOGA Coupled Ocean‐Atmosphere Response Experiment (COARE), and Central Equatorial Pacific Experiment (CEPEX) radiation data to examine the details of the dynamical processes that lead to this net positive forcing. The period chosen is the period of two field experiments: TOGA COARE and CEPEX during December 1992‐March 1993. The four months used in the study were sufficient to establish that the warm pool atmosphere (WPA) was close to a state of radiative‐convective‐dynamic equilibrium. The analysis suggests that the large-scale circulation imports about 200 W m22 of sensible heat and about 140 W m22 of latent energy into the WPA mainly through the low-level mass convergence and exports about 420 W m22 potential energy mainly through the upper-level mass divergence. Thus the net effect of the large-scale dynamics is to export about 80 W m22 energy out of the WPA and cool the WPA by about 0.8 K day21. The dynamic cooling in addition to the radiative cooling of about 0.4 K day21 or 40 W m22 leads to a net radiative‐dynamic cooling of about 1.2 K day21 or 120 W m22, which should be balanced by convective heating of the same magnitude. The WPA radiative cooling is only about 0.4 K day21, which is considerably smaller than previously cited values in the Tropics. This difference is largely due to the cloud radiative forcing (CRF), about 70 W m 22, associated with the deep convective cirrus clouds in the WPA, which compensates the larger clear sky radiative cooling. Thus moist convection heats the WPA, not only through the direct convective heating, that is, the vertical eddy sensible heat and latent energy transport, but also through the indirect convective heating, that is, the CRF of deep convective clouds. The CRF of the deep convective clouds has a dipole structure, in other words, strong heating of the atmosphere through convergence of longwave radiation and a comparable cooling of the surface through the reduction of shortwave radiation at the surface. As a result, the deep convective clouds enhance the required atmospheric heat transport and reduce the required oceanic heat transport significantly in the WP. A more detailed understanding of these convective processes is required to improve our understanding of the heat transport by the large-scale circulation in the Tropics.

Determination of Ice Water Path and Mass Median Particle Size Using Multichannel Microwave Measurements

The method of simultaneously retrieving ice water path and mass median diameter using microwave data at two frequencies is examined and implemented for tropical nonprecipitating clouds. To develop the retrieval algorithm, the authors first derived a bulk mass–size relation for ice particles in tropical clouds based on microphysical data collected during the Central Equatorial Pacific Experiment. This relation effectively allows ice particle density to decrease with particle size. In implementing the retrieval algorithm, 150- and 220-GHz Millimeter-Wave Imaging Radiometer data collected during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment were used. Ice water path and mass median diameter are determined based on a lookup table generated by a radiative transfer model. The lookup table depends on cloud type, cloud liquid water path, and atmospheric temperature and humidity profiles. Only nonprecipitating clouds are studied in this paper. Error analyses were performed by a Monte Carlo procedure in which atmospheric profiles, ice cloud height, liquid water content, surface temperature, and instrument noise vary randomly within their uncertainty range through a Latin hypercube sampling scheme. The rms error in the retrievals is then assessed and presented in a two-dimensional diagram of ice water path and mass median diameter. It is shown that the simultaneous retrieval method using 150 and 220 GHz may be used for clouds with ice water path larger than 200 g m−2 and mass median diameter larger than 200 μm. To obtain meaningful retrievals for “thinner” clouds, higher microwave frequencies are needed. It is also shown that liquid water clouds that are at the same altitude as ice clouds interfere with the retrievals to a significant degree. To obtain reasonable ice water path and mass median size retrievals, it is necessary first to group clouds into several classes, then to apply separate algorithms to the different classes. The accuracy of the retrievals also depends on cloud type, with the best accuracy for cirrus and the worst for the midtop mixed-phase cloud among the clouds investigated in this study.

Determination of surface heating by convective cloud systems in the central equatorial Pacific from surface and satellite measurements

The heating of the ocean surface by longwave radiation from convective clouds has been estimated using measurements from the Central Equatorial Pacific Experiment (CEPEX). The ratio of the surface longwave cloud forcing to the cloud radiative forcing on the total atmospheric column is parameterized by the ƒ factor. The ƒ factor is a measure of the partitioning of the cloud radiative effect between the surface and the troposphere. Estimates of the ƒ factor have been obtained by combining simultaneous observations from ship, aircraft, and satellite instruments. The cloud forcing near the ocean surface is determined from radiometers on board the National Oceanic and Atmospheric Administration P‐3 aircraft and the R/V John Vickers. The longwave cloud forcing at the top of the atmosphere has been estimated from data obtained from the Japanese Geostationary Meteorological Satellite GMS 4. A new method for estimating longwave fluxes from satellite narrowband radiances is described. The method is based upon calibrating the satellite radiances against narrowband and broadband infrared measurements from the high‐altitude NASA ER‐2 aircraft. The average value of ƒ derived from the surface and satellite observations of convective clouds is 0.15±0.02. The area‐mean top‐of‐atmosphere longwave forcing by convective clouds in the region 10°S–10°N, 160°E–160°W is 40 W/m2 during CEPEX. These results indicate that the surface longwave forcing by convective clouds was approximately 5 W/m2 in the central equatorial Pacific and that this forcing is the smallest radiative component of the surface energy budget.

Thin and Subvisual Tropopause Tropical Cirrus: Observations and Radiative Impacts

Abstract In situ microphysical, remote sensing, and satellite observations of thin and subvisible cirrus have been used to establish their frequency of occurrence, determine their mean optical depths and radiative forcings, and to analyze their association with deep convection. A spatially thin layer of cirrus, with both base and top above 15 km, was observed in the central Pacific Tropics 29% of the time, with a mean thickness of 0.47 km, using a nadir-pointing Nd:YAG lidar operating at 1.064 μm during the Central Equatorial Pacific Experiment (CEPEX). In situ microphysical data collected in the mid-1970s and mid-1980s by a WB-57 and Learjet near Kwajalein, Marshall Islands, are revisited to determine typical ice crystal sizes and shapes that occur in this cloud type. Three observed vertical profiles, obtained from ascents/descents through cloud, are used with a δ-four-stream radiative transfer model to calculate observed heating rates of up to 1.0 K day−1, principally in the infrared, and cloud radiativ...

Use of observed ice crystal sizes and shapes to calculate mean‐scattering properties and multispectral radiances: CEPEX April 4, 1993, case study

During the Central Equatorial Pacific Experiment, ice crystal sizes and shapes were measured in an outflow anvil. A habit (i.e., column, bullet rosette, Koch fractal polycrystal, sphere) was assigned to each particle using a self‐organized neural network based on simulations of how the maximum particle dimension and area ratio varied for random orientations of these crystals. Average ice crystal size and shape distributions were calculated for 25 km long segments at six altitudes using measurements from a two‐dimensional cloud probe for crystals larger than 90 μm and a parameterization for smaller crystals based on measurements from the Video Ice Particle Sampler (VIPS). Mean‐scattering properties were determined by weighting the size and shape dependent single‐scattering properties computed with ray‐tracing algorithms according to scattering cross‐section. Reflectances at 0.664, 0.875, 1.621, and 2.142 μm were then calculated using a Monte Carlo radiative transfer routine. Although these reflectances agree reasonably with those measured by the MODIS airborne simulator (MAS) above the anvil, uncertainties in cloud base and system evolution prevent a determination of whether ray‐tracing or anomalous diffraction theory better predict reflectance. The calculated reflectances are as sensitive to the numbers and shapes of crystals smaller than 90 μm as to those of larger crystals. The calculated reflectances were insensitive to the classification scheme (i.e., neural network, discriminator analysis, and previously used classification scheme) for assigning particle shape to observed crystals. However, the reflectances significantly depended on assumed particle shape.

Analysis of the CEPEX ozone data using a 3D chemistry-meteorology model

The ozone (O3) data collected during the Central Equatorial Pacific Experiment (CEPEX) are analysed with the aid of the 3D global photochemistry model MATCH-MPIC (Model of Atmospheric Transport and Chemistry-Max-Planck-Institute for Chemistry version). This study focuses on using MATCH-MPIC to address three specific questions: (1) are the individual CEPEX O3 soundings, in particular the extremely low O3 levels occasionally observed in the upper troposphere (UT), reproducible by a state-of-the-art global photochemical model driven with analysed meteorological data from the same time period as the measurements (March 1993): (2) are the CEPEX O3 data likely to be representative of the mean state of the regional atmosphere, or do they instead indicate the degree of variability in this region of the atmosphere; and (3) what causes the UT O3 minima? It is found that MATCH-MPIC is not able to reproduce the soundings obtained during CEPEX on an individual basis; however, the model does reproduce some of the key features present in the observations, such as UT minima and mid-tropospheric maxima similar to those observed during CEPEX. the UT O3 minima computed by the model are mainly due to convective pumping of low-O3 marine boundary-layer air, as is demonstrated by comparison with the results of a run in which convective transport of O3 is suppressed. the UT O3 minima simulated by MATCH-MPIC (with O3 between 5 and 10 nmol mol-1) are less intense than those observed (with O3 < 5 nmol mol-1), even at relatively high model spatial and temporal resolution, and with a convection scheme that would be expected to readily produce UT O3 minima via intense pumping of low-O3 marine boundary-layer air directly into the UT convective outflow regions. This may indicate that additional photochemical loss processes are involved, either in situ in the UT or, in particular, near the surface in the convective inflow regions. where the model tends to overestimate the observed O3 levels. In addition, although a significant temporal variability for O3 in this region is computed, it is still less than indicated by the observations in the UT. This high degree of variability implies that individual O3 soundings will often differ considerably from the mean over a longer period. For instance, 20-50% less O3 in the UT during the CEPEX period is computed with MATCH-MPIC than the average for March. Thus, the CEPEX expedition was perhaps fortunate in encountering extreme conditions which produced extensive UT O3 minima. which have helped to demonstrate one end of the large degree of variability of O3 in the tropical troposphere.

Analysis of the CEPEX ozone data using a 3D chemistry‐meteorology model

AbstractThe ozone (O3) data collected during the Central Equatorial Pacific Experiment (CEPEX) are analysed with the aid of the 3D global photochemistry model MATCH‐MPIC (Model of Atmospheric Transport and Chemistry‐Max‐Planck‐Institute for Chemistry version). This study focuses on using MATCH‐MPIC to address three specific questions: (1) are the individual CEPEX O3 soundings, in particular the extremely low O3 levels occasionally observed in the upper troposphere (UT), reproducible by a state‐of‐the‐art global photochemical model driven with analysed meteorological data from the same time period as the measurements (March 1993): (2) are the CEPEX O3 data likely to be representative of the mean state of the regional atmosphere, or do they instead indicate the degree of variability in this region of the atmosphere; and (3) what causes the UT O3 minima? It is found that MATCH‐MPIC is not able to reproduce the soundings obtained during CEPEX on an individual basis; however, the model does reproduce some of the key features present in the observations, such as UT minima and mid‐tropospheric maxima similar to those observed during CEPEX. the UT O3 minima computed by the model are mainly due to convective pumping of low‐O3 marine boundary‐layer air, as is demonstrated by comparison with the results of a run in which convective transport of O3 is suppressed. the UT O3 minima simulated by MATCH‐MPIC (with O3 between 5 and 10 nmol mol‐1) are less intense than those observed (with O3 &lt; 5 nmol mol‐1), even at relatively high model spatial and temporal resolution, and with a convection scheme that would be expected to readily produce UT O3 minima via intense pumping of low‐O3 marine boundary‐layer air directly into the UT convective outflow regions. This may indicate that additional photochemical loss processes are involved, either in situ in the UT or, in particular, near the surface in the convective inflow regions. where the model tends to overestimate the observed O3 levels. In addition, although a significant temporal variability for O3 in this region is computed, it is still less than indicated by the observations in the UT. This high degree of variability implies that individual O3 soundings will often differ considerably from the mean over a longer period. For instance, 20‐50% less O3 in the UT during the CEPEX period is computed with MATCH‐MPIC than the average for March. Thus, the CEPEX expedition was perhaps fortunate in encountering extreme conditions which produced extensive UT O3 minima. which have helped to demonstrate one end of the large degree of variability of O3 in the tropical troposphere.

Long-Term Behavior of Cloud Systems in TOGA COARE and Their Interactions with Radiative and Surface Processes. Part II: Effects of Ice Microphysics on Cloud–Radiation Interaction

Abstract A two-dimensional cloud-resolving model with a large domain is integrated for 39 days during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) to study the effects of ice phase processes on cloud properties and cloud radiative properties. The ice microphysical parameterization scheme is modified based on microphysical measurements from the Central Equatorial Pacific Experiment. A nonlocal boundary layer diffusion scheme is included to improve the simulation of the surface heat fluxes. The modified ice scheme produces fewer ice clouds during the 39-day simulation. The cloud radiative properties show significant improvement and compare well with various observations. Both the 39-day mean value (202 W m−2) and month-long evolution of outgoing longwave radiative flux from the model are comparable with satellite observations. The 39-day mean surface shortwave cloud forcing is −110 W m−2, consistent with other estimates obtained for TOGA COARE. The 39-day me...

Critical levels and mixing layers induced by convectively generated gravity waves during CEPEX

AbstractWe study data from the Central Equatorial Pacific Experiment, in particular the soundings providing temperature, pressure, wind and ozone profiles, and we link them to the large‐scale conditions deduced from the analyses of the European Centre for Medium‐Range Weather Forecasts. to the west of the date line, the mean wind shows a strong shear at the tropopause between the easterlies of the upper troposphere and the westerlies of the lower stratosphere. These are conditions propitious to the formation of critical levels for quasi‐stationary gravity waves generated by deep convection. the measured wind profiles show the main characteristics of the encounter of a wave with its critical level and, in the ozone and potential‐temperature profiles, the existence of a conspicuous mixing layer in the vicinity of this critical level. Our interpretation that the mixing layer is a consequence of the critical level is supported by numerical simulations with a nonlinear time‐dependent two‐dimensional model. the occurrence of a mixing layer in the tropopause structure seems to be characteristic of large‐scale equatorial convective areas during westerly phases of the quasi‐biennial oscillation.

Detecting tropical convection using AVHRR satellite data

A new method is introduced to identify active convective cloud regions in high‐resolution AVHRR satellite data. This method is developed to overcome several limitations of existing visible/infrared techniques. Steep temperature gradients between the convective overshoots of cumulonimbus clouds and the surrounding cirrus are used to identify the edges of active convective regions. The limitations of previous methods are shown using up to 1.1 km resolution AVHRR data of the NOAA‐11 and NOAA‐12 satellites collected during the Central Equatorial Pacific Experiment (CEPEX), a measurement campaign that took place over the central Pacific Ocean between March 7 and April 6, 1993. Differences in the identification of convection between the new algorithm and the existing algorithms are clarified by a case study and further illustrated by three collocations between NOAA‐11 and NOAA‐12 overpasses and observations made with the Massachusetts Institute of Technology (MIT) 5 cm Doppler radar onboard the research vessel R/V Vickers.

Potential retrieval of dominating crystal habit and size using radiance data from a dual‐view and multiwavelength instrument: A tropical cirrus anvil case

The potential to retrieve dominating crystal habit, size and bulk optical depth for tropical anvil cirrus using radiance measurements from a dual‐view and multiwavelength instrument is demonstrated. The radiance measurements are taken from the along‐track scanning radiometer (ATSR‐2) instrument at wavelengths of 0.87 and 1.6 μm. The ATSR‐2 has a dual‐view capability being able to make radiance measurements at near nadir and at a forward view of 55° of the same cloud. The dual‐view radiance measurements at 0.87 μm are best fitted by a polycrystal phase function and estimates of crystal size (crystal size in this paper is defined in terms of the crystal maximum dimension) using the 1.6 μm channel vary between 25 and 100 μm. Two other tropical cirrus anvil cases are used to show qualitatively the variation of crystal size with anvil maturity. Quantitative comparisons are made with observations taken from the Central Equatorial Pacific Experiment with regard to dominating habit and cloud‐top crystal size.