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Mark. A. Miller and Anthony Slingo

The Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF) was recently developed to enable collection of detailed climate data in locations not currently sampled by ARM's five fixed sites. The AMF includes a comprehensive suite of active and passive remote sensors, including cloud radar, that sample the atmosphere in a narrow column above its location. Surface radiation, aerosols, and fluxes are also measured and there is an ancillary measurement facility to help quantify local gradients. The AMF is deployed at no cost to the principal investigator or institution for periods from six months to one year on the basis of an international proposal competition judged by a nonpartisan board. The proposal to ARM that led to the initial international deployment of the AMF in Niamey, Niger, was titled “Radiative Atmospheric Divergence Using the AMF, GERB Data, and AMMA Stations (RADAGAST).” This paper provides a description of the instruments that compose the AMF, its charter, a description of its deployment in support of RADAGAST, and examples of data that have been collected in Africa.

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Melissa kazemirad and Mark A. Miller

Abstract

Marine boundary layer (MBL) cloud morphology associated with two summertime cold fronts over the eastern North Atlantic (ENA) is investigated using high-resolution simulations from the Weather Research and Forecasting (WRF) Model and observations from the Atmospheric Radiation Measurement (ARM) ENA Climate Research Facility. Lagrangian trajectories are used to study the evolution of post-cold-frontal MBL clouds from solid stratocumulus to broken cumulus. Clouds within specified domains in the vicinity of transitions are classified according to their degree of decoupling, and cloud-base and cloud-top breakup processes are evaluated. The Lagrangian derivative of the surface latent heat flux is found to be strongly correlated with that of the cloud fraction at cloud base in the simulations. Cloud-top entrainment instability (CTEI) is shown to operate only in the decoupled MBL. A new indicator of inversion strength at cloud top that employs the vertical gradients of equivalent potential temperature and saturation equivalent potential temperature, which can be computed directly from soundings, is proposed as an alternative to CTEI. Overall, results suggest that the deepening–warming hypothesis suggested by Bretherton and Wyant explains many of the characteristics of the summertime postfrontal MBL evolution of cloud structure over the ENA, thereby widening the phase space over which the hypothesis may be applied. A subset of the deepening–warming hypothesis involving warming initially dominating over moistening is proposed. It is postulated that changes in climate change–induced modifications in cold-frontal structure over the ENA may be accompanied by coincident changes in the location and timing of MBL cloud transitions in the post-cold-frontal environment.

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Laury Miller, D. Randolph Watts, and Mark Wimbush

Abstract

For 14 months in 1980–81, surface dynamic height was monitored with inverted echo sounders at five sites from 0 to 9°N along 110°W. These records show that the SEC/NECC equatorial current system was well-developed during the boreal summer and fall, but weak and irregular during winter and spring when westward flow associated with the NEC extended as far south as 6°N.

Superimposed on the mean dynamic topography of this region are energetic 20-to-80-day oscillations, longer periods being associated with higher latitudes. Near the equatorial ridge (∼5°N), these oscillations have predominantly monthly periods, and amplitude of ∼10 dyn cm comparable to the mean dynamic-height difference across the NECC. The broad in-phase meridional extent of these monthly oscillations implies that the principal mode of ridge variation is vertical undulation rather than meridional meandering, producing large in-phase monthly modulations in transport of the SEC and NECC.

Oscillations or the equatorial ridge are correlated with propagating ∼1000-km wavelength sea surface temperature (SST) wave patterns observed in satellite infrared imagery. Passage of a northerly SST crest on the equatorial front at 110°W corresponds to a dynamic height minimum on the equatorial ridge. The relative phase and trochoidal shape of these crests is explained kinematically by superposition of the observed mean and oscillatory dynamic-height fields.

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Mark A. Miller and Bruce A. Albrecht

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Data obtained from the island of Santa Maria in the Azores, during the Atlantic Stratocumulus Transition Experiment (ASTEX) are used to describe cloud and boundary-layer structure for a 24-h period on 15 June 1992 over the east-central Atlantic (37°N, 25°10′W). The evolution of the mesoscale cloud structure during a 24-h period in the vertical column above the surface site was characterized using a 94-GHz radar, a laser ceilometer, 3-h radiosonde ascents, and surface micrometeorological instrumentation. Mesoscale circulations and drizzle were found to be key elements of the boundary-layer clouds observed in this region.

During the late-night and predawn hours of the study period, a single layer of stratocumulus that averaged ∼200 m in thickness topped a well-mixed marine boundary layer. Mesoscale cellular convection (MCC), which had ascending regions with horizontal dimensions of ∼7 km, was observed during this period. At sunrise, decoupling was imposed on this MCC, and extreme mesoscale variations in the cloud thickness and surface precipitation rate were observed. These variations included mesoscale patches of cumulus that rose from the surface-lifting condensation level into the overlying stratocumulus (cumulus–stratocumulus interaction), co-existing with patches of decoupled stratocumulus that occasionally had small, shallow cumulus beneath. The average horizontal scale of cumulus–stratocumulus interaction regions was found to be on the order of ∼12 km, and mesoscale variations in the cloud thickness of as much as 400 m were indicated in the remote sensor data during the daytime.

Major drizzle events observed at the surface were shown to correspond with the deepening of the cumulus layer. Evidence was presented that mesoscale cumulus-stratocumulus interaction regions were affecting the surrounding decoupled regions through (i) the vertical transport of properties of the surface moist layer to decoupled stratocumulus by nearby cumulus and (ii) the extended influence of subcloud-layer wakes induced by major precipitation events to nearby decoupled regions. It was suggested that the enhanced precipitation due to cumulus convection may be an important feedback mechanism between the cloud and subcloud layers in the transition region.

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Allison B. Marquardt Collow and Mark A. Miller

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Changes in the climate system of the Amazon rain forest of Brazil can impact factors that influence the radiation budget such as clouds, atmospheric moisture, and the surface albedo. This study examines the relationships between clouds and radiation in this region using surface observations from the first year of the deployment of the Atmospheric Radiation Measurement (ARM) Program’s Mobile Facility 1 (AMF1) in Manacapuru, Brazil, and satellite measurements from the Clouds and the Earth’s Radiant Energy System (CERES). The seasonal cycles of the radiation budget and cloud radiative effects (CREs) are evaluated at the top of the atmosphere (TOA), at the surface, and within the atmospheric column using these observations and are placed into a regional context using the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). Water vapor and clouds are abundant throughout the year, even though slight decreases are observed in the dry season. The column water vapor load is large enough that the longwave radiative flux divergence is nearly constant throughout the year. Clouds produce a significant shortwave CRE at the surface and TOA, exceeding 200 W m−2 during the wet season. Discrepancies, especially in column shortwave radiative absorption, between the observations and MERRA-2 are demonstrated that warrant additional analysis of the microphysical and macrophysical cloud properties in MERRA-2. More trustworthy fields in the MERRA-2 product suggest that the expansive nearby river system impacts the regional radiation budget and thereby renders AMF1 observations potentially biased relative to regions farther removed from rivers within the Amazon rain forest.

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Robert N. Miller and Mark A. Cane

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The Kalman filter is implemented and tested for a simple model of sea level anomalies in the tropical Pacific, using tide gauge data from six selected island stations to update the model. The Kalman filter requires detailed statistical assumptions about the errors in the model and the data. In this study, it is assumed that the model errors are dominated by the errors in the wind stress analysis. The error model is a simple covariance function with parameters fit from the observed differences between the tide gauge data and the model output. The fitted parameters are consistent with independent estimates of the errors in the wind stress analysis. The calibrated error model is used in a Kalman filtering scheme to generate monthly sea level height anomaly maps for the tropical Pacific. The filtered maps, i.e., those which result from data assimilation, exhibit fine structure that is absent from the unfiltered model output, even in regions removed from the data insertion points. Error estimates, an important byproduct of the scheme, suggest that the filter reduces the error in the equatorial wave guide by about 1 cm. The few independent verification points available are consistent with this estimate. Given that only six data points participate in the data assimilation, the results are encouraging, but it is obvious that model errors cannot be substantially reduced without more data.

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Mark A. Miller, Mary Jane Bartholomew, and R. Michael Reynolds

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An analytical uncertainty propagation model is used in conjunction with laboratory and field data to quantify the uncertainty in measurements of the direct-normal irradiance, aerosol optical thickness, and Ångström exponent made with a ship-mounted fast-rotating shadow-band radiometer (FRSR). Total uncertainties in FRSR measurements of aerosol optical thickness are found to be 0.02–0.03 at the 95% confidence level (two standard deviations). The “lever-arm” effect, a salient characteristic of the Langely technique in which uncertainties in aerosol optical thickness measurements are reduced as the solar zenith angle increases, is essentially offset by orientation uncertainty. Lack of a lever-arm effect precludes Langley calibration of FRSRs while at sea; they must be calibrated on land. Uncertainties in FRSR measurements of the two-wavelength Ångström exponent are shown to depend strongly on the aerosol optical thickness, with the maximum uncertainty of 0.6 associated with clean, maritime air masses.

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R. Michael Reynolds, Mark A. Miller, and Mary J. Bartholomew

Abstract

This paper describes the design, calibration, and deployment of a fast-rotating shadowband radiometer (FRSR) that accurately decomposes downward shortwave (solar) irradiance into direct-beam and diffuse components from a moving platform, such as a ship on the ocean. The FRSR has seven channels, one broad-band silicone detector, and six 10-nm-wide channels at 415, 500, 610, 660, 860, and 940 nm. The shadowband technique produces estimates of the direct-beam normal irradiance, the diffuse irradiance (sky component), and the total irradiance. The direct-beam normal irradiances produce time series of aerosol optical thickness. A proven ability to derive meaningful at-sea estimates of aerosol optical depth from an economical, automated, and reliable instrument opens the way to a distributed network of such measurements from volunteer observing ships in all areas of the World Ocean. The processing algorithms are key to the instrument’s ability to derive direct-normal beam irradiance without gimbals and a gyro-stabilized table. At-sea Langley plots were produced during the Aerosols99 cruise of the R/V Ronald H. Brown from Norfolk, Virginia, to Cape Town, South Africa. A Langley calibration of the instrument at the Mauna Loa Observatory confirmed prior calibrations and demonstrated that the calibration was stable over the duration of the cruise. The standard deviation in all plots was of the order 2% for all channels.

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John N. Porter, Mark Miller, Christophe Pietras, and Craig Motell

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The use of hand-held Microtops II sun photometers (built by Solar Light Inc.) on ship platforms is discussed. Their calibration, filter stability, and temperature effects are also described. It is found that under rough conditions, the ship motion causes the largest error, which can result in a bias toward higher optical depths. In order to minimize this bias, a large number of sun photometer measurements (∼25) should be taken in a short period of time, and the higher values should be discarded. Under rough ocean conditions, it is also best to shorten the Microtops sun photometer sampling period (less than 5 s) and save only a single value (no averaging) and remove the high optical depths in postprocessing. It is found that the Microtops should be turned off frequently to correct for zero drift caused by temperature effects. Calibration is maintained by routine Langley plot calibrations at the Mauna Loa Observatory for each unit or through cross calibration.

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Mark A. Miller, Michael P. Jensen, and Eugene E. Clothiaux

Abstract

Radiosonde, in situ, and surface-based remote sensor data from the Atlantic Stratocumulus Transition Experiment are used to study the diurnal cycle of cloud and thermodynamic structure. A cloud layer and decoupled subcloud layer separated by a stable transition layer, often observed in the vicinity of cumulus cloud base, characterizes the thermodynamic structure during the study period. The mode of cloud structure is cumulus with bases below decoupled stratus. Data are presented that support the hypothesis that diurnal variations in cumulus development are modulated by the stability in the transition layer.

The frequency of cumulus convection decreases during the afternoon, but mesoscale regions of vigorous cumulus with cloud tops overshooting the base of the trade inversion and increased surface drizzle rates are present during the late afternoon and early evening, when the transition layer is the most stable. It is postulated that mesoscale organization may be required to accumulate enough water vapor in the subcloud layer to produce the convective available potential energy needed for developing cumulus to overcome transition layer stability. The mesoscale regions appear to fit the description of cyclic cumulus convection proposed in a previous study, and this theory is expanded to account for diurnal variations in the stability of the transition layer. The occurrence of these mesoscale clusters of vigorous convection makes it difficult to determine if the latent heat flux in the cloud layer has actually decreased in the late afternoon and early evening, when the transition layer is the most stable.

Liquid water structure was examined and no pronounced diurnal signal was found. Results showed that clouds thicker than approximately 450 m tended to have subadiabatic integrated liquid water contents, presumably due to evaporation of drizzle in the subcloud layer, removal of liquid water at the surface, and the evaporation of cloud water at cloud top. A significant fraction of clouds less than 450 m thick produced liquid water contents that were greater than adiabatic, and there may be a physical mechanism that could produce such values in this cloud system (i.e., lateral detrainment of cloud water from convective elements mixing with existing liquid water in decoupled stratus or with liquid water detrained by nearby convective elements). Unfortunately, instrument limitations may have also produced these greater-than-adiabatic values and the extent of instrument artifacts in these results is unclear.

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