Abstract

The datasets currently being collected by the Atmospheric Radiation Measurement (ARM) program on the islands of Nauru and Manus represent the longest time series of ground-based cloud measurements in the tropical western Pacific region. In this series of papers, a shortwave flux closure study is presented using observations collected at the Nauru site between June 1999 and May 2000. The first paper presented frequency of occurrence of nonprecipitating clouds detected by the millimeter-wavelength cloud radar (MMCR) at Nauru and statistics of their retrieved microphysical properties. This paper presents estimates of the cloud radiative effect over the study period and results from a closure study in which retrieved cloud properties are input to a radiative transfer model and the modeled surface fluxes are compared to observations.

The average surface shortwave cloud radiative forcing is 48.2 W m⁻², which is significantly smaller than the cloud radiative forcing estimates found during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) field project. The difference in the estimates during the two periods is due to the variability in cloud amount over Nauru during different phases of the El Niño–Southern Oscillation (ENSO). In the closure study, modeled and observed surface fluxes show large differences at short time scales, due to the temporal and spatial variability of the clouds observed at Nauru. Averaging over 60 min reduces the average root-mean-square difference in total flux to 10% of the observed flux. Modeled total downwelling fluxes are unbiased with respect to the observed fluxes while direct fluxes are underestimated and diffuse fluxes are overestimated. Examination of the differences indicates that cloud amount derived from the ground-based measurements is an overestimate of the radiatively important cloud amount due to the anisotropy of the cloud field at Nauru, interpolation of the radar data, uncertainty in the microwave brightness temperature measurements for thin clouds, and the uncertainty in relating the sixth moment of the droplet size distribution observed by the radar to the more radiatively important moments.

Abstract

The datasets currently being collected at the Atmospheric Radiation Measurement (ARM) program's sites on the islands of Nauru and Manus represent the longest time series of ground-based cloud measurements available in the tropical western Pacific region. In this and a companion paper, a shortwave flux closure study is presented using observations collected at the Nauru site between June 1999 and May 2000. This paper presents frequency of occurrence of nonprecipitating liquid and ice clouds detected by the millimeter wavelength cloud radar (MMCR) and statistics of retrieved microphysical properties. The companion paper presents results from a closure study in which the retrieved cloud properties are input to a radiative transfer model, and the modeled surface fluxes are compared to observations. The liquid cloud properties are retrieved from MMCR and microwave radiometer (MWR) measurements using a Bayesian retrieval technique. Properties of ice phase clouds are retrieved from MMCR measurements using regression equations based on in situ observations taken during the Central Equatorial Pacific Experiment (CEPEX). Nonprecipitating liquid clouds were observed at Nauru in 35% of the radar observations. These clouds were primarily shallow cumulus with bases less than 1 km. Of the retrieved liquid clouds, 90% had liquid water path less than 100 g m⁻². The average retrieved effective radius was 7.5 μm. The frequency of liquid cloud detection and height of the liquid cloud base showed a clear diurnal cycle, which is most likely related to the island effect and the existence of the Nauru cloud plume. Ice clouds with no underlying liquid clouds were detected in 16.5% of the radar observations and ice clouds above liquid clouds in 7.7% of the observations. The mean retrieved IWP of the radar-detected ice clouds was 22.1 g m⁻², and the mean effective diameter retrieved was 72 μm. Large monthly variability was seen in both the amount of cirrus detected and the retrieved ice water path. Ice clouds were observed by the radar more frequently at night than during the day at Nauru, but there was no clear diurnal trend in the retrieved microphysical properties.

Abstract

Nauru Island at times generates low clouds that impact low-level cloud statistics and downwelling shortwave radiation measurements made at the Atmospheric Radiation Measurement Program (ARM) site. This study uses five years of Nauru data to quantify the island impact on the site measurements. The results indicate that the solar-heating-produced Nauru island effect occurs about 11% of the time during daylight hours. The island effect increases the 500–1000-m cloud base occurrence by 15%–20% when clouds occur, but because the island effect only occurs 11% of the time the overall increase in daylight low-cloud statistics is 2%, or 1% for 24-h statistics. In a similar way, the island effect produces a reduction of about 17% in the downwelling shortwave (SW) radiation across the daylight hours during the 11% of the time it occurs, an overall 2% daylight (or 1% for 24 h) average reduction. The island effect produces frequent positive downwelling SW cloud effects, in particular during the morning, which tend to somewhat mitigate the overall decrease in downwelling SW radiation that is due to clouds. This produces 17 W m⁻² less daylight average SW cloud effect relative to non-island-effect times, in particular for the convectively suppressed regime that typifies island-effect-producing conditions. For long-term overall statistical studies such as model and satellite comparisons, the 2% daylight (or 1% per 24 h) average increase in low-level cloud occurrence and decrease in downwelling SW are not of large concern as long as researchers are aware of them. For shorter-term studies, however, or those that separate data by conditions such as convectively active/suppressed regimes, the Nauru island effect can have significant impacts.

Abstract

Ground-based high temporal and vertical resolution datasets from observations during 2002–07 at the Atmospheric Radiation Measurement (ARM) tropical western Pacific (TWP) site on Manus Island are used to examine the characteristics of clouds and rainfall associated with the active phase of the Madden–Julian oscillation (MJO) passing over Manus. A composite MJO event at Manus is developed based on the NOAA MJO index 4 and precipitation using 13 events. The cloud characteristics associated with the active phase of the MJO at Manus show a two-phase structure as the wave passes over Manus. During the development phase, congestus plays an important role, and the enhanced convection is located between surface westerly and easterly wind anomalies (type-I structure). During the mature phase, deep convection is the dominant cloud type, and the enhanced convection is collocated with the westerly wind anomalies (type-II structure). Consistent with this two-phase structure, the heavy rainfall frequency also shows a two-peak structure during the MJO disturbance, while light rainfall does not show a clear relation to the intraseasonal disturbance associated with the MJO. In addition, a positive relationship between the precipitation rate and precipitable water vapor exists at Manus, and the atmospheric column is less moist after the passing of the MJO convection center than before.

Abstract

An Atmospheric Radiation and Cloud Station (ARCS) was established on the island of Nauru by the Atmospheric Radiation Measurement (ARM) Program. Analysis of the Nauru99 field experiment data indicated that measurements at the ARCS were affected by a cloud plume that was induced by diurnal heating of the island. During the Nauru Island Effects Study, instrumentation was installed at a second site to develop criteria for identifying when the cloud plume occurs and to quantify its effect on ARCS measurements. The plume directional heading and frequency of occurrence are affected by the large-scale tropical circulation. During the present study, in which an El Niño was developing, Nauru was in a region of active convection, and easterly trade winds were not dominant; plumes were observed in 25% of satellite images, and only one-half of the observed plumes were downwind of the ARCS site. Surface wind direction, surface air temperature, and downwelling solar radiation at the two sites were used to identify periods when the cloud plume affected surface measurements. Differences in low-cloud frequency and surface radiation between plume-affected and non-plume-affected periods were examined. Existence of the cloud plume increased the average low-cloud frequency of occurrence from 20% to 35%, decreased the average downwelling shortwave radiation by 50–60 W m⁻², and increased the average downwelling longwave radiation by 5–10 W m⁻². Installing a suite of surface meteorological instruments and a global shortwave radiometer at a second site will allow for the long-term quantification of the cloud plume effect on the radiation field at the ARCS site.

Abstract

Cloud radiative effects on surface downwelling fluxes are investigated using datasets from the Atmospheric Radiation Measurement Program (ARM) sites in the tropical western Pacific Ocean (TWP) region. The Nauru Island (Republic of Nauru) and Darwin, Australia, sites show large variability in sky cover, downwelling radiative fluxes, and surface cloud radiative effect (CRE) that is due to El Niño–Southern Oscillation (ENSO) and the Australian monsoon, respectively, whereas the Manus Island (Papua New Guinea) site shows little intraseasonal or interannual variability. At Nauru, the average shortwave (SW) surface CRE varies from −38.2 W m⁻² during La Niña conditions to −90.6 W m⁻² during El Niño conditions. The average longwave (LW) CRE ranges from 9.5 to 15.8 W m⁻² during La Niña and El Niño conditions, respectively. At Manus, the average SW and LW CREs vary by less than 5 and 2 W m⁻², respectively, between the ENSO phases. The variability at Darwin is even larger than at Nauru, with average SW (LW) CRE ranging from −27.0 (8.6) W m⁻² in the dry season to −95.8 (17.0) W m⁻² in the wet season. Cloud radar measurements of cloud-base and cloud-top heights are used to define cloud types to examine the effect of cloud type on the surface CRE. Clouds with low bases contribute 71%–75% of the surface SW CRE and 66%–74% of the surface LW CRE at the three TWP sites, clouds with midlevel bases contribute 8%–9% of the SW CRE and 12%–14% of the LW CRE, and clouds with high bases contribute 16%–19% of the SW CRE and 15%–21% of the LW CRE.

Abstract

A 4-yr climatology of midlevel clouds is presented from vertically pointing cloud lidar and radar measurements at the Atmospheric Radiation Measurement Program (ARM) site at Darwin, Australia. Few studies exist of tropical midlevel clouds using a dataset of this length. Seventy percent of clouds with top heights between 4 and 8 km are less than 2 km thick. These thin layer clouds have a peak in cloud-top temperature around the melting level (0°C) and also a second peak around −12.5°C. The diurnal frequency of thin clouds is highest during the night and reaches a minimum around noon, consistent with variation caused by solar heating. Using a 1.5-yr subset of the observations, the authors found that thin clouds have a high probability of containing supercooled liquid water at low temperatures: ~20% of clouds at −30°C, ~50% of clouds at −20°C, and ~65% of clouds at −10°C contain supercooled liquid water. The authors hypothesize that thin midlevel clouds formed at the melting level are formed differently during active and break monsoon periods and test this over three monsoon seasons. A greater frequency of thin midlevel clouds are likely formed by increased condensation following the latent cooling of melting during active monsoon periods when stratiform precipitation is most frequent. This is supported by the high percentage (65%) of midlevel clouds with preceding stratiform precipitation and the high frequency of stable layers slightly warmer than 0°C. In the break monsoon, a distinct peak in the frequency of stable layers at 0°C matches the peak in thin midlevel cloudiness, consistent with detrainment from convection.

Abstract

Vertically pointing millimeter-wavelength radar observations of anvil clouds extending from mesoscale convective systems (MCSs) that pass over an Atmospheric Radiation Measurement Program (ARM) field site in Niamey, Niger, are compared to anvil structures generated by the Weather Research and Forecasting (WRF) mesoscale model using six different microphysical schemes. The radar data provide the statistical distribution of the radar reflectivity values as a function of height and anvil thickness. These statistics are compared to the statistics of the modeled anvil cloud reflectivity at all altitudes. Requiring the model to be statistically accurate at all altitudes is a stringent test of the model performance. The typical vertical profile of radiative heating in the anvil clouds is computed from the radar observations. Variability of anvil structures from the different microphysical schemes provides an estimate of the inherent uncertainty in anvil radiative heating profiles. All schemes underestimate the optical thickness of thin anvils and cirrus, resulting in a bias of excessive net anvil heating in all of the simulations.

Abstract

To improve understanding of the convective processes key to the Madden–Julian oscillation (MJO) initiation, the Dynamics of the MJO (DYNAMO) and the Atmospheric Radiation Measurement Program (ARM) MJO Investigation Experiment (AMIE) collected 4 months of observations from three radars—the S-band dual-polarization Doppler radar (S-Pol), the C-band Shared Mobile Atmospheric Research and Teaching Radar (SMART-R), and Ka-band ARM zenith radar (KAZR)—along with radiosonde and comprehensive surface meteorological instruments on Addu Atoll, Maldives, in the tropical Indian Ocean. One DYNAMO/AMIE hypothesis suggests that the evolution of shallow and congestus cloud populations is essential to the initiation of the MJO. This study focuses on evaluating the ability of these three radars to document the full spectrum of cloud populations and to construct a merged cloud–precipitation radar dataset that can be used to test this hypothesis. Comparisons between collocated observations from the three radars show that KAZR provides the only reliable estimate of shallow clouds, while S-Pol/SMART-R can reasonably detect congestus within the 30–50-km range in addition to precipitating deep clouds. On the other hand, KAZR underestimates cloud-top heights due to rainfall attenuation in ~34% of the precipitating clouds, and an empirical method to correct KAZR cloud-top height bias is proposed. Finally, a merged KAZR–S-Pol dataset is produced to provide improved cloud-top height estimates, total hydrometeor microphysics, and radiative heating rate retrievals. With this dataset the full spectrum of tropical convective clouds during DYNAMO/AMIE can be reliably constructed and, together with complimentary radiosonde data, it can be used to study the role of shallow and congestus clouds in the initiation of the MJO.

The large horizontal extent, with its location in the cold upper troposphere, and ice composition make cirrus clouds important modulators of the Earth's radiation budget and climate. Cirrus cloud microphysical properties are difficult to measure and model because they are inhomogeneous in nature and their ice crystal size distribution and habit are not well characterized. Accurate retrievals of cloud properties are crucial for improving the representation of cloud-scale processes in largescale models and for accurately predicting the Earth's future climate. A number of passive and active remote sensing retrieval algorithms exist for estimating the microphysical properties of upper-tropospheric clouds. We believe significant progress has been made in the evolution of these retrieval algorithms in the last decade; however, there is room for improvement. Members of the Atmospheric Radiation Measurement (ARM) program Cloud Properties Working Group are involved in an intercomparison of optical depth τ and ice water path in ice clouds retrieved using ground-based instruments. The goals of this intercomparison are to evaluate the accuracy of state-of-the-art algorithms, quantify the uncertainties, and make recommendations for their improvement.

Currently, there are significant discrepancies among the algorithms for ice clouds with very small optical depths (τ < 0.3) and those with 1 < τ < 5. The good news is that for thin clouds (0.3 < τ < 1), the algorithms tend to converge. In this first stage of the intercomparison, we present results from a representative case study, compare the retrieved cloud properties with aircraft and satellite measurements, and perform a radiative closure experiment to begin gauging the accuracy of these retrieval algorithms.