Understanding orographic effects on surface observations at Macquarie Island

The meteorological observations on Macquarie Island have become of increasing value for efforts to understand the unique nature of atmospheric processes over the Southern Ocean. While the island is of modest elevation (peak altitude of 410 m), the orographic effects on observations on this island are still not clear. High-resolution numerical simulations [Weather Research and Forecasting (WRF) Model] with and without terrain have been used to identify orographic effects for four cases representing common synoptic patterns at Macquarie Island: a cold front, a warm front, postfrontal drizzle, and a midlatitude cyclone. Although the simulations cannot capture every possible feature of the precipitation, preliminary results show that clouds and precipitation can readily be perturbed by the island with the main enhancement of precipitation normally in the lee in accordance with the nondimensional mountain height being much less than 1. The weather station is located at the far north end of the island and is only in the lee to southerly and southwesterly winds, which are normally associated with drizzle. The station is on the upwind side for strong northwesterly winds, which are most common and can bring heavier frontal precipitation. Overall the orographic effect on the precipitation record is not found to be significant, except for the enhancement of drizzle found in southwesterly winds. Given the strong winds over the Southern Ocean and the shallow height of the island, the 3D nondimensional mountain height is smaller than 1 in 93.5% of the soundings. As a result, boundary layer flow commonly passes over the island, with the greatest impact in the lee

Lidar ratios of stratospheric volcanic ash and sulfate aerosols retrieved from CALIOP measurements

We apply a two-way transmittance constraint to nighttime CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) observations of volcanic aerosol layers to retrieve estimates of the particulate lidar ratio (Sp) at 532 nm. This technique is applied to three volcanic eruption case studies that were found to have injected aerosols directly into the stratosphere. Numerous lidar observations permitted characterization of the optical and geometric properties of the volcanic aerosol layers over a time period of 1–2 weeks. For the volcanic ash-rich layers produced by the Puyehue-Cordón Caulle eruption (June 2011), we obtain mean and median particulate lidar ratios of 69 ± 13 sr and 67 sr, respectively. For the sulfate-rich aerosol layers produced by Kasatochi (August 2008) and Sarychev Peak (June 2009), the means of the retrieved lidar ratios were 66 ± 19 sr (median 60 sr) and 63 ± 14 sr (median 59 sr), respectively. The 532 nm layer-integrated particulate depolarization ratios (δp) observed for the Puyehue layers (δp = 0.33 ± 0.03) were much larger than those found for the volcanic aerosol layers produced by the Kasatochi (δp = 0.09 ± 0.03) and Sarychev (δp = 0.05 ± 0.04) eruptions. However, for the Sarychev layers we observe an exponential decay (e-folding time of 3.6 days) in δp with time from 0.27 to 0.03. Similar decreases in the layer-integrated attenuated colour ratios with time were observed for the Sarychev case. In general, the Puyehue layers exhibited larger colour ratios (χ′ = 0.53 ± 0.07) than what was observed for the Kasatochi (χ′ = 0.35 ± 0.07) and Sarychev (χ′ = 0.32 ± 0.07) layers, indicating that the Puyehue layers were generally composed of larger particles. These observations are particularly relevant to the new stratospheric aerosol subtyping classification scheme, which has been incorporated into version 4 of the level 2 CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) data products

Constraints on droplet growth in radiatively cooled stratocumulus clouds

Radiative cooling near the top of a layer cloud plays a dominant role in droplet condensation growth. The impact of this cooling on the evolution of small droplets and the formation of precipitation-sized drops is calculated using a microphysical model that includes radiatively driven condensation and coalescence. The cloud top radiative environment used for these calculations is determined using a mixed-layer model of a marine stratocumulus cloud with a subsiding, radiatively cooled inversion. Calculations of the radiatively driven equilibrium supersaturation show that net long wave emission by cloud droplets produces supersaturations below 0.04% for typical nocturnal conditions. While supersaturations as low as this will force evaporation for droplets smaller than ≈ 5 μm, radiatively enhanced growth for larger droplets can reduce the time required to produce precipitation-sized particles by a factor of 2–4, compared with droplets in a quiescent cloud without flux divergence. The impact of this radiative enhancement on the acceleration of coalescence is equivalent to that produced in updrafts of 0.1–0.5 ms−1, and varies linearly with the total emitted flux (the “radiative exchange”). An edited version of this paper was published by AGU. Copyright 1995 American Geophysical Union.Science, Faculty ofEarth and Ocean Sciences, Department ofReviewedFacult

A study on the low-altitude clouds over the Southern Ocean using the DARDAR-MASK

International audienceA climatology of the thermodynamic phase of the clouds over the Southern Ocean (40-65S,100-160E) has been constructed with the A-Train merged data product DARDAR-MASK for the four-year period 2006-2009 during Austral winter and summer. Low-elevation clouds with little seasonal cycle dominate this climatology, with the cloud-tops commonly found at heights less than 1km. Such clouds are problematic for the DARDAR-MASK in that the Cloud Profiling Radar (CPR) of CloudSat is unable to distinguish returns from the lowest four bins (heights up to 720 - 960m), and the CALIOP lidar of CALIPSO may suffer from heavy extinction. The CPR is further limited for all of the low-altitude clouds (tops below 3km) as they are predominantly in the temperature range from freezing to -20 C, where understanding the CPR reflectivity becomes difficult due to the unknown thermodynamic phase. These shortcomings are seen to flow through to the merged CloudSat-CALIPSO product. A cloud-top phase climatology comparison has been made between CALIPSO, the DARDAR-MASK and MODIS. All three products highlight the extensive presence of supercooled liquid water over the Southern Ocean, particularly during summer. The DARDAR-MASK recorded substantially more ice at cloud-tops as well as mixed phase in the low-elevation cloud-tops in comparison to CALIPSO and MODIS. Moving beneath the cloud-top, the DARDAR-MASK finds ice to be dominant at heights greater than 1 km, once the lidar signal is attenuated. The limitations demonstrated in this study highlight the enormous challenge that remains in better defining the energy and water budget over the Southern Ocean

The Queensland cloud seeding research program

In late 2006 the Queensland government decided to establish the Queensland Cloud Seeding Research Program (QCSRP) in southeastern Queensland to determine the feasibility of cloud seeding as a component of its long-term water management strategy. The Queensland water management strategy recognizes the need for a broad portfolio of water sources to account for the uncertainties and costs associated with each type of source. While it was not expected that cloud seeding would restore southeastern Queensland's water supply levels to pre-drought values, it seemed valuable to determine whether certain types of seeding techniques might impact rainfall and water supplies in the region and whether that impact could be quantified. The project was developed as a collaboration between a number of institutions from Australia, the United States, and South Africa, and included field measurements over the course of two wet seasons. A two-pronged approach was taken to a) conduct a randomized cloud seeding experiment and b) assemble state-of-the-art instrumentation systems to collect data on the complete physical process from cloud formation to seeding to precipitation