School of Earth Sciences - Research Publications

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    A Comparison of Cloud Microphysical Properties Derived From MODIS and CALIPSO With In Situ Measurements Over the Wintertime Southern Ocean
    Ahn, E ; Huang, Y ; Siems, ST ; Manton, MJ (AMER GEOPHYSICAL UNION, 2018-10-16)
    Abstract In situ observations of cloud effective radius (reff), droplet number concentration (Nd), and thermodynamic phase from 11 wintertime flights over the Southern Ocean (43–45°S, 145–148°E) are compared to products from MODerate‐resolution Imaging Spectroradiometer (MODIS) and Cloud‐Aerosol Lidar with Orthogonal Polarization. The in situ observations were in close alignment with A‐train overpasses for a 30‐min window. For open mesoscale cellular convection, which was predominantly observed, clouds were commonly found to be intermittently drizzling, patchy, and mixed phase. Compared to the in situ observations of the cloud thermodynamic phase, the Cloud‐Aerosol Lidar with Orthogonal Polarization and MODIS cloud phase optical property products consistently underestimated the occurrence of mixed‐phase clouds, whereas the MODIS infrared‐based phase product showed a better qualitative agreement despite a frequent classification of uncertainty. The MODIS reff_2.1 overestimated the in situ reff for nondrizzling clouds (by ~13 μm on average) and, to a lesser extent, for lightly drizzling cases. Conversely, MODIS reff_2.1 underestimated the in situ reff for heavily drizzling cases by ~10 μm on average. The overestimation of reff is much greater than that for the stratocumulus over the Southeast Pacific shown in other studies. An examination on subpixel heterogeneity, droplet size variability, a bimodal distribution, and solar zenith angle suggests that all of these factors have measurable impacts on the MODIS reff bias. The MODIS Nd is largely consistent with the in situ observations. However, the Nd of the two high Nd cases (closed mesoscale cellular convection) are highly underestimated. An error analysis suggests that the Nd biases are likely a result of a compensating error effect.
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    Air-sea heat and momentum fluxes in the Southern Ocean
    Bharti, V ; Fairall, C ; Blomquist, B ; Huang, Y ; Protat, A ; Sullivan, P ; Siems, S ; Manton, M (American Geophysical Union, 2019)
    The Clouds, Aerosols, Precipitation, Radiation, and atmospheric Composition Over the southeRn oceaN (CAPRICORN) experiment was carried out in March-April 2016 onboard R/V Investigator studying momentum (τ), sensible heat (Hs) and latent heat (Hi) fluxes over the Australian sector of the Southern Ocean including over one cyclonic cold-core and one anticyclonic warm-core mesoscale oceanic eddy. The turbulence-based flux measurements obtained with the NOAA PSD flux system employing eddy covariance (EC) and inertial dissipation (ID) methods are compared with those obtained by the Coupled Ocean-Atmosphere Response Experiment (COARE) 3.5 bulk model, and the neutral transfer coefficients are studied. The relative uncertainty between the turbulence-based and COARE 3.5 estimates of τ, Hs and Hi are 22%, 70% and 26%, respectively at 1-hour timescale over the Southern Ocean. Further, the variability in bulk fluxes is investigated with respect to oceanic eddies, precipitation events, atmospheric stability and extratropical cyclones encountered during the voyage. The main observed variability is an increase in significant wave height or γw (∼33%), τ (∼89%), Hs (∼187%) and Hi (∼79%) over the warm eddy as compared to average voyage values. During the passage of 6 extratropical cyclones, an increase in τ (∼62% average) and a decrease in Hs (∼235%) and Hi (∼79%) is noted in the warm sector, compared to pre-storm conditions, but the pattern reverses behind the cold front.
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    Assessing surface heat flux products with in situ observations over the Australian sector of the Southern Ocean
    Bharti, V ; Schulz, E ; Fairall, CW ; Blomquist, BW ; Huang, Y ; Protat, A ; Siems, ST ; Manton, MJ (American Meteorological Society, 2019-09-13)
    Given the large uncertainties in surface heat fluxes over the Southern Ocean, an assessment of fluxes obtained by European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) product, the Australian Integrated Marine Observing System (IMOS) routine observations, and the Objectively Analyzed Air–Sea Heat Fluxes (OAFlux) project hybrid dataset is performed. The surface fluxes are calculated using the COARE 3.5 bulk algorithm with in situ data obtained from the NOAA Physical Sciences Division flux system during the Clouds, Aerosols, Precipitation, Radiation, and Atmospheric Composition over the Southern Ocean (CAPRICORN) experiment on board the R/V Investigator during a voyage (March–April 2016) in the Australian sector of the Southern Ocean (43°–53°S). ERA-Interim and OAFlux data are further compared with the Southern Ocean Flux Station (SOFS) air–sea flux moored surface float deployed for a year (March 2015–April 2016) at ~46.7°S, 142°E. The results indicate that ERA-Interim (3 hourly at 0.25°) and OAFlux (daily at 1°) estimate sensible heat flux Hs accurately to within ±5 W m−2 and latent heat flux Hl to within ±10 W m−2. ERA-Interim gives a positive bias in Hs at low latitudes (<47°S) and in Hl at high latitudes (>47°S), and OAFlux displays consistently positive bias in Hl at all latitudes. No systematic bias with respect to wind or rain conditions was observed. Although some differences in the bulk flux algorithms are noted, these biases can be largely attributed to the uncertainties in the observations used to derive the flux products.
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    Evaluating Himawari-8 Cloud Products Using Shipborne and CALIPSO Observations: Cloud-top Height and Cloud-top Temperature
    Huang, Y ; Siems, S ; Manton, M ; Majewski, L ; Protat, A ; Nguyen, H (American Meteorological Society, 2019-09-03)
    Cloud-top height (CTH) and cloud-top temperature (CTT) retrieved from the Himawari-8 observations are evaluated using the active shipborne radar-lidar observations derived from the 31-day “Clouds, Aerosols, Precipitation Radiation and atmospherIc Composition Over the southeRn oceaN” (CAPRICORN) experiment in 2016 and one-year observations from the space-borne Cloud- Aerosol Lidar with Orthogonal Polarization (CALIOP) cloud product over a large sector of the Southern Ocean. The results show that the Himawari-8 CTH (CTT) retrievals agree reasonably well with both the shipborne estimates, with a correlation coefficient of 0.837 (0.820), a mean bias error of 0.226 km (-2.526°C), and an RMSE of 1.684 km (10.069°C), respectively. In the comparison with CALIOP, the corresponding quantities are found to be 0.786 (0.480), -0.570 km (1.343°C), and 2.297 km (25.176°C). The Himawari-8 CTH (CTT) generally falling between the physical CTHs observed by CALIOP and the ship-borne radar-lidar estimates. However, major systematic biases are also identified. These errors include (i) a low (warm) bias in CTH (CTT) for warm liquid cloud type, (ii) a cold bias in CTT for supercooled liquid water cloud type, (iii) a lack of CTH at ~3 km that does not have a corresponding gap in CTT, (iv) a tendency of misclassifying some low- / mid-top clouds as cirrus and overlap cloud types, and (v) a saturation of CTH (CTT) around 10 km (-40°C), particularly for cirrus and overlap cloud types. Various challenges that underpin these biases are also explored, including the potential of parallax bias, low-level inversion, and cloud heterogeneity.
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    Characteristics of the Marine Atmospheric Boundary Layer Over the Southern Ocean in Response to the Synoptic Forcing
    Lang, F ; Huang, Y ; Siems, ST ; Manton, MJ (American Geophysical Union (AGU), 2018)
    The characteristics of the marine atmospheric boundary layer (MABL) in relation to synoptic meteorology over the Southern Ocean are examined using upper‐air soundings and surface precipitation at Macquarie Island (54.62°S, 158.85°E), with a primary focus on the post‐cold‐frontal environment where large cloud and radiative biases are presented in a multitude of climate models. Thermodynamic profiles from the European Centre for Medium‐Range Weather Forecasts ERA‐Interim reanalyses are compared with the observations to evaluate their representation of the MABL characteristics. Observations confirm that boundary layer clouds over the Southern Ocean commonly reside within a shallow MABL under the influence of frequent midlatitude cyclones and fronts. The evaluation of MABL height shows that, for both observations and reanalysis, the inversion is higher northward of the low center and under postcold front conditions. Under cold frontal passages, however, the main inversions are not well represented by ERA‐Interim, which is featured by an underestimate of the MABL height by 22%. Significant differences are found in the moisture profiles within the MABL between the observations and ERA‐Interim soundings within the context of cold frontal passages. The moisture in the ERA‐Interim is found to be too confined to the surface layer, which is consistent with the shallower MABL represented by the ERA‐Interim. Analysis of the surface precipitation shows that ERA‐Interim overestimates the amount of precipitation over Macquarie Island in the vicinity of cyclone cores but underestimates the precipitation not immediately associated with cold fronts, leading to an overall underestimate of the annual precipitation by 11%.