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Title: Cloud properties and associated radiative heating rates in the tropical western Pacific

Abstract

Radiative heating of the atmosphere affects cloud evolution on the cloud scale and it influences large-scale vertical motion. Obtaining good estimates of radiative heating rate profiles has been difficult due to a lack of cloud profile observations. The Atmospheric Radiation Measurement (ARM) program has been measuring cloud property distributions at sites around the globe including three in the tropical western Pacific (TWP) region. We have analyzed a month of these remote sensing observations at Manus and Nauru to calculate time series of vertical cloud property profiles and radiative heating rates. This data set will be an important tool for describing radiative processes in the tropics and assessing the simulation of these processes in dynamical models.

Authors:
; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
964627
Report Number(s):
PNNL-SA-43260
KP1201030
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Geophysical Research. D. (Atmospheres), 112 (Article number: D05201)
Country of Publication:
United States
Language:
English

Citation Formats

Mather, Jim H., McFarlane, Sally A., Miller, Mark A., and Johnson, Karen L. Cloud properties and associated radiative heating rates in the tropical western Pacific. United States: N. p., 2007. Web. doi:10.1029/2006JD007555.
Mather, Jim H., McFarlane, Sally A., Miller, Mark A., & Johnson, Karen L. Cloud properties and associated radiative heating rates in the tropical western Pacific. United States. doi:10.1029/2006JD007555.
Mather, Jim H., McFarlane, Sally A., Miller, Mark A., and Johnson, Karen L. Thu . "Cloud properties and associated radiative heating rates in the tropical western Pacific". United States. doi:10.1029/2006JD007555.
@article{osti_964627,
title = {Cloud properties and associated radiative heating rates in the tropical western Pacific},
author = {Mather, Jim H. and McFarlane, Sally A. and Miller, Mark A. and Johnson, Karen L.},
abstractNote = {Radiative heating of the atmosphere affects cloud evolution on the cloud scale and it influences large-scale vertical motion. Obtaining good estimates of radiative heating rate profiles has been difficult due to a lack of cloud profile observations. The Atmospheric Radiation Measurement (ARM) program has been measuring cloud property distributions at sites around the globe including three in the tropical western Pacific (TWP) region. We have analyzed a month of these remote sensing observations at Manus and Nauru to calculate time series of vertical cloud property profiles and radiative heating rates. This data set will be an important tool for describing radiative processes in the tropics and assessing the simulation of these processes in dynamical models.},
doi = {10.1029/2006JD007555},
journal = {Journal of Geophysical Research. D. (Atmospheres), 112 (Article number: D05201)},
number = ,
volume = ,
place = {United States},
year = {Thu Mar 01 00:00:00 EST 2007},
month = {Thu Mar 01 00:00:00 EST 2007}
}
  • Radiative heating of the atmosphere affects cloud evolution and atmospheric dynamics. The most direct means available for determining radiative heating profiles is to measure profiles of thermodynamic and cloud properties (temperature, humidity, liquid and ice water content) and use these profiles to calculate radiative fluxes. Obtaining accurate, high resolution profiles of these properties requires active remote sensing instruments. Instruments capable of making these measurements and the techniques for interpreting these measurements for meteorological applications have only recently become available. The Atmospheric Radiation Measurement (ARM) program operates instruments including millimeter wavelength radars and microwave radiometers to measure cloud property distributions atmore » sites around the world including three in the tropical western Pacific region. We have analyzed several months of ARM observations from Manus and Nauru to calculate time series of vertical cloud property profiles and associated radiative fluxes and heating rates. To test the validity of these radiative profiles, we have conducted closure tests that compare calculated radiative fluxes at the surface and top of atmosphere to measurements from the ARM sites and from geostationary satellite. The cloud and radiation profiles exhibit distinct vertical structure with strong boundary layer and cirrus features at both sites. Manus, which was much more convectively active than Nauru during the study period, also exhibits a mid-level cloud feature near the melting level. The two sites exhibit very different diurnal cycles. This data set will be an important tool for describing radiative processes in the tropics and assessing the simulation of these processes in dynamical models.« less
  • Cloud radiative effects are examined using long-term datasets collected at the three Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Climate Research Facilities in the tropical western Pacific. We quantify the surface radiation budget, cloud populations, and cloud radiative effects by partitioning the data by cloud type, time of day, and as a function of large scale modes of variability such as El Niño Southern Oscillation (ENSO) phase and wet/dry seasons at Darwin. The novel facet of our analysis is that we break aggregate cloud radiative effects down by cloud type across the diurnal cycle. The Nauru cloud populations andmore » subsequently the surface radiation budget are strongly impacted by ENSO variability whereas the cloud populations over Manus only shift slightly in response to changes in ENSO phase. The Darwin site exhibits large seasonal monsoon related variations. We show that while deeper convective clouds have a strong conditional influence on the radiation reaching the surface, their limited frequency reduces their aggregate radiative impact. The largest source of shortwave cloud radiative effects at all three sites comes from low clouds. We use the observations to demonstrate that potential model biases in the amplitude of the diurnal cycle and mean cloud frequency would lead to larger errors in the surface energy budget compared to biases in the timing of the diurnal cycle of cloud frequency. Our results provide solid benchmarks to evaluate model simulations of cloud radiative effects in the tropics.« less
  • Cloud radiative effects on surface downwelling fluxes are investigated using long-term datasets from the three Atmospheric Radiation Measurement (ARM) sites in the Tropical Western Pacific (TWP) region. The Nauru and Darwin sites show significant variability in sky cover, downwelling radiative fluxes, and surface cloud radiative effect (CRE) due to El Niño and the Australian monsoon, respectively, while the Manus site shows little intra-seasonal or interannual variability. Cloud radar measurement of cloud base and top heights are used to define cloud types so that the effect of cloud type on the surface CRE can be examined. Clouds with low bases contributemore » 71-75% of the surface shortwave (SW) CRE and 66-74% of the surface longwave (LW) CRE at the three TWP sites, while clouds with mid-level 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.« less
  • Here, this study evaluates the ability of the Community Atmospheric Model version 5 (CAM5) to reproduce low clouds observed by the Atmospheric Radiation Measurement (ARM) cloud radar at Manus Island of the tropical western Pacific during the Years of Tropical Convection. Here low clouds are defined as clouds with their tops below the freezing level and bases within the boundary layer. Low-cloud statistics in CAM5 simulations and ARM observations are compared in terms of their general occurrence, mean vertical profiles, fraction of precipitating versus nonprecipitating events, diurnal cycle, and monthly time series. Other types of clouds are included to putmore » the comparison in a broader context. The comparison shows that the model overproduces total clouds and their precipitation fraction but underestimates low clouds in general. The model, however, produces excessive low clouds in a thin layer between 954 and 930 hPa, which coincides with excessive humidity near the top of the mixed layer. This suggests that the erroneously excessive low clouds stem from parameterization of both cloud and turbulence mixing. The model also fails to produce the observed diurnal cycle in low clouds, not exclusively due to the model coarse grid spacing that does not resolve Manus Island. Lastly, this study demonstrates the utility of ARM long-term cloud observations in the tropical western Pacific in verifying low clouds simulated by global climate models, illustrates issues of using ARM observations in model validation, and provides an example of severe model biases in producing observed low clouds in the tropical western Pacific.« less