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Title: A New Scheme for Predicting Fair-Weather Cumulus

Abstract

A new parameterization for boundary layer cumulus clouds, called the cumulus potential (CuP) scheme, is introduced. Unlike many other parameterizations, the CuP scheme explicitly links the fair-weather clouds to the boundary-layer turbulence and accounts for the non-local nature of the turbulence. This scheme uses joint probability density functions (JPDFs) of virtual potential temperature and water-vapor mixing ratio, as well as the mean vertical profiles of virtual potential temperature, to predict the amount and size distribution of boundary layer cloud cover. This model considers the diversity of air parcels over a heterogeneous surface, and recognizes that some parcels rise above their lifting condensation level to become cumulus, while other parcels might rise as clear updrafts. This model has several unique features: 1) surface heterogeneity and boundary-layer turbulence is represented using the boundary layer JPDF of virtual potential temperature versus water-vapor mixing ratio, 2) clear and cloudy thermals are allowed to coexist at the same altitude, and 3) a range of cloud-base heights, cloud-top heights, and cloud thicknesses are predicted within any one cloud field, as observed. Using data from Boundary Layer Experiment 1996 and a model intercomparsion study using large eddy simulation (LES) based on the Barbados Oceanographic and Meteorological Experimentmore » (BOMEX), the CuP scheme is compared to three other cumulus parameterizations: one based on relative humidity, a statistical scheme based on the saturation deficit, and a slab model. It is shown that the CuP model does a better job predicting the cloud-base height and the cloud-top height than three other parameterizations. The model also shows promise in predicting cloud cover, and is found to give better cloud-cover estimates than the three other cumulus parameterizations. In ongoing work supported by the US Department of Energy¹s Atmospheric Radiation Measurement Program, the CuP scheme is being implemented in the Weather Research and Forecasting (WRF) model, in which it replaces the ad-hoc trigger function in an existing cumulus parameterization.« less

Authors:
;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
944510
Report Number(s):
PNNL-SA-53276
Journal ID: ISSN 0003-0007; BAMIAT; KP1205010; TRN: US200902%%645
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Bulletin of the American Meteorological Society, 88(4):486-487; Journal Volume: 88; Journal Issue: 4
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; BOUNDARY LAYERS; CLOUD COVER; CLOUDS; FORECASTING; HUMIDITY; PROBABILITY DENSITY FUNCTIONS; TURBULENCE; WATER VAPOR; WEATHER

Citation Formats

Berg, Larry K., and Stull, Roland B.. A New Scheme for Predicting Fair-Weather Cumulus. United States: N. p., 2007. Web.
Berg, Larry K., & Stull, Roland B.. A New Scheme for Predicting Fair-Weather Cumulus. United States.
Berg, Larry K., and Stull, Roland B.. Sun . "A New Scheme for Predicting Fair-Weather Cumulus". United States. doi:.
@article{osti_944510,
title = {A New Scheme for Predicting Fair-Weather Cumulus},
author = {Berg, Larry K. and Stull, Roland B.},
abstractNote = {A new parameterization for boundary layer cumulus clouds, called the cumulus potential (CuP) scheme, is introduced. Unlike many other parameterizations, the CuP scheme explicitly links the fair-weather clouds to the boundary-layer turbulence and accounts for the non-local nature of the turbulence. This scheme uses joint probability density functions (JPDFs) of virtual potential temperature and water-vapor mixing ratio, as well as the mean vertical profiles of virtual potential temperature, to predict the amount and size distribution of boundary layer cloud cover. This model considers the diversity of air parcels over a heterogeneous surface, and recognizes that some parcels rise above their lifting condensation level to become cumulus, while other parcels might rise as clear updrafts. This model has several unique features: 1) surface heterogeneity and boundary-layer turbulence is represented using the boundary layer JPDF of virtual potential temperature versus water-vapor mixing ratio, 2) clear and cloudy thermals are allowed to coexist at the same altitude, and 3) a range of cloud-base heights, cloud-top heights, and cloud thicknesses are predicted within any one cloud field, as observed. Using data from Boundary Layer Experiment 1996 and a model intercomparsion study using large eddy simulation (LES) based on the Barbados Oceanographic and Meteorological Experiment (BOMEX), the CuP scheme is compared to three other cumulus parameterizations: one based on relative humidity, a statistical scheme based on the saturation deficit, and a slab model. It is shown that the CuP model does a better job predicting the cloud-base height and the cloud-top height than three other parameterizations. The model also shows promise in predicting cloud cover, and is found to give better cloud-cover estimates than the three other cumulus parameterizations. In ongoing work supported by the US Department of Energy¹s Atmospheric Radiation Measurement Program, the CuP scheme is being implemented in the Weather Research and Forecasting (WRF) model, in which it replaces the ad-hoc trigger function in an existing cumulus parameterization.},
doi = {},
journal = {Bulletin of the American Meteorological Society, 88(4):486-487},
number = 4,
volume = 88,
place = {United States},
year = {Sun Apr 01 00:00:00 EDT 2007},
month = {Sun Apr 01 00:00:00 EDT 2007}
}
  • Continental fair-weather cumuli exhibit significant diurnal, day-to-day, and year-to-year variability. This study describes the climatology of cloud macroscale properties, including the cloud-base height, cloud-top height, cloud thickness, and cloud chord length over the U.S. Department of Energy¹s Atmospheric Radiation Measurement (ARM) Climate Research Facility (ARCF) Southern Great Plains (SGP) site. The diurnal cycle of cloud fraction, cloud-base height, and cloud-thickness were well defined. The cloud fraction reached its maximum value near 14:00 CST. The average cloud-base height increased throughout the day, while the average cloud thickness decreased with time. In contrast to the other cloud properties, the average cloud-chord lengthmore » remained nearly constant throughout the day. The sensitivity of the cloud properties to the year-to-year variability and to changes in low-level moisture were compared. The cloud-base height was found to be sensitive to both the year and the low-level moisture, the cloud thickness was much more sensitive to the year then to the low-level moisture, and the cloud fraction and cloud chord length were more sensitive to the low-level moisture than to the year. Distributions of the cloud-chord length over the ARCF SGP site were computed and were well fit by an exponential distribution. The contribution of clouds of each cloud-chord length to the total cloud fraction was computed, and it was found the clouds with a chord length of about 1 km contributed the most to the observed cloud fraction.« less
  • A new parameterization for boundary layer cumulus clouds, called the cumulus potential (CuP) scheme, is introduced. This scheme uses joint probability density functions (JPDFs) of virtual potential temperature and water-vapor mixing ratio, as well as the mean vertical profiles of virtual potential temperature, to predict the amount and size distribution of boundary layer cloud cover. This model considers the diversity of air parcels over a heterogeneous surface, and recognizes that some parcels rise above their lifting condensation level to become cumulus, while other parcels might rise as clear updrafts. This model has several unique features: 1) surface heterogeneity is representedmore » using the boundary layer JPDF of virtual potential temperature versus water-vapor mixing ratio, 2) clear and cloudy thermals are allowed to coexist at the same altitude, and 3) a range of cloud-base heights, cloud-top heights, and cloud thicknesses are predicted within any one cloud field, as observed. Using data from Boundary Layer Experiment 1996 and a model intercomparsion study using large eddy simulation (LES) based on Barbados Oceanographic and Meteorological Experiment (BOMEX), it is shown that the CuP model does a good job predicting cloud-base height and cloud-top height. The model also shows promise in predicting cloud cover, and is found to give better cloud-cover estimates than three other cumulus parameterizations: one based on relative humidity, a statistical scheme based on the saturation deficit, and a slab model.« less
  • A three-dimensional ensemble-mean mesoscale atmospheric model, with simplified second-moment turbulence closure equations and a statistical treatment for the condensation process, is used to stimulate a fair weather marine boundary layer observed during the GATE (GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment). The data deduced from airborne and surface-based instrumentation provided not only comprehensive initial and boundary conditions for the model but also permitted detailed comparisons between modeled and observed turbulence quantities.
  • A new parameterization for boundary-layer cumulus clouds, called the Cumulus Potential (CuP) scheme is introduced. This scheme uses Joint Probability Density Functions (JPDFs) of virtual potential temperature and water-vapor mixing ratio, as well as the mean vertical profiles of virtual potential temperature to predict the amount and size distribution of boundary-layer cloud cover. This model considers the diversity of air parcels over a heterogeneous surface, and recognizes that some parcels rise above their lifting condensation level to become cumulus, while other parcels might rise as non-cloud updrafts. This model has several unique features: (1) cloud cover is determined from themore » boundary-layer JPDF of virtual potential temperature vs. water-vapor mixing ratio , (2) clear and cloudy thermals are allowed to coexist at the same altitude, and (3) a range of cloud-base heights, cloud-top heights, and cloud thicknesses are predicted within any one cloud field, as observed. Using data from Boundary Layer Experiment 1996, and a model intercomparsion study using Large Eddy Simulation (LES) based on BOMEX, it is shown that the CuP model does a good job predicting cloud-base height and cloud-top height. The model also shows promise in predicting cloud cover, and is found to give better cloud-cover estimates than three other cumulus parameterizations: one based on relative humidity, a statistical scheme based on the saturation deficit, and a slab model.« less