Evaluation of autoconversion and accretion enhancement factors in general circulation model warm-rain parameterizations using ground-based measurements over the Azores

Abstract. A great challenge in climate modeling is how to parameterize subgrid cloud processes, such as autoconversion and accretion in warm-rain formation. In this study, we use ground-based observations and retrievals over the Azores to investigate the so-called enhancement factors, Eauto and Eaccr, which are often used in climate models to account for the influence of subgrid variance of cloud and precipitation water on the autoconversion and accretion processes. Eauto and Eaccr are computed for different equivalent model grid sizes. The calculated Eauto values increase from 1.96 (30 km) to 3.2 (180 km), and the calculated Eaccr values increase from 1.53 (30 km) to 1.76 (180 km). Comparing the prescribed enhancement factors in Morrison and Gettleman (2008, MG08) to the observed ones, we found that a higher Eauto (3.2) at small grids and lower Eaccr (1.07) are used in MG08, which might explain why most of the general circulation models (GCMs) produce too-frequent precipitation events but with too-light precipitation intensity. The ratios of the rain to cloud water mixing ratio (qr/qc) at Eaccr=1.07 and Eaccr=2.0 are 0.063 and 0.142, respectively, from observations, further suggesting that the prescribed value of Eaccr=1.07 used in MG08 is too small to simulate precipitation intensity correctly. Both Eauto and Eaccr increase when the boundary layer becomes less stable, and the values are larger in precipitating clouds (CLWP>75 gm−2) than those in non-precipitating clouds (CLWP<75 gm−2). Therefore, the selection of Eauto and Eaccr values in GCMs should be regime- and resolution-dependent.

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Evaluate autoconversion and accretion enhancement factors in GCM warm-rain parameterizations using ground-based measurements at the Azores

10.5194/acp-2018-499 ◽

2018 ◽

Author(s):

Peng Wu ◽

Baike Xi ◽

Xiquan Dong ◽

Zhibo Zhang

Keyword(s):

Climate Models ◽

Spatial Scales ◽

Precipitation Intensity ◽

Climate Modelling ◽

Warm Rain ◽

Enhancement Factors ◽

Rain Formation ◽

Precipitating Clouds ◽

Light Precipitation ◽

Precipitation Events

Abstract. A great challenge in climate modelling is how to parametrize sub-grid cloud processes, such as autoconversion and accretion in warm rain formation. In this study, we use ground-based observations and retrievals over the Azores to investigate the so-called enhancement factors, Eauto and Eaccr, which are often used in climate models to account for the influences of sub-grid variances of cloud and precipitation water on the autoconversion and accretion processes. Eauto and Eaccr are computed at a variety of tempo-spatial scales corresponding to different model resolutions. The calculated Eauto increase from 1.79 (0.5-hr/36 km) to 3.15 (3.5-hr/126 km), and the calculated Eaccr increases from 1.25 (0.5-hr/36 km) to 1.6 (5-hr/180 km). Comparing the prescribed enhancement factors to the values from observations shows that GCMs are using a much higher Eauto (3.2) and lower Eaccr (1.07). This helps to explain why most of the GCMs produce too frequent precipitation events but with too light precipitation intensity. The ratios of rain to cloud liquid water at Eaccr = 1.07 and Eaccr = 2.0 are 0.048 and 0.119, respectively, further proving that the prescribed value of Eaccr = 1.07 used in GCMs is too small to simulate correct precipitation intensity. Both Eauto and Eaccr increase when the boundary layer becomes less stable, and the values are larger in precipitating clouds (CLWP > 75 gm−2) than those in nonprecipiting clouds (CLWP

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Potential changes in the number of wet days and its effect on future intense and annual precipitation in northern Oman

Hydrology Research ◽

10.2166/nh.2017.188 ◽

2017 ◽

Vol 49 (1) ◽

pp. 237-250 ◽

Cited By ~ 3

Author(s):

Luminda Niroshana Gunawardhana ◽

Ghazi A. Al-Rawas ◽

Andy Y. Kwarteng ◽

Malik Al-Wardy ◽

Yassine Charabi

Keyword(s):

General Circulation ◽

Climate Models ◽

Coastal Region ◽

General Circulation Models ◽

Precipitation Intensity ◽

Annual Precipitation ◽

Monsoon Period ◽

Precipitation Characteristics ◽

The Future ◽

Precipitation Records

Abstract The changes in the number of wet days (NWD) in Oman projected by climate models was analyzed, focusing mostly on variation of precipitation intensity and its effect on total annual precipitation (PTOT) in the future. The daily precipitation records of 49 gage stations were divided into five regions. Of the five general circulation models studied, two of them were selected based on their performance to simulate local-scale precipitation characteristics. All regions studied, except the interior desert region of the country, could experience fewer wet days in the future, with the most significant decreases estimated in southern Oman. The contribution from the cold frontal troughs to the PTOT in the northeast coastal region would decrease from 85% in the 1985–2004 period to 79% during the 2040–2059 period and further decrease to 77% during the 2080–2099 period. In contrast, results depict enhanced tropical cyclone activities in the northeast coastal region during the post-monsoon period. Despite the decreases in the NWD, PTOT in all regions would increase by 6–29% and 35–67% during the 2040–2059 and 2080–2099 periods, respectively. These results, therefore, show that increases in precipitation intensity dominate the changes in PTOT.

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Downscaling general circulation model output: a review of methods and limitations

Progress in Physical Geography Earth and Environment ◽

10.1177/030913339702100403 ◽

1997 ◽

Vol 21 (4) ◽

pp. 530-548 ◽

Cited By ~ 835

Author(s):

R.L. Wilby ◽

T.M.L. Wigley

Keyword(s):

General Circulation ◽

Climate Models ◽

Daily Precipitation ◽

Global Climate ◽

Circulation Model ◽

General Circulation Models ◽

Limited Area ◽

Regression Methods ◽

Present Generation ◽

Precipitation Characteristics

General circulation models (GCMs) suggest that rising concentrations of greenhouse gases may have significant consequences for the global climate. What is less clear is the extent to which local (subgrid) scale meteorological processes will be affected. So-called 'downscaling' techniques have subsequently emerged as a means of bridging the gap between what climate modellers are currently able to provide and what impact assessors require. This article reviews the present generation of downscaling tools under four main headings: regression methods; weather pattern (circulation)-based approaches; stochastic weather generators; and limited-area climate models. The penultimate section summarizes the results of an international experiment to intercompare several precipitation models used for downscaling. It shows that circulation-based downscaling methods perform well in simulating present observed and model-generated daily precipitation characteristics, but are able to capture only part of the daily precipitation variability changes associated with model-derived changes in climate. The final section examines a number of ongoing challenges to the future development of climate downscaling.

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Implementing the Delta-Four-Stream Approximation for Solar Radiation Computations in an Atmosphere General Circulation Model

Journal of the Atmospheric Sciences ◽

10.1175/2007jas2526.1 ◽

2008 ◽

Vol 65 (7) ◽

pp. 2448-2457 ◽

Cited By ~ 7

Author(s):

Tarek Ayash ◽

Sunling Gong ◽

Charles Q. Jia

Keyword(s):

Solar Radiation ◽

General Circulation ◽

Climate Modeling ◽

Circulation Model ◽

General Circulation Models ◽

Computational Method ◽

Radiation Budget ◽

High Latitudes ◽

Clear Sky ◽

Winter Hemisphere

Abstract Proper quantification of the solar radiation budget and its transfer within the atmosphere is of utmost importance in climate modeling. The delta-four-stream (DFS) approximation has been demonstrated to offer a more accurate computational method of quantifying the budget than the simple two-stream approximations widely used in general circulation models (GCMs) for radiative-transfer computations. Based on this method, the relative improvement in the accuracy of solar flux computations is investigated in the simulations of the third-generation Canadian Climate Center atmosphere GCM. Relative to the computations of the DFS-modified radiation scheme, the GCM original-scheme whole-sky fluxes at the top of the atmosphere (TOA) show the largest underestimations at high latitudes of a winter hemisphere on the order of 4%–6% (monthly means), while the largest overestimations of the same order are found over equatorial regions. At the surface, even higher overestimations are found, exceeding 20% at subpolar regions of a winter hemisphere. Flux differences between original and DFS schemes are largest in the tropics and at high latitudes, where the monthly zonal means and their dispersions are within 5 W m−2 at the TOA and 10 W m−2 at the surface in whole sky, but differences may be as large as 20 and −40 W m−2. In clear sky, monthly zonal means and their dispersions remain within 2 W m−2, but may be as large as 25 and −12 W m−2. Such differences are found to be mostly determined by variations in cloud optical depth and solar zenith angle, and by aerosol loading in a clear sky.

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Multiscale Impacts of Variable Heating in Climate

Journal of Climate ◽

10.1175/2007jcli1411.1 ◽

2007 ◽

Vol 20 (23) ◽

pp. 5677-5695 ◽

Cited By ~ 17

Author(s):

Prashant D. Sardeshmukh ◽

Philip Sura

Keyword(s):

General Circulation ◽

Climate Models ◽

Climate Model ◽

Circulation Model ◽

General Circulation Models ◽

Heat Fluxes ◽

Winter Climate ◽

The Mean ◽

Mean Climate ◽

Diabatic Forcing

Abstract While it is obvious that the mean diabatic forcing of the atmosphere is crucial for maintaining the mean climate, the importance of diabatic forcing fluctuations is less evident in this regard. Such fluctuations do not appear directly in the equations of the mean climate but affect the mean indirectly through their effects on the time-mean transient-eddy fluxes of heat, momentum, and moisture. How large are these effects? What are the effects of tropical phenomena associated with substantial heating variations such as ENSO and the MJO? To what extent do variations of the extratropical surface heat fluxes and precipitation affect the mean climate? What are the effects of the rapid “stochastic” components of the heating fluctuations? Most current climate models misrepresent ENSO and the MJO and ignore stochastic forcing; they therefore also misrepresent their mean effects. To what extent does this contribute to climate model biases and to projections of climate change? This paper provides an assessment of such impacts by comparing with observations a long simulation of the northern winter climate by a dry adiabatic general circulation model forced only with the observed time-mean diabatic forcing as a constant forcing. Remarkably, despite the total neglect of all forcing variations, the model reproduces most features of the observed circulation variability and the mean climate, with biases similar to those of some state-of-the-art general circulation models. In particular, the spatial structures of the circulation variability are remarkably well reproduced. Their amplitudes, however, are progressively underestimated from the synoptic to the subseasonal to interannual and longer time scales. This underestimation is attributed to the neglect of the variable forcing. The model also excites significant tropical variability from the extratropics on interannual scales, which is overwhelmed in reality by the response to tropical heating variability. It is argued that the results of this study suggest a role for the stochastic, and not only the coherent, components of transient diabatic forcing in the dynamics of climate variability and the mean climate.

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Investigation of Regional and Seasonal Variations in Marine Boundary Layer Cloud Properties from MODIS Observations

Journal of Climate ◽

10.1175/2008jcli1974.1 ◽

2008 ◽

Vol 21 (19) ◽

pp. 4955-4973 ◽

Cited By ~ 34

Author(s):

Michael P. Jensen ◽

Andrew M. Vogelmann ◽

William D. Collins ◽

Guang J. Zhang ◽

Edward P. Luke

Keyword(s):

Boundary Layer ◽

General Circulation ◽

Climate Models ◽

Marine Boundary Layer ◽

General Circulation Models ◽

Test Bed ◽

Liquid Water Path ◽

Microphysical Properties ◽

Cloud Properties ◽

Peak Versus

Abstract To aid in understanding the role that marine boundary layer (MBL) clouds play in climate and assist in improving their representations in general circulation models (GCMs), their long-term microphysical and macroscale characteristics are quantified using observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard the National Aeronautics and Space Administration’s (NASA’s) Terra satellite. Six years of MODIS pixel-level cloud products are used from oceanic study regions off the west coasts of California, Peru, the Canary Islands, Angola, and Australia where these cloud types are common. Characterizations are given for their organization (macroscale structure), the associated microphysical properties, and the seasonal dependencies of their variations for scales consistent with the size of a GCM grid box (300 km × 300 km). MBL mesoscale structure is quantified using effective cloud diameter CD, which is introduced here as a simplified measure of bulk cloud organization; it is straightforward to compute and provides descriptive information beyond that offered by cloud fraction. The interrelationships of these characteristics are explored while considering the influences of the MBL state, such as the occurrence of drizzle. Several commonalities emerge for the five study regions. MBL clouds contain the best natural examples of plane-parallel clouds, but overcast clouds occur in only about 25% of the scenes, which emphasizes the importance of representing broken MBL cloud fields in climate models (that are subgrid scale). During the peak months of cloud occurrence, mesoscale organization (larger CD) increases such that the fractions of scenes characterized as “overcast” and “clumped” increase at the expense of the “scattered” scenes. Cloud liquid water path and visible optical depth usually trend strongly with CD, with the largest values occurring for scenes that are drizzling. However, considerable interregional differences exist in these trends, suggesting that different regression functionalities exist for each region. For peak versus off-peak months, the fraction of drizzling scenes (as a function of CD) are similar for California and Angola, which suggests that a single probability distribution function might be used for their drizzle occurrence in climate models. The patterns are strikingly opposite for Peru and Australia; thus, the contrasts among regions may offer a test bed for model simulations of MBL drizzle occurrence.

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Using Different Formulations of the Transformed Eulerian Mean Equations and Eliassen–Palm Diagnostics in General Circulation Models

Journal of the Atmospheric Sciences ◽

10.1175/2010jas3355.1 ◽

2010 ◽

Vol 67 (6) ◽

pp. 1983-1995 ◽

Cited By ~ 16

Author(s):

Steven C. Hardiman ◽

David G. Andrews ◽

Andy A. White ◽

Neal Butchart ◽

Ian Edmond

Keyword(s):

General Circulation Model ◽

General Circulation ◽

Circulation Model ◽

General Circulation Models ◽

Primitive Equations ◽

Model Output ◽

Vertical Coordinate ◽

Order Of Magnitude ◽

Transformed Eulerian Mean

Abstract Transformed Eulerian mean (TEM) equations and Eliassen–Palm (EP) flux diagnostics are presented for the general nonhydrostatic, fully compressible, deep atmosphere formulation of the primitive equations in spherical geometric coordinates. The TEM equations are applied to a general circulation model (GCM) based on these general primitive equations. It is demonstrated that a naive application in this model of the widely used approximations to the EP diagnostics, valid for the hydrostatic primitive equations using log-pressure as a vertical coordinate and presented, for example, by Andrews et al. in 1987 can lead to misleading features in these diagnostics. These features can be of the same order of magnitude as the diagnostics themselves throughout the winter stratosphere. Similar conclusions are found to hold for “downward control” calculations. The reasons are traced to the change of vertical coordinate from geometric height to log-pressure. Implications for the modeling community, including comparison of model output with that from reanalysis products available only on pressure surfaces, are discussed.

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Tropical Atmospheric Variability Forced by Oceanic Internal Variability

Journal of Climate ◽

10.1175/jcli4044.1 ◽

2007 ◽

Vol 20 (4) ◽

pp. 765-771 ◽

Cited By ~ 17

Author(s):

Markus Jochum ◽

Clara Deser ◽

Adam Phillips

Keyword(s):

General Circulation ◽

Climate Models ◽

Rainfall Variability ◽

Circulation Model ◽

Internal Variability ◽

Tropical Pacific ◽

Atmospheric Variability ◽

Instability Waves ◽

Tropical Instability Waves ◽

The Tropical Pacific

Abstract Atmospheric general circulation model experiments are conducted to quantify the contribution of internal oceanic variability in the form of tropical instability waves (TIWs) to interannual wind and rainfall variability in the tropical Pacific. It is found that in the tropical Pacific, along the equator, and near 25°N and 25°S, TIWs force a significant increase in wind and rainfall variability from interseasonal to interannual time scales. Because of the stochastic nature of TIWs, this means that climate models that do not take them into account will underestimate the strength and number of extreme events and may overestimate forecast capability.

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Coupled ice sheet–climate modeling under glacial and pre-industrial boundary conditions

Climate of the Past ◽

10.5194/cp-10-1817-2014 ◽

2014 ◽

Vol 10 (5) ◽

pp. 1817-1836 ◽

Cited By ~ 22

Author(s):

F. A. Ziemen ◽

C. B. Rodehacke ◽

U. Mikolajewicz

Keyword(s):

Boundary Conditions ◽

General Circulation ◽

Climate Modeling ◽

Ice Sheets ◽

Ice Sheet ◽

General Circulation Models ◽

Quasi Equilibrium ◽

Ice Streams ◽

Ice Stream ◽

First Results

Abstract. In the standard Paleoclimate Modelling Intercomparison Project (PMIP) experiments, the Last Glacial Maximum (LGM) is modeled in quasi-equilibrium with atmosphere–ocean–vegetation general circulation models (AOVGCMs) with prescribed ice sheets. This can lead to inconsistencies between the modeled climate and ice sheets. One way to avoid this problem would be to model the ice sheets explicitly. Here, we present the first results from coupled ice sheet–climate simulations for the pre-industrial times and the LGM. Our setup consists of the AOVGCM ECHAM5/MPIOM/LPJ bidirectionally coupled with the Parallel Ice Sheet Model (PISM) covering the Northern Hemisphere. The results of the pre-industrial and LGM simulations agree reasonably well with reconstructions and observations. This shows that the model system adequately represents large, non-linear climate perturbations. A large part of the drainage of the ice sheets occurs in ice streams. Most modeled ice stream systems show recurring surges as internal oscillations. The Hudson Strait Ice Stream surges with an ice volume equivalent to about 5 m sea level and a recurrence interval of about 7000 yr. This is in agreement with basic expectations for Heinrich events. Under LGM boundary conditions, different ice sheet configurations imply different locations of deep water formation.

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Boundary conditions for the Middle Miocene Climate Transition (MMCT v1.0)

10.5194/gmd-2017-171 ◽

2017 ◽

Author(s):

Amanda Frigola ◽

Matthias Prange ◽

Michael Schulz

Keyword(s):

Boundary Conditions ◽

General Circulation ◽

Middle Miocene ◽

Climate Models ◽

Global Climate ◽

Co2 Concentration ◽

General Circulation Models ◽

Global Climate Models ◽

Ice Volume ◽

Climate Transition

Abstract. The Middle Miocene Climate Transition was characterized by major Antarctic ice-sheet expansion and global cooling during the interval ~ 15–13 Ma. Here we present two sets of boundary conditions for global general circulation models characterizing the periods before (Middle Miocene Climatic Optimum; MMCO) and after (Middle Miocene Glaciation; MMG) the transition. These boundary conditions include Middle Miocene global topography, bathymetry and vegetation. Additionally, Antarctic ice volume and geometry, sea-level and atmospheric CO2 concentration estimates for the MMCO and the MMG are reviewed. The boundary-condition files are available for use as input in a wide variety of global climate models and constitute a valuable tool for modeling studies with a focus on the Middle Miocene.

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