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James A. Ridout

Abstract

Numerical forecast experiments are carried out to investigate the implications of observed moisture variability in the tropical Pacific for deep convection. The study uses a series of quasi-cloud-resolving model forecasts with a triple-nested version of the Naval Research Laboratory's Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS). The forecasts are carried out for a dry tongue episode in the tropical western Pacific in November 1998. During a 24-h forecast, a number of convective cells develop in an area of deep convection near the edge of the simulated dry tongue in the COAMPS 3-km mesh inner grid, which is located in the southern portion of the domain of the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) field program. The forecast is repeated multiple times using a moisture profile from a TOGA COARE dry tongue episode to modify the lateral boundary conditions for the 3-km mesh grid over layers of varying depth and altitude. The entrainment of drier air results in a 37% reduction in rainfall when the dry layer extends from the 600-hPa level to the model top. Tests show that entrainment of dry air at mid levels has a major impact in suppressing precipitation production due to cold rain processes. It should be noted that conditions where warm rain processes dominate are not addressed by the present experiments. Associated with the suppression of deep convection by dry layers in these forecasts, there is a large increase in stored buoyant energy. The presence of dry midtropospheric air may thus serve to help permit the buildup of buoyant energy for subsequent episodes of deep convection, such as associated with the onset of the Madden–Julian oscillation.

In order to investigate the ability of convective parameterizations to represent the moisture sensitivity observed in the COAMPS experiments, data from the COAMPS 3-km mesh grid were used for tests of convective parameterizations in both semiprognostic model (SPM) and single-column model (SCM) mode. Both the Emanuel and the Kain–Fritsch convective parameterizations are unable to account in the SPM tests for the reduction in rainfall and cloud-base mass flux obtained on the COAMPS 3-km mesh grid when dry lateral boundary conditions are imposed above the 600-hPa level. The results are consistent with the interpretation that inadequacies in the convective closure assumptions play a signficant role. Comparisons of updraft mass flux profiles in the SPM tests point to considerable sensitivity to the details of the implementation of the stochastic buoyancy-sorting model in these schemes. In the SCM tests, the parameterized rainfall amounts are better than in semiprognostic mode, but apparently too low in one case by a substantial amount. In addition, large errors are observed in the simulated stored buoyant energy. The convective schemes are shown to perform best for greater horizontal resolutions of the forcing data.

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James A. Ridout and Carolyn A. Reynolds

Abstract

The sensitivity of the atmospheric general circulation model of the Navy Operational Global Atmospheric Prediction System to a parameterization of convective triggering by atmospheric boundary layer thermals is investigated. The study focuses on the western Pacific warm pool region and examines the results of seasonal integrations of the model for the winter of 1987/88. A parameterization for thermal triggering of deep convection is presented that is based on a classification of the unstable boundary layer. Surface-based deep convection is allowed only for boundary layer regimes associated with the presence of thermals. The regime classification is expressed in terms of a Richardson number that reflects the relative significance of buoyancy and shear in the boundary layer. By constraining deep convection to conditions consistent with the occurrence of thermals (high buoyancy to shear ratios), there is a significant decrease in precipitation over the southern portion of the northeast trade wind zone in the tropical Pacific and along the ITCZ. This decrease in precipitation allows for an increased flux of moisture into the region south of the equator corresponding to the warmest portion of the Pacific warm pool. Improvements in the simulated distribution of precipitation, precipitable water, and low-level winds in the tropical Pacific are demonstrated. Over the western Pacific, the transition from free convective conditions associated with thermals to forced convective conditions is found to be primarily due to variations in mixed layer wind speed. Low-level winds thus play the major role in regulating the ability of thermals to initiate deep convection. The lack of coupling with the ocean in these simulations may possibly produce a distorted picture in this regard.

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James A. Ridout and Thomas E. Rosmond

Abstract

Cloud radiative effects are represented in simulations with the general circulation model of the Navy Operational Global Atmospheric Prediction System (NOCAPS) using ingested cloud field data from the ISCCP dataset rather than model-diagnosed cloud fields. The primary objective is to investigate the extent to which the high temporal resolution ISCCP data can be used to improve the simulation of cloud radiative effects on the general circulation in GCM simulations much as observed sea surface temperatures (SSTs) have been used to avoid simulation errors resulting from inaccurately modeled SSTs. Experiments are described that examine the degree to which uncertainties in cloud field vertical structure impair the utility of the observed cloud data in this regard, as well as the extent to which unrealistic combinations of cloud radiative forcing and other physical processes may affect GCM simulations. The potential for such unrealistic combinations stems from the lack of feedback to the cloud fields in simulations using ingested cloud data in place of model-predicted cloud fields.

Simulations for the present work were carried out for three April through July periods (1986–1988) using prescribed sea surface temperatures. Analysis of the model results concentrated primarily on the month of July, allowing for a 3-month spinup period. Comparisons with ERBE data show the expected improvement in the simulation of top of the atmosphere radiation fields using the observed cloud data. Three experiments are described that examine the model sensitivity to the vertical structure assumed for the cloud fields. The authors show that although uncertainties in assumed vertical profiles of cloudiness may possibly have significant effects on certain aspects of our simulations, such effects do not appear to be large in terms of monthly mean quantities except in the case of large errors in cloud field vertical profiles. Precipitation fields are particularly insensitive to such uncertainties. A preliminary investigation of potential inaccuracies in our representation of cloud radiative effects with ISCCP data resulting from unrealistic combinations of cloud radiative forcing and other physical processes is made by comparing simulations with 3-hourly and monthly mean cloud fraction data. The authors find little difference in the simulation of monthly mean quantities in spite of large differences in the temporal variability of the imposed ISCCP-based cloud radiative forcing in these simulations. These results do not preclude the importance of simulating the correct temporal relationship between cloud radiative forcing and other physical processes in climate model simulations, but they do support the assumption that a correct simulation of that relationship is not essential for the simulation of certain monthly mean quantities. The present results point favorably to the use of the ISCCP cloud data for climate model testing, as well as further GCM experiments examining the radiative effects of clouds on the general circulation.

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Melinda S. Peng, James A. Ridout, and Timothy F. Hogan

Abstract

The convective parameterization of Emanuel has been employed in the forecast model of the Navy Operational Global Atmospheric Prediction System (NOGAPS) since 2000, when it replaced a version of the relaxed Arakawa–Schubert scheme. Although in long-period data assimilation forecast tests the Emanuel scheme has been found to perform quite well in NOGAPS, particularly for tropical cyclones, some weaknesses have also become apparent. These weaknesses include underprediction of heavy-precipitation events, too much light precipitation, and unrealistic heating at upper levels. Recent research efforts have resulted in modifications of the scheme that are designed to reduce such problems. One change described here involves the partitioning of the cloud-base mass flux into mixing cloud mass flux at individual levels. The new treatment significantly reduces a heating anomaly near the tropopause that is associated with a large amount of mixing cloud mass flux ascribed to that region in the original Emanuel scheme. In another modification, the selection of the updraft source level is changed in a manner that takes into consideration the assumed connection between updraft mass flux and parcel buoyancy at cloud-base level in the Emanuel scheme. Test results suggest that the modified scheme may in some cases better represent precipitation during the middle and latter stages of convective events. The scheme has also been modified to eliminate cloud-top overshooting. The parameterization changes are supported in part by diagnostic tests, including semiprognostic model tests using observed data and single-column model tests using cloud-resolving-scale simulation data. The modifications showed significant positive impacts in forecast experiments over the original designs and have been implemented into the operational NOGAPS.

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Weiwei Li, Zhuo Wang, Melinda S. Peng, and James A. Ridout

Abstract

Navy Operational Global Atmospheric Prediction System (NOGAPS) analysis and operational forecasts are evaluated against the Interim ECMWF Re-Analysis (ERA-Interim; ERAI) and satellite data, and compared with the Global Forecast System (GFS) analysis and forecasts, using both performance- and physics-based metrics. The NOGAPS analysis captures realistic Madden–Julian oscillation (MJO) signals in the dynamic fields and the low-level premoistening leading to active convection, but the MJO signals in the relative humidity (RH) and diabatic heating rate (Q1) fields are weaker than those in the ERAI or the GFS analysis. The NOGAPS forecasts, similar to the GFS forecasts, have relatively low prediction skill for the MJO when the MJO initiates over the Indian Ocean and when active convection is over the Maritime Continent. The NOGAPS short-term precipitation forecasts are broadly consistent with the Climate Prediction Center (CPC) morphing technique (CMORPH) precipitation results with regionally quantitative differences. Further evaluation of the precipitation and column water vapor (CWV) indicates that heavy precipitation develops too early in the NOGAPS forecasts in terms of the CWV, and the NOGAPS forecasts show a dry bias in the CWV increasing with forecast lead time. The NOGAPS underpredicts light and moderate-to-heavy precipitation but overpredicts extremely heavy rainfall. The vertical profiles of RH and Q1 reveal a dry bias within the marine boundary layer and a moist bias above. The shallow heating mode is found to be missing for CWV < 50 mm in the NOGAPS forecasts. The diabatic heating biases are associated with weaker trade winds, weaker Hadley and Walker circulations over the Pacific, and weaker cross-equatorial flow over the Indian Ocean in the NOGAPS forecasts.

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James A. Ridout, Yi Jin, and Chi-Sann Liou

Abstract

A quasi balance with respect to parcel buoyancy at cloud base between destabilizing processes and convection is imposed as a constraint on convective cloud-base mass flux in a modified version of the Kain–Fritsch cumulus parameterization. Supporting evidence is presented for this treatment, showing a cloud-base quasi balance (CBQ) on a time scale of approximately 1–3 h in explicit simulations of deep convection over the U.S. Great Plains and over the tropical Pacific Ocean with the Naval Research Laboratory’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS). With the exception of the smaller of two convective events in the Great Plains simulation, a CBQ is still observed upon restriction of the data analysis to instances where the available buoyant energy (ABE) exceeds a threshold value of 1000 J kg−1. This observation is consistent with the view that feedbacks between convection and cloud-base parcel buoyancy can control the rate of convection on shorter time scales than those associated with the elimination of buoyant energy and supports the addition of a CBQ constraint to the Kain–Fritsch mass-flux closure.

Tests of the modified Kain–Fritsch scheme in single-column-model simulations based on the explicit three-dimensional simulations show a significant improvement in the representation of the main convective episodes, with a greater amount of convective rainfall. The performance of the scheme in COAMPS precipitation forecast experiments over the continental United States is also investigated. Improvements are obtained with the modified scheme in skill scores for middle to high rainfall rates.

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Stephanie S. Rushley, Matthew A. Janiga, James A. Ridout, and Carolyn A. Reynolds

Abstract

The Madden–Julian oscillation (MJO) is a key source of predictability in the subseasonal time scale (weeks to months) and influences a wide range of weather and climate phenomena. Although there have been enormous gains in simulating the MJO, many climate and forecast models still have biases in MJO behavior and structure. In this study, we examine the MJO in the Navy Earth System Prediction Capability (Navy ESPC) forecasts performed for the Subseasonal Experiment (SubX) using process-based diagnostics and a moisture budget analysis that uses wavenumber–frequency filtering to isolate the MJO. The MJO in the Navy ESPC is too strong in both boreal winter and summer. This amplitude bias is driven by biases in the vertical moisture advection in the Navy ESPC, which is too strong and deep, driven by a more bottom-heavy vertical motion profile and too steep lower-tropospheric vertical moisture gradient. Additionally, the convective moisture adjustment time scale in the Navy ESPC is faster than observed, such that for a given moisture anomaly the precipitation response is greater than observed. In the Navy ESPC, the MJO propagation shows strong agreement with observations in the Indian Ocean, followed by too rapid propagation east of the Maritime Continent in both seasons. This MJO acceleration east of the Maritime Continent is linked to an acceleration of moisture anomalies driven by biases in anomalous moisture tendency. The mechanisms that drive this bias have seasonal differences, with excess evaporation in the western Pacific dominating in boreal winter and horizontal moisture advection dominating in boreal summer.

Open access
William A. Komaromi, Xiaodong Hong, Matthew A. Janiga, Carolyn A. Reynolds, James A. Ridout, and James D. Doyle

Abstract

Given the prohibitive expense of running a global coupled high-resolution model for multiweek forecasts, we explore the feasibility of running a limited-area model forced by a global model on monthly time scales. Specifically, we seek to understand the constraints of the accuracy of lateral boundary conditions (LBCs) produced by NAVGEM on the skill of limited-area COAMPS forecasts. In this study, we analyze simulations of the successive MJO events of November 2011. In the NAVGEM simulations, the effect of ocean boundary conditions are examined, including fixed sea surface temperature (SST), observed SST, and coupled SST with HYCOM. With fixed SST, the second MJO fails to develop, highlighting the importance of the ocean response in the ability to model successive MJO events. Next, we examine the dependence of the regional COAMPS skill on the global model forecast performance. It is found that even when using the inferior but computationally inexpensive uncoupled NAVGEM for LBCs, coupled COAMPS can accurately predict the successive MJO events. A well-resolved atmospheric Rossby wave that slowly propagates westward in the COAMPS domain contributes to increased predictive skill. Ocean coupling and the ability of the model to sufficiently warm the ocean during the convectively suppressed phase also appears to be critical. Last, while COAMPS exhibits a significant moist bias, the sign and magnitude of the vertical and horizontal moisture flux appear to be consistent with reanalysis, a necessary attribute of any model to be used in multiweek MJO prediction.

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Matthew A. Janiga, Carl J. Schreck III, James A. Ridout, Maria Flatau, Neil P. Barton, E. Joseph Metzger, and Carolyn A. Reynolds

Abstract

In this study, the contribution of low-frequency (>100 days), Madden–Julian oscillation (MJO), and convectively coupled equatorial wave (CCEW) variability to the skill in predicting convection and winds in the tropics at weeks 1–3 is examined. We use subseasonal forecasts from the Navy Earth System Model (NESM); NCEP Climate Forecast System, version 2 (CFSv2); and ECMWF initialized in boreal summer 1999–2015. A technique for performing wavenumber–frequency filtering on subseasonal forecasts is introduced and applied to these datasets. This approach is better able to isolate regional variations in MJO forecast skill than traditional global MJO indices. Biases in the mean state and in the activity of the MJO and CCEWs are smallest in the ECMWF model. The NESM overestimates cloud cover as well as MJO, equatorial Rossby, and mixed Rossby–gravity/tropical depression activity over the west Pacific. The CFSv2 underestimates convectively coupled Kelvin wave activity. The predictive skill of the models at weeks 1–3 is examined by decomposing the forecasts into wavenumber–frequency signals to determine the modes of variability that contribute to forecast skill. All three models have a similar ability to simulate low-frequency variability but large differences in MJO skill are observed. The skill of the NESM and ECMWF model in simulating MJO-related OLR signals at week 2 is greatest over two regions of high MJO activity, the equatorial Indian Ocean and Maritime Continent, and the east Pacific. The MJO in the CFSv2 is too slow and too weak, which results in lower MJO skill in these regions.

Open access