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D. Gregory

Abstract

The parameterization of evaporation of rain and snow in large-scale numerical models of the atmosphere is considered. Evaporation coefficients dependent on the precipitation rate are derived following the method of Kessler for both stratiform and convective precipitation and compared with the calculations of more detailed models and observations using passive models. The derived “bulk” parameterizations are in good agreement with the evaporation rates derived from the microphysical model of Clough and Franks, showing more rapid evaporation of snow than rain. Comparison is made to other recent evaporation parameterizations and the sensitivity of the estimated evaporation rate to the nature of precipitation, and the motion of the air through which it falls is also studied. The impact of the inclusion of different rates for the evaporation of stratiform rain and snow upon climate simulations by the Meteorological Office Unified Model (the Hadley Centre Climate Model) is considered.

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D. Gregory and P. R. Rowntree

Abstract

The convection scheme used at the UK Meteorological Office in large-scale numerical models is described. The scheme uses a “bulk” cloud model to represent an ensemble of convective clouds and aims to represent shallow, deep and midlevel convection. A simple closure is employed, the initial convective mass flux being related to the stability of the initial convecting layer. The ability of the scheme to represent convective processes in a variety of situations is evaluated using GATE, BOMEX, and ATEX data. In each case realistic heating rates are simulated and although the closure of the scheme does not demand a balance between convective and large-scale forcings as in many other types of convection scheme (for example the Arakawa–Schubert scheme), a quasi-equilibrium is established while retaining realistic atmospheric structure.

The performance of the scheme in an 11-layer atmospheric general circulation model used in climate research at the UK Meteorological Office is also evaluated by comparing aspects of the simulated tropical flow from a recent 4-year integration with observed data. The scheme simulates the main areas of latent heat release and their variation throughout the year, although the Indian Monsoon is poorly simulated. The upper level divergent circulation is also well simulated, although too weak in northern summer. The zonally averaged tropospheric temperature structure is reasonable indicating that the interaction of convective and radiative processes is reasonably modeled. The variation of outgoing longwave radiation (a proxy for convective rainfall in the tropics) with sea surface temperature agrees with recent observational studies.

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Gregory S. Elsaesser and Christian D. Kummerow

Abstract

In light of the upcoming launch of the Global Precipitation Measurement (GPM) mission, a parametric retrieval algorithm of the nonraining parameters over the global oceans is developed with the ability to accommodate all currently existing and planned spaceborne microwave window channel sensors and imagers. The physical retrieval is developed using all available sensor channels in a full optimal estimation inversion. This framework requires that retrieved parameters be physically consistent with all observed satellite radiances regardless of the sensor being used. The retrieval algorithm has been successfully applied to the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E), the Special Sensor Microwave Imager (SSM/I), and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) with geophysical parameter retrieval results comparable to independent studies using sensor-optimized algorithms. The optimal estimation diagnostics characterize the retrieval further, providing errors associated with each of the retrieved parameters, indicating whether the retrieved state is physically consistent with observed radiances, and yielding information on how well simulated radiances agree with observed radiances. This allows for the quantitative assessment of potential calibration issues in either the model or sensor. In addition, there is an expected, consistent response of these diagnostics based on the scene being observed, such as in the case of a raining scene, allowing for the emergence of a rainfall detection scheme providing a new capability in rainfall identification for use in passive microwave rainfall and cloud property retrievals.

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Julxo C. Calvo and James D. Gregory

Abstract

Monthly and annual air temperature characteristics and their relationship to elevation and latitude were examined for North Carolina. Regression equations to predict monthly and annual mean temperatures and mean temperature ranges were derived using 57 stations with a 30-yr record of temperatures. A procedure for predicting maximum and minimum temperatures was also developed. An estimate of the temperature lapse rates due to elevation and latitude was proposed. Regression diagnostics, as well as validation analysis against 25 independent stations, indicated that the proposed equations reliably predict monthly and annual air temperature characteristics in North Carolina.

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William H. Jasperson and Gregory D. Nastrom

Abstract

Comparison between in situ aircraft observations of temperature and National Meteorological Center and Global Weather Central analysis fields of temperature is presented for a continental and oceanic flight route. The standard deviations of the temperature differences over several hundred flights are found to be 2.5 and 3.5°C for the continental and oceanic route, respectively. A bias towards warm temperatures of about 0.85°C for the analysis fields was found for the oceanic route. Only small differences are found between the NMC and GWC analysis field temperatures.

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Gregory M. Flato and William D. Hibler III

Abstract

Polar ocean circulation is influenced by fluxes of salt and freshwater at the surface as ice freeze in one location, is transported by the winds and currents, and melts again elsewhere. The motion of sea ice, moreover, is strongly affected by internal stresses that arise from the mechanical strength of the ice cover. A simple sea-ice dynamics model, allowing these effects to be included in large-scale climate studies, is presented. In this model a cavitating fluid behaviour is assumed whereby the ice pack does not resist divergence or shear, but does resist convergence. While less realistic than other rheologies that include shear strength, this assumption has certain advantages for long-term climate studies. First, it allows a simple and efficient numerical scheme, in both rectangular and spherical coordinates, which as developed here along with a generation to include shear strength via the Mohr-Coulomb failure criteria. Second, realistic ice transport is maintained, even when the model is driven by smoothed wind forcing–a feature that may be useful in coupled ice-ocean climate models using mean monthly or mean annual winds. Finally, the lack of shear strength allows smooth flow past an obstacle, making the scheme attractive for coupling to a global ocean circulation model using an artificial island to avoid the mathematical singularity at the North Pole. Noteworthy. however, is the fact that the numerical scheme developed here does not require an island at the pole, making the model equally suited for coupling to a global atmospheric circulation model.

Three-year dynamic-themodynamic simulations using observed forcing from 1981 to 1983 are performed using the cavitating fluid model and a more complete viscous-plastic model for comparison. The thickness buildup patterns, net ice growth, atmospheric heat flux, and total ice volume calculated by the cavitating fluid model are very similar to the viscous-plastic model results; however, the cavitating fluid model substantially overestimates local ice drift when compared to observed buoy drift. A 3-year simulation using the spherical grid version of the model, both with and without an artificial island at the pole, shows that the island has little impact on the thickness buildup and ice transport. Overall, the cavitating fluid approximation is shown to be a useful simplification, allowing essential feedback between ocean circulation and ice transport to be efficiently included in large-scale climate studies.

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J. B. Gregory and D. T. Rees

Abstract

Spaced-antenna drift measurements between 60 and 100 km, from radiowave partial reflections, are presented in the form of zonal and meridional profiles for May and June 1969, and as a time cross section of zonal winds, from February–June 1969, for 52N, 107W. ROCOB data from 54N, 110W, up to 60 km, are used to complete the profiles. The profiles show satisfactory agreement with profiles derived from a recent model of zonal and meridional winds.

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Gregory D. Bierly and Julie A. Winkler

Abstract

Relative wind isentropic analysis was employed to investigate the evolution of airstreams and airstream boundaries within midlatitude cyclones that formed in the Colorado cyclogenesis region of the United States. This study attempts to verify and expand upon existing conceptual models of three-dimensional airflow, while describing how such models vary at different times during cyclone development and when the intensification history of the storm is considered. Forty-nine cyclone events were first divided into three categories: early-developing cyclones (those that intensify with 24 h of cyclogenesis), late-developing cyclones (those that intensify 24–48 h after cyclogenesis), and nondeveloping cyclones (those that either display little change in intensity or weaken with time). Composite isentropic surfaces for multiple levels (315–290 K, separated by 5 K) were constructed by cyclone category for six 12-h time periods within the cyclone life cycle.

Three distinct airstreams and four types of airstream boundaries were identified on the composite isentropic surfaces. Two of the airstreams closely resemble the “drystream” and “warm conveyor belt (WCB)” described in previous studies. The third airstream is referred to here as the cyclonically turning moist airstream (CMA). Until approximately 24 h after cyclogenesis, the CMA and WCB originate at similar latitudes although the CMA occurs at a lower elevation. Later in the storm life cycle, the CMA originates at a more northerly latitude than the WCB and in comparison is a relatively cold airstream. Airstream boundaries separating the WCB and the drystream are seen at almost all time periods. This feature acquires a forward-leaning orientation with time with only the lowermost boundaries being accompanied by a modest to strong temperature gradient. Two airstream boundaries involve the CMA. The first separates the CMA and the drystream and is a lower-tropospheric feature, particularly late in the storm life cycle. The second boundary is located north or northwest of the cyclone center and separates the CMA from northerly descending air. This midtropospheric feature occurs along a relatively weak temperature gradient. The fourth type of airstream boundary is referred to as a southwest confluence zone and separates northerly, descending airflow southwest of the cyclone center from easterly, rising airflow to the southeast. At the middle and later stages of the cyclone life cycle, this boundary is a lower-tropospheric feature. It is often associated with a relatively strong temperature gradient. The composites indicate that the evolution of the airstreams and airstream boundaries is remarkably similar for the three cyclone types, except that they are out of synchrony by one or more 12-h time steps. In particular, all three airstreams are evident on the precyclogenesis (time t − 12) composite surfaces for the nondeveloping cyclones, whereas the full suite of airstreams does not appear until 12 h later for the developing cyclones.

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Gregory J. Hakim and and Ryan D. Torn

Abstract

Synoptic and mesoscale meteorology underwent a revolution in the 1940s and 1950s with the widespread deployment of novel weather observations, such as the radiosonde network and the advent of weather radar. These observations provoked a rapid increase in our understanding of the structure and dynamics of the atmosphere by pioneering analysts such as Fred Sanders. The authors argue that we may be approaching an analogous revolution in our ability to study the structure and dynamics of atmospheric phenomena with the advent of probabilistic objective analyses. These probabilistic analyses provide not only best estimates of the state of the atmosphere (e.g., the expected value) and the uncertainty about this state (e.g., the variance), but also the relationships between all locations and all variables at that instant in time. Up until now, these relationships have been determined by sampling in time by, for example, case studies, composites, and time-series analysis. Here the authors propose a new approach, ensemble synoptic analysis, which exploits the information contained in probabilistic samples of analyses at one or more instants in time.

One source of probabilistic analyses is ensemble-based state-estimation methods, such as ensemble-based Kalman filters. Analyses from such a filter may be used to study atmospheric phenomena and the relationships between fields and locations at one or more instants in time. After a brief overview of a research-based ensemble Kalman filter, illustrative examples of ensemble synoptic analysis are given for an extratropical cyclone, including relationships between the cyclone minimum sea level pressure and other synoptic features, statistically determined operators for potential-vorticity inversion, and ensemble-based sensitivity analysis.

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Ryan D. Torn and Gregory J. Hakim

Abstract

An ensemble Kalman filter (EnKF) based on the Weather Research and Forecasting model is applied to generate ensemble analyses and forecasts of Hurricane Katrina (2005) and the surrounding area every 6 h over the lifetime of the storm on a nested domain. Analyses are derived from assimilating conventional in situ observations, reconnaissance dropsondes, including data taken during the Hurricane Rainband and Intensity Exchange Experiment (RAINEX), and tropical cyclone position estimates. Observation assimilation at individual times consistently reduces errors in tropical cyclone position, but not necessarily in intensity; however, withholding observations leads to significantly larger errors in both quantities. Analysis increments for observations near the tropical cyclone are dominated by changes in vortex position, and these increments increase the asymmetric structure of the storm. Data denial experiments indicate that dropsondes deployed in the synoptic environment provide minimal benefit to the outer domain; however, dropsondes deployed within the tropical cyclone lead to significant reductions in position and intensity errors on the inner domain. Specifically, errors in the inner domain ensemble-mean 6-h forecasts of minimum pressure are 70% larger when dropsonde data is not assimilated. Precipitation fields are qualitatively similar to Tropical Rainfall Measuring Mission (TRMM) satellite estimates, although model values are double the values of the satellite estimate. Moreover, the spinup period and initial imbalance in EnKF-initialized WRF forecasts is less than starting the model from a GFS analysis. Ensemble-mean 48-h forecasts initialized with EnKF analyses have track and intensity errors that are 50% smaller than GFS and NHC official forecasts.

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