Monthly Weather Review

Saito, Kazuo; Keenan, Tom; Holland, Greg; Puri, Kamal

doi: 10.1175/1520-0493(2001)129<0378:NSOTDE>2.0.CO;2pmid: N/A

Numerical simulations of the diurnal evolution of tropical island convection observed during the Maritime Continent Thunderstorm Experiment (MCTEX) are performed using the Meteorological Research Institute nonhydrostatic model (MRI NHM). The MRI NHM is double-nested within a form of the Australian Bureau of Meteorology Research Centre’s Limited-Area Assimilation and Prediction System specially operated for the MCTEX period. Excellent agreement is found between the simulation and observed evolution of the convective clouds over the Tiwi Islands on 27 November 1995. A transition from horizontal convection occurring during the morning to vertical convection in afternoon is evident. In the morning, the sea breeze appears along the coastlines, with a clear contrast evident in structure between the windward and leeward sides. At the windward coast, the sea breeze intrudes inland more rapidly, where the larger surface heat flux modifies the lowest air mass and makes the sea breeze front (SBF) indistinct. On the other hand, at the leeward coast, the upward motion at the head of the SBF is larger and deeper. Shallow convective clouds therefore have a preference for alignment along the leeward SBF. Over the interior of the islands ahead of SBFs, shallow convective clouds corresponding to the Rayleigh–Benard convection occur at corners of open polygonal shaped cells and seem randomly distributed. Within the SBFs, organization of convection characteristic of horizontal convective rolls (HCRs) is evident. These HCRs are preferred at the windward coast and occur within cloud-free regions. Clouds associated with the SBFs appear to develop preferentially at the cross points of the SBFs and HCRs where the surface convergence is enhanced. Following further inland propagation of SBFs, weak precipitation starts and the Rayleigh–Benard convection is disturbed by resulting outflows. At the merging stage, the clouds organize at the leeward central part of the islands in the form of an east–west line. In this convergence zone between the two SBFs, explosive growth of convection occurs and cloud top reaches the tropopause. In the case simulated here, the associated downdrafts are not strong compared with the upward motion due to a lack of the midlevel dry air necessary to enhance evaporative cooling. The inclusion of ice phase physics in the simulation produces little qualitative difference in storm development and the associated surface rainfall distribution, but yields stronger updrafts and higher cloud-top heights. The vertical profile of the apparent heat source ( Q 1 ) in the ice phase experiment shows double peaks corresponding to the condensation and freezing levels. Sensitivity experiments show that the orographic undulations as well as the horizontal scale of the island are important factors determining the timing of cloud merger and convective intensity. Without hills, the transition to the explosive growth in the merger stage is delayed. This results in weaker rainfall, even if the hills are relatively flat. A smaller island produced weaker convection, which means that the total rain produced by each island is not proportional to island area. These results suggest that the intensity of tropical island convection is determined not only by the convective stability of the environmental atmosphere but is influenced significantly by the island-scale circulations, that is, horizontal convection in the morning that ultimately forces the deep convection during the afternoon.

Sousounis, Peter J.; Wallman, James; Mann, Greg E.; Miner, Todd J.

doi: 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2pmid: N/A

An intense cutoff low developed over the Great Lakes during the period 13–15 September 1996. The low developed as unseasonably cool air spread over the relatively warm water of the Great Lakes aggregate (i.e., all the Great Lakes). It eventually developed an eye, spiral rainbands, and a warm core, similar to those in a hurricane. This event presented some forecast challenges for the Nested Grid Model (NGM) and Eta Model and hence for the National Weather Service. The NGM model forecasted a weaker low (999 vs 993 hPa) to be centered east of the observed location, over Lake Huron. The Eta Model forecasted a slightly stronger low (991 vs 993 hPa) to be centered even farther east than did the NGM, over southern Ontario. As a result of the sea level pressure errors, both models also forecasted much weaker winds than were observed over the lakes and much less precipitation around the lakeshores. The coarse resolution in both models likely contributed significantly to these errors. With-lake (WL) and no-lake (NL) simulations were performed with the National Center for Atmospheric Research–Pennsylvania State University mesoscale model MM5 to determine the impacts of the Great Lakes on development of the low. The WL simulation agreed well with the observations. At the surface, the intensity and position of the WL low was within 1.7 hPa and 70 km at 30 h into the simulation (1800 UTC 14 September 1996), when the observed low was most intense. To the extent that the impact of the Great Lakes can be ascertained through comparison of the simulations, selected WL–NL differences at the surface revealed that the lakes deepened the WL low by ∼5–7 hPa and restricted its movement. A comparison of WL and NL simulations at upper levels revealed equally impressive differences (e.g., lake-induced perturbations). Strong negative (positive) height and meso- α -scale cyclonic (anticyclonic) wind perturbations at 850 (300) hPa support the hypothesis that the Great Lakes were instrumental in generating a warm core and strong winds near the surface. A comparison of WL–NL differences for this case are compared with those from a more typical wintertime case to illustrate that the WL–NL perturbations can be more intense and can extend to considerably greater depths than in typical winter cases. Strong latent heat fluxes, low static stability, and slow movement (e.g., the cut-off nature) of the synoptic-scale low allowed the strong heating and moistening from the Great Lakes to extend to midtropospheric levels for an extended period of time.

Bishop, Craig H.; Etherton, Brian J.; Majumdar, Sharanya J.

doi: 10.1175/1520-0493(2001)129<0420:ASWTET>2.0.CO;2pmid: N/A

A suboptimal Kalman filter called the ensemble transform Kalman filter (ET KF) is introduced. Like other Kalman filters, it provides a framework for assimilating observations and also for estimating the effect of observations on forecast error covariance. It differs from other ensemble Kalman filters in that it uses ensemble transformation and a normalization to rapidly obtain the prediction error covariance matrix associated with a particular deployment of observational resources. This rapidity enables it to quickly assess the ability of a large number of future feasible sequences of observational networks to reduce forecast error variance. The ET KF was used by the National Centers for Environmental Prediction in the Winter Storm Reconnaissance missions of 1999 and 2000 to determine where aircraft should deploy dropwindsondes in order to improve 24–72-h forecasts over the continental United States. The ET KF may be applied to any well-constructed set of ensemble perturbations. The ET KF technique supercedes the ensemble transform (ET) targeting technique of Bishop and Toth. In the ET targeting formulation, the means by which observations reduced forecast error variance was not expressed mathematically. The mathematical representation of this process provided by the ET KF enables such things as the evaluation of the reduction in forecast error variance associated with individual flight tracks and assessments of the value of targeted observations that are distributed over significant time intervals. It also enables a serial targeting methodology whereby one can identify optimal observing sites given the location and error statistics of other observations. This allows the network designer to nonredundantly position targeted observations. Serial targeting can also be used to greatly reduce the computations required to identify optimal target sites. For these theoretical and practical reasons, the ET KF technique is more useful than the ET technique. The methodology is illustrated with observation system simulation experiments involving a barotropic numerical model of tropical cyclonelike vortices. These include preliminary empirical tests of ET KF predictions using ET KF, 3DVAR, and hybrid data assimilation schemes—the results of which look promising. To concisely describe the future feasible sequences of observations considered in adaptive sampling problems, an extension to Ide et al.’s unified notation for data assimilation is suggested.

Fox-Rabinovitz, Michael S.; Takacs, Lawrence L.; Govindaraju, Ravi C.; Suarez, Max J.

doi: 10.1175/1520-0493(2001)129<0453:AVRSGG>2.0.CO;2pmid: N/A

The development of and results obtained with a variable-resolution stretched-grid GCM for the regional climate simulation mode are presented. A global variable-resolution stretched grid used in the study has enhanced horizontal resolution over the United States as the area of interest. The stretched-grid approach is an ideal tool for representing regional- to global-scale interactions. It is an alternative to the widely used nested-grid approach introduced over a decade ago as a pioneering step in regional climate modeling. The major results of the study are presented for the successful stretched-grid GCM simulation of the anomalous climate event of the 1988 U.S. summer drought. The straightforward (with no updates) 2-month simulation is performed with 60-km regional resolution. The major drought fields, patterns, and characteristics, such as the time-averaged 500-hPa heights, precipitation, and the low-level jet over the drought area, appear to be close to the verifying analyses for the stretched-grid simulation. In other words, the stretched-grid GCM provides an efficient downscaling over the area of interest with enhanced horizontal resolution, in spite of degradation of skill over the coarser resolution far away from the area of interest. It is also shown that the stretched-grid GCM skill is sustained over the area of interest throughout the simulation extended to 1 yr. The stretched-grid GCM, developed and tested in a simulation mode, is a viable tool for regional and subregional climate studies and applications.

Jensen, Tommy G.

doi: 10.1175/1520-0493(2001)129<0470:AOTGMT>2.0.CO;2pmid: N/A

The gravity wave retardation (GWR) method is a simple technique that allows layer models to include bottom topography. Here the method is applied, and its accuracy is evaluated, for monthly climatological wind forcing in an Indian Ocean model with realistic bottom topography. This is an extension of previous studies where the GWR method was applied to idealized wind forcing in oceans with idealized basin geometry. Comparison to a model integration with a flat bottom demonstrates that GWR integrations with speedup factors of up to 16 indeed capture the influence of the bottom relief and have less error in the deep volume transports. For a speedup factor of that magnitude, a GWR integration is also found to have less error than a reduced gravity model simulation. It is concluded that integrations using the GWR method give remarkably good results for the upper-layer circulation as well as the deep flow with a speedup factor of up to 8.

Trapp, R. Jeffrey; Schultz, David M.; Ryzhkov, Alexander V.; Holle, Ronald L.

doi: 10.1175/1520-0493(2001)129<0486:MSAEOA>2.0.CO;2pmid: N/A

A significant winter precipitation event occurred on 8–9 March 1994 in Oklahoma. Snow accumulations greater than 30 cm (12 in.) were measured within a narrow corridor in northern Oklahoma. On the synoptic scale and mesoscale, a correspondence between large snow accumulations and 600-hPa frontogenesis was revealed; the precipitation was formed above the cold frontal surface, owing to midtropospheric ascent associated with the cross-frontal circulation in a region of elevated conditional instability. The location of such a narrow corridor of large accumulations was not, however, disclosed by any patterns in the radar reflectivity data. Indeed, during this event, an elongated maximum of snow accumulation was not associated with a persistent “band” of enhanced reflectivity and vice versa. Dual-polarization and dual-Doppler radar data allowed for a novel analysis of winter precipitation processes and structures, within the context of the larger-scale diagnosis. It was possible to identify, in order of distance southward toward the surface cold front: (i) an elevated convective element, which was classified as an elevated thunderstorm and may have functioned as an ice crystal “generator” cell, embedded within a broad region of generally stratiform precipitation; (ii) a reflectivity band and associated rain–snow transition zone, the evolution and structure of which apparently were coupled to the effects of melting precipitation and strong vertical wind shear; and (iii) a mixed-phase precipitation-generating, prolific lightning-producing, non elevated thunderstorm cell that was sustained in the postfrontal air in part by virtue of its rotational dynamics.

Holmlund, Kenneth; Velden, Christopher S.; Rohn, Michael

doi: 10.1175/1520-0493(2001)129<0517:EAQCAT>2.0.CO;2pmid: N/A

The coverage and quality of atmospheric motion vectors (AMVs) derived from geostationary satellite imagery have improved considerably over the past few years. This is due not only to the deployment of the new generation of satellites, but is also a result of improved data processing and automated quality control (AQC) schemes. The postprocessing of the Geostationary Operational Environmental Satellite (GOES) derived displacement vectors at the National Oceanic and Atmospheric Administration/National Environmental Satellite, Data, and Information Service (NOAA/NESDIS) has been fully automated since early 1996. At the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) AQC was used as support to the manual quality control (MQC) for Meteosat vector fields until September 1998 when the MQC was discontinued and fully replaced with the automated procedure. The AQC schemes at the two organizations are quite diverse. Based on a method developed at the University of Wisconsin–Cooperative Institute for Meteorological Satellite Studies, the NOAA/NESDIS AQC involves an objective three-dimensional recursive filter analysis of the derived wind fields. The fit of each vector to that analysis yields a recursive filter flag (RFF). The AQC scheme employed at EUMETSAT derives a quality indicator (QI) for each individual vector based on the properties of the vector itself and its consistency with other AMVs in close proximity. Mainly relying on satellite data, this QI-based scheme has been proven to provide a good estimate of the reliability of the derived displacements, but it fails to identify fleets of winds that are consistently assigned to a wrong height. The RFF-based scheme is capable of readjusting the heights attributed to the wind vectors, which yields a better fit to the analysis and ancillary data. These quality estimates can be employed by the user community to select the part of the vector field that best suits their application, as well as in data assimilation schemes for optimizing the data selection procedures. Even though both schemes have already been successfully implemented into operational environments, the possibility exists to exploit the advantages of both approaches to create a superior combined methodology. In order to substantiate the differences of the two methodologies, the two schemes were applied to the high-density AMV fields derived during the 1998 North Pacific Experiment from GOES and the Geostationary Meteorological Satellite (controlled by the Japan Meteorological Agency) multispectral imagery data. The performance of the two schemes was evaluated by examining departures from the analysis fields of the European Centre for Medium-Range Weather Forecasts (ECMWF) assimilation scheme, and a new combined methodology was further evaluated by looking at the impact on medium-range forecasts verified against the operational forecasts. It will be shown that the new combined AQC approach yields superior ECMWF forecast results.

Pomeroy, Kenneth R.; Parish, Thomas R.

doi: 10.1175/1520-0493(2001)129<0530:ACSOTI>2.0.CO;2pmid: N/A

Coast-parallel low-level jets are commonplace in the marine boundary layer off the west coast of the United States during summer. A field study was conducted in early summer of 1997 to document the forcing of boundary layer winds in the near-coastal environment off California. On 8 June 1997 the Wyoming King Air collected data along a 350-km stretch of coastal margin from Cape Mendocino to San Francisco in order to examine the interaction between the coastal topography and the low-level jet. During the course of the flight, 32 soundings were conducted. The maximum speed of the coastal jet was found near the top of the marine boundary layer at altitudes from 200 to 600 m. Analysis of the data revealed a westward increase in the height of the marine boundary layer and maximum jet wind speeds. Strongest jet winds were observed southwest of Cape Mendocino with a maximum speed of 28 m s −1 . The coastal jet was characterized by a broad horizontal extent. Wind maxima were found at distances approximately 30 km to more than 100 km offshore. Hydraulic features such as jumps and expansion fans have previously been observed downwind of coastal capes and points along the California coast. The flow upwind of Cape Mendocino and Point Arena was found to be supercritical, but the King Air data showed that accelerations associated with possible expansion fan phenomena were minimal. It is proposed that the sloping inversion at the top of the marine boundary layer and attendant coastal jet are fundamentally the result of a geostrophic adjustment process arising because of the horizontal temperature contrast between the cool ocean and warm continent. This view emphasizes that the coastal jet is a ubiquitous, large-scale feature of the summertime coastal environment. Terrain-induced wind speed variations associated with expansion fans and hydraulic jumps only modulate the primary jet structure.