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L. Cucurull and R. A. Anthes

Abstract

A comparison of the impact of infrared (IR), microwave (MW), and radio occultation (RO) observations on NCEP’s operational global forecast model over the month of March 2013 is presented. Analyses and forecasts with only IR, MW, and RO observations are compared with analyses and forecasts with no satellite data and with each other. Overall, the patterns of the impact of the different satellite systems are similar, with the MW observations producing the largest impact on the analyses and RO producing the smallest. Without RO observations, satellite radiances are over– or under–bias corrected and RO acts as an anchor observation, reducing the forecast biases globally. Positive correlation coefficients of temperature impacts are generally found between the different satellite observation analyses, indicating that the three satellite systems are affecting the global temperatures in a similar way. However, the correlation in the lower troposphere among all three systems is surprisingly small. Correlations for the moisture field tend to be small in the lower troposphere between the different satellite analyses. The impact of the satellite observations on the 500-hPa geopotential height forecasts is much different in the Northern and Southern Hemispheres. In the Northern Hemisphere, all the satellite observations together make a small positive impact compared to the base (no satellite) forecasts. The IR and MW, but not the RO, make a small positive impact when assimilated alone. The situation is considerably different in the Southern Hemisphere, where all the satellite observations together make a much larger positive impact, and all three observation types (IR, MW, and RO) make similar and significant impacts.

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RICHARD A. ANTHES and DONALD R. JOHNSON

Abstract

The energetics of hurricane Hilda (1964) are studied through the theory of available potential energy applied to a limited fixed region surrounding the storm. The generation of available potential energy is shown to be closely dependent on differential heating within the baroclinic structure of the hurricane, occurring primarily in the middle and upper troposphere of the core of the storm. The diabatic heating components (condensation, emission of long wave radiation, direct absorption of solar radiation, and sensible heating) are modeled and the contribution to the total generation from each component computed. The results from the latent heating model based on Kuo's work portray the dependence of the deep cumulus convection on the sea surface temperature. The best estimate of the total generation of available potential energy within the hurricane scale is 10.3 × 1012 watts, of which 77 percent is generated by latent heating, 17 percent by infrared cooling, and 6 percent by direct solar absorption. The total generation compares favorably with estimates of kinetic energy production in mature hurricanes. Energy considerations in the steady state condition of the hurricane system are discussed within the framework of the available potential energy theory.

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X. Zou, Hui Liu, and R. A. Anthes

Abstract

Atmospheric data from National Centers for Environmental Prediction (NCEP) analyses and orbital parameters from 133 real Global Positioning System (GPS) meteorological data soundings are used to compute the “true” bending angle profiles using an accurate 3D ray-tracing procedure. They are then compared with approximate profiles obtained using the spherical symmetry assumption and an efficient 2D ray-tracing model. The average fractional error of the bending angles due to the spherical symmetry assumption is less than 0.15%. The average fractional error due to the use of the 2D ray-tracing model is slightly greater than that due to the spherical symmetry assumption. The vertical error correlations due to the spherical symmetry assumption are sharp between 6 and 8 km and broader above and below this layer. The vertical error correlations associated with the 2D forward model show a nearly diagonal structure below 15 km with high correlation confined to a 2-km layer centered at the observation level.

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David R. Rodenhuis and Richard A. Anthes

A few examples of scientific accomplishments in tropical meteorology and hurricane research are presented. Tropical field experiments such as GATE have greatly influenced observational studies of convection and tropical easterly waves. One application of the study of convection is the attempt to estimate precipitation from satellite platforms.

Research in tropical cyclones has further improved the definition of large-scale structure and the environment in which the hurricane grows. Radiation, convection, and air-sea interaction studies are directed at the forcing and possible feedback of the hurricane with its environment. With this improved physical understanding, numerical modeling of hurricanes can now produce position forecasts of reasonable accuracy that are becoming competitive with current statistical-dynamical methods. There is a continuing effort to attempt hurricane modification experiments in conjunction with an adequate measurement program.

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Richard A. Anthes, Ying-Hwa Kuo, and John R. Gyakum

Abstract

The extratropical cyclone which damaged the liner Queen Elizabeth II in September 1978 is a well-documented example of explosive marine cyclogenesis in which the 24 h surface central pressure fall was 60 mb commencing 1200 GMT 9 September. Operational models of both the National Meteorological Center (NMC) and Fleet Numerical Weather Central (FNWC) predicted virtually none of the observed surface intensification. This study reports on results of simulations performed with a primitive equation model. Emphasis will be placed on discovering why such poor forecasts were made of this storm. The extensive data set compiled by Gyakum (1983a, b) is used both to initialize and verify the model in a series of 24 h simulations, in order to assess the impact of initializing the model with these supplementary data. Physical processes identified observationally by Gyakum as being important in the storm's evolution are also examined numerically for their relative importance. In a series of seven simulations in which initial condition, horizontal resolution, and physics are varied, the model storm intensity varies considerably. In the weakest, the minimum pressure and maximum boundary-layer wind speeds are 1001 mb and 15.0 m s−1; in the strongest, these parameters are 960 mb and 50.2 m s−1.

The model simulations without the supplementary data set show little improvement over the forecasts of NMC and FNWC. Those simulations with the supplementary data produce improvements in the S 1 score, the intensity of the storm and the track of the storm. The improvement in the model simulations with the introduction of the supplementary data appears due to their more realistic documentation of the shallow cyclonic circulation, the small low-level static stability, and enhanced lower-tropospheric water vapor content.

Physical processes also played a major role in the simulators. The effect of surface fluxes of sensible and latent heat were moderate on the 24 h pressure and wind forecasts. In addition, these fluxes produced large changes in the temperature and moisture structure of the planetary boundary layer over a large area of cold northerly flow to the rear of the cyclone.

Latent heating was important in determining the storm intensity and track. Including latent heating through a cumulus parameterization scheme with a horizontal resolution of 90 km produced an improvement in the simulated intensity and position, with a reduction in minimum pressure of 7 mb and an increase in boundary-layer wind speed of 5 m s−1. With 45 km horizontal resolution, use of explicit condensation heating rather than the cumulus parameterization produced a further reduction in minimum pressure of 12 mb. Although experiments with explicit rather than parameterized latent heating produced more intense storms, in agreement with observations, the model storm motion was slowed considerably during the last 6 h of the simulation, resulting in an increased position error.

The model storm showed a small increase in intensity when the horizontal grid length reduced from 90 km to 45 km, with the minimum pressure decreasing by 3 mb. A further reduction in horizontal resolution to 22.5 km produced only minor differences in storm intensity.

The most intense model storm was simulated when an explicit medium-resolution planetary boundary-layer formulation replaced the bulk formulation used in most of the experiments. With 45 km resolution, explicit latent heating, and the medium-resolution boundary-layer model, a storm with minimum pressure of 960 mb and a maximum wind speed of 50.2 m s−1 was obtained.

This study suggests that baroclinic instability in the weakly stratified lower troposphere is the major mechanism of growth for this cyclone, as discussed by Reed, although latent heat plays an important role in the later stages of development. The development of this strong, yet relatively shallow, storm has three major implications for improving operational forecasts Of similar storms. First, the vertical resolution of the model must be adequate; our estimate is that at least four model layers are required below 700 mb. Second, the lower-tropospheric winds, static stability, water vapor content, and sea-surface temperature must be resolved accurately in the initial analysis because of the sensitivity of the model storm to these fields. Third, continued improvement of modeling planetary boundary-layer and latent heating processes is likely to be important in cases of this type.

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Gary D. Fried, John J. Cahir, and R. A. Anthes

Abstract

A method of introducing subsynoptic-scale initial data into a six-level primitive equation model is presented. Vorticity perturbations on the scale of subsynoptic disturbances are added to the initial winds; the amplitudes and slopes of the perturbations are subjectively determined from physical, climatological and observational considerations. Balanced winds, heights and temperatures are then derived from the enhanced vorticity fields.

Two 12 h forecasts are made from two different initial analyses. The first

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L. Cucurull, R. A. Anthes, and L.-L. Tsao

Abstract

Satellite radiance measurements are used daily at numerical weather prediction (NWP) centers around the world, providing a significant positive impact on weather forecast skill. Owing to the existence of systematic errors, either in the observations, instruments, and/or forward models, which can be larger than the signal, the use of infrared or microwave radiances in data assimilation systems requires significant bias corrections. As most bias-correction schemes do not correct for biases that exist in the model forecasts, the model needs to be grounded by an unbiased observing system. These reference measurements, also known as “anchor observations,” prevent a drift of the model to its own climatology and associated biases, thus avoiding a spurious drift of the observation bias corrections.

This paper shows that the assimilation of global positioning system (GPS) radio occultation (RO) observations over a 3-month period in an operational NWP system results in smaller, more accurate bias corrections in infrared and microwave observations, resulting in an overall more effective use of satellite radiances and a larger number of radiance observations that pass quality control. A full version of the NCEP data assimilation system is used to evaluate the results on the bias corrections for the High Resolution Infrared Radiation Sounder-3 (HIRS-3) on NOAA-17 and the Advanced Microwave Sounding Unit-A (AMSU-A) on NOAA-15 in an operational environment.

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Y.-H. Kuo, X. Zou, S. J. Chen, W. Huang, Y.-R. Guo, R. A. Anthes, M. Exner, D. Hunt, C. Rocken, and S. Sokolovskiy

A Global Positioning System Meteorology (GPS/MET) proof-of-concept experiment became a reality on 3 April 1995. A small satellite carrying a modified GPS receiver was launched into earth orbit to demonstrate the feasibility of active limb sounding of the earth's neutral atmosphere and ionosphere using the radio occultation method. On 22 October 1995, a GPS/MET occultation took place over northeastern China where a dense network of radiosonde observations was available within an hour of the occultation. The GPS/MET refractivity profile shows an inflection, and the corresponding temperature retrieval displays a sharp temperature inversion around 310 mb. Subjective analyses based on radiosonde observations indicate that the GPS/MET occultation went through a strong upper-level front. In this paper, the GPS/MET sounding is compared with nearby radiosonde observations to assess its accuracy and ability to resolve a strong mesoscale feature. The inflection in the refractivity profile and the sharp frontal inversion seen in the GPS/MET sounding were verified closely by a radiosonde located about 150 km to the east of the GPS/MET occultation site. A similar frontal structure was also found in other nearby radiosonde observations. These results showed that high-quality GPS/MET radio occultation data can be obtained even when the occultation goes through a sharp temperature gradient associated with an upper-level front.

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Steven Businger, Steven R. Chiswell, Michael Bevis, Jingping Duan, Richard A. Anthes, Christian Rocken, Randolph H. Ware, Michael Exner, T. VanHove, and Fredrick S. Solheim

This paper provides an overview of applications of the Global Positioning System (GPS) for active measurement of the Earth's atmosphere. Microwave radio signals transmitted by GPS satellites are delayed (refracted) by the atmosphere as they propagate to Earth-based GPS receivers or GPS receivers carried on low Earth orbit satellites.

The delay in GPS signals reaching Earth-based receivers due to the presence of water vapor is nearly proportional to the quantity of water vapor integrated along the signal path. Measurement of atmospheric water vapor by Earth-based GPS receivers was demonstrated during the GPS/STORM field project to be comparable and in some respects superior to measurements by ground-based water vapor radiometers. Increased spatial and temporal resolution of the water vapor distribution provided by the GPS/STORM network proved useful in monitoring the moisture-flux convergence along a dryline and the decrease in integrated water vapor associated with the passage of a midtropospheric cold front, both of which triggered severe weather over the area during the course of the experiment.

Given the rapid growth in regional networks of continuously operating Earth-based GPS receivers currently being implemented, an opportunity exists to observe the distribution of water vapor with increased spatial and temporal coverage, which could prove valuable in a range of operational and research applications in the atmospheric sciences.

The first space-based GPS receiver designed for sensing the Earth's atmosphere was launched in April 1995. Phase measurements of GPS signals as they are occluded by the atmosphere provide refractivity profiles (see the companion article by Ware et al. on page 19 of this issue). Water vapor limits the accuracy of temperature recovery below the tropopause because of uncertainty in the water vapor distribution. The sensitivity of atmospheric refractivity to water vapor pressure, however, means that refractivity profiles can in principle yield information on the atmospheric humidity distribution given independent information on the temperature and pressure distribution from NWP models or independent observational data.

A discussion is provided of some of the research opportunities that exist to capitalize on the complementary nature of the methods of active atmospheric monitoring by GPS and other observation systems for use in weather and climate studies and in numerical weather prediction models.

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R. A Anthes, P. A Bernhardt, Y. Chen, L. Cucurull, K. F. Dymond, D. Ector, S. B. Healy, S.-P. Ho, D. C Hunt, Y.-H. Kuo, H. Liu, K. Manning, C. McCormick, T. K. Meehan, W J. Randel, C. Rocken, W S. Schreiner, S. V. Sokolovskiy, S. Syndergaard, D. C. Thompson, K. E. Trenberth, T.-K. Wee, N. L. Yen, and Z Zeng

The radio occultation (RO) technique, which makes use of radio signals transmitted by the global positioning system (GPS) satellites, has emerged as a powerful and relatively inexpensive approach for sounding the global atmosphere with high precision, accuracy, and vertical resolution in all weather and over both land and ocean. On 15 April 2006, the joint Taiwan-U.S. Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)/Formosa Satellite Mission 3 (COSMIC/FORMOSAT-3, hereafter COSMIC) mission, a constellation of six microsatellites, was launched into a 512-km orbit. After launch the satellites were gradually deployed to their final orbits at 800 km, a process that took about 17 months. During the early weeks of the deployment, the satellites were spaced closely, offering a unique opportunity to verify the high precision of RO measurements. As of September 2007, COSMIC is providing about 2000 RO soundings per day to support the research and operational communities. COSMIC RO data are of better quality than those from the previous missions and penetrate much farther down into the troposphere; 70%–90% of the soundings reach to within 1 km of the surface on a global basis. The data are having a positive impact on operational global weather forecast models.

With the ability to penetrate deep into the lower troposphere using an advanced open-loop tracking technique, the COSMIC RO instruments can observe the structure of the tropical atmospheric boundary layer. The value of RO for climate monitoring and research is demonstrated by the precise and consistent observations between different instruments, platforms, and missions. COSMIC observations are capable of intercalibrating microwave measurements from the Advanced Microwave Sounding Unit (AMSU) on different satellites. Finally, unique and useful observations of the ionosphere are being obtained using the RO receiver and two other instruments on the COSMIC satellites, the tiny ionosphere photometer (TIP) and the tri-band beacon.

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