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F. C. W. Olson

Abstract

The square of the wavelength λ of a surface gravity wave is expressed as an eighth-degree polynomial in u=4π2 h/(gT 2), where h is the depth and T the period. The relative error in λ/λ0, where λ0 is the deep water wavelength, is less than 0.0004% for u<l and less than 1% for u<2.

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Robert W. Houghton, Donald B. Olson, and Peter J. Celone

Abstract

A small scale but energetic and highly nonlinear anticyclonic eddy was observed near the New England Shelf Break in July 1983. Satellite images show a tongue of shelf water 15 km wide flowing offshore then turning anticyclonically westward. Subsequent images show the circulation closing on itself as the eddy expands and slowly drifts southwestward along the shelf break. Acoustic Doppler current data show anticyclonic circulation extending to at least 200 m depth. The center of the eddy appears to undergo solid body rotation with a peak azimuthal velocity of 50 cm s−1 at a radius of 7 km to give an angular velocity ω = −7 × 10−5 s−1 and Rossby number Ro = |ω|/f = 0.74. The resulting nonlinearity from the cyclostropic term produces a shoaling of the near surface pycnocline and a vertical current shear with sign opposite to that of the thermal wind shear.

The eddy is not a warm core ring. Its size, lifetime and water type composition suggest a local origin. It is possible that the anticyclonic vorticity is generated by the thinning of a lense of shelf water as it moves offshore. Pressure work then leads to anticyclogenisis deeper in the water column. Comparison of the observed velocity field with calculations from a two-layer eddy model of Olson and co-authors agree within a factor of 3. The energy source for this feature is not readily identified. Wind stress is not a likely source. The proximity of warm core ring 83-E suggests a potential role in the eddy formation, perhaps through the generation of baroclinic instability of the shelf/slope water front.

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David A. Olson, Norman W. Junker, and Brian Korty

Abstract

The National Meteorological Center (NMC) initiated Quantitative Precipitation Forecasts (QPF) and an intensive QPF verification program in 1960. These forecast products have evolved from a manual effort, relying on extensive forecast experience to one that placed much greater reliance on the interpretation and modification of numerical models.

Verification graphs show steady improvements in forecast accuracy, especially for the longer-range forecasts, which in this context am those in the 24–60-h range. During the 1960s the Threat Score (TS) for day-2 forecasts for 1 in or more of precipitation averaged approximately 0.07. During recent years, that score has nearly doubled, and the 36–60-h period forecast in 1993 had a TS comparable to that for the 12–36-h period during the 1960s. Improvement in accuracy is probably related to a number of diverse factors including improved numerical models, increased forecaster knowledge of the strengths and weaknesses of the operational models, and an increased understanding of precipitation processes. The verification results have been used to track individual and group progress.

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Roy W. Spencer, Barry B. Hinton, and William S. Olson

Abstract

In a comparison between 37 GHz brightness temperatures from the Nimbus 7 Scanning Multichannel Microwave Radiometer and rain rates derived from the WSR-57 radars at Galveston, Texas and Apalachicola, Florida, it was found that the brightness temperatures explained 72% of the variance of the rain rates. The functional form relating these two types of data was significantly different from that predicted by models of radiative transfer through plane-parallel clouds. Most of the difference can be explained in terms of the partial coverage of footprints by convective showers. Because residual polarization is always present, even for large obscuring storms over land and water, it is hypothesized that emission by nonspherical hydrometeors is at least partly responsible for the observed polarization.

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Dale E. Linvill, W. J. Hooker, and Brian Olson

Abstract

Atmospheric ozone was monitored in Michigan during the late 1880's using Schoenbein's test paper. A conversion chart was constructed to relate the Schoenbein ozone scale at various relative humidifies to ozone levels indicated by a Dasibi ozone monitor. Average monthly ozone values were highest during the spring months and lowest during the winter season.

Highest recorded ozone levels accompanied southwest wind flow into Michigan. Ozone level increased with time as an air mass passed over Michigan and dropped immediately after frontal passage. These patterns are similar to patterns observed in today's data. These results strongly suggest that anthropogenic nitrogen emitted by actively growing green plants and soils is the major source of photochemical ozone precursors in earth's boundary layer.

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S. Lang, W-K. Tao, J. Simpson, R. Cifelli, S. Rutledge, W. Olson, and J. Halverson

Abstract

The 3D Goddard Cumulus Ensemble model is used to simulate two convective events observed during the Tropical Rainfall Measuring Mission Large-Scale Biosphere–Atmosphere (TRMM LBA) experiment in Brazil. These two events epitomized the type of convective systems that formed in two distinctly different environments observed during TRMM LBA. The 26 January 1999 squall line formed within a sheared low-level easterly wind flow. On 23 February 1999, convection developed in weak low-level westerly flow, resulting in weakly organized, less intense convection. Initial simulations captured the basic organization and intensity of each event. However, improvements to the model resolution and microphysics produced better simulations as compared to observations. More realistic diurnal convective growth was achieved by lowering the horizontal grid spacing from 1000 to 250 m. This produced a gradual transition from shallow to deep convection that occurred over a span of hours as opposed to an abrupt appearance of deep convection. Eliminating the dry growth of graupel in the bulk microphysics scheme effectively removed the unrealistic presence of high-density ice in the simulated anvil. However, comparisons with radar reflectivity data using contoured-frequency-with-altitude diagrams (CFADs) revealed that the resulting snow contents were too large. The excessive snow was reduced primarily by lowering the collection efficiency of cloud water by snow and resulted in further agreement with the radar observations. The transfer of cloud-sized particles to precipitation-sized ice appears to be too efficient in the original scheme. Overall, these changes to the microphysics lead to more realistic precipitation ice contents in the model. However, artifacts due to the inability of the one-moment scheme to allow for size sorting, such as excessive low-level rain evaporation, were also found but could not be resolved without moving to a two-moment or bin scheme. As a result, model rainfall histograms underestimated the occurrence of high rain rates compared to radar-based histograms. Nevertheless, the improved precipitation-sized ice signature in the model simulations should lead to better latent heating retrievals as a result of both better convective–stratiform separation within the model as well as more physically realistic hydrometeor structures for radiance calculations.

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G. M. Heymsfield, J. M. Shepherd, S. W. Bidwell, W. C. Boncyk, I. J. Caylor, S. Ameen, and W. S. Olson

Abstract

This paper presents an analysis of a unique radar and radiometer dataset from the National Aeronautics and Space Administration (NASA) ER-2 high-altitude aircraft overlying Florida thunderstorms on 5 October 1993 during the Convection and Moisture Experiment (CAMEX). The observations represent the first ER-2 Doppler radar (EDOP) measurements and perhaps the most comprehensive multispectral precipitation measurements collected from a single aircraft. The objectives of this paper are to 1) examine the relation of the vertical radar reflectivity structure to the radiometric responses over a wide range of remote sensing frequencies, 2) examine the limitations of rain estimation schemes over land and ocean backgrounds based on the observed vertical reflectivity structures and brightness temperatures, and 3) assess the usefulness of scattering-based microwave frequencies (86 GHz and above) to provide information on vertical structure in the ice region. Analysis focused on two types of convection: a small group of thunderstorms over the Florida Straits and sea-breeze-initiated convection along the Florida Atlantic coast.

Various radiometric datasets are synthesized including visible, infrared (IR), and microwave (10–220 GHz). The rain cores observed over an ocean background by EDOP, compared quite well with elevated brightness temperatures from the Advanced Microwave Precipitation Radiometer (AMPR) 10.7-GHz channel. However, at higher microwave frequencies, which are ice-scattering based, storm evolution and vertical wind shear were found to be important in interpretation of the radiometric observations. As found in previous studies, the ice-scattering region was displaced significantly downshear of the convective and surface rainfall regions due to upper-level wind advection. The ice region above the rain layer was more opaque in the IR, although the 150- and 220-GHz brightness temperatures Tb approached the IR measurements and both corresponded well with the radar-detected ice regions. It was found that ice layer reflectivities and thicknesses were approximately 15 dBZ and a few kilometers, respectively, for detectable ice scattering to be present at these higher microwave frequencies.

The EDOP-derived rainfall rates and the simultaneous microwave Tb's were compared with single-frequency forward radiative transfer calculations using a family of vertical cloud and precipitation water profiles derived from a three-dimensional cloud model. Over water backgrounds, the lower-frequency emission-based theoretical curves agreed in a rough sense with the observed radar rainfall rate–Tb data points, in view of the uncertainties in the measurements and the scatter of the cloud model profiles.

The characteristics of the ice regions of the thunderstorms were examined using brightness temperature differences ΔTb such as Tb(37 GHz)–Tb(220 GHz). The Δ Tb's (150–220, 89–220, and 37–86 GHz) suggested a possible classification of the clouds and precipitation according to convective cores, elevated ice layers, and rain without significant ice above the melting layer. Although some qualitative classification of the ice is possible, the quantitative connection with ice path was difficult to obtain from the present observations.

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R. W. Spencer, W. S. Olson, Wu Rongzhang, D. W. Martin, J. A. Weinman, and D. A. Santek

Abstract

In an examination of microwave data from the Nimbus 7 satellite, brightness temperatures were found that were much lower than those expected for the radiation emanating from rain-producing clouds. Every case of very cold brightness temperature coincided with heavy thunderstorm rainfall. The cold temperatures can be attributed to scattering by a layer of ice hydrometeors in the upper parts of the storms. Thus it appears that brightness temperatures observed by satellite microwave radiometers can at times distinguish heavy rain over land.

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Barry B. Hinton, William S. Olson, David W. Martin, and Brian Auvine

Abstract

This study discusses a rainfall algorithm utilizing six channels of microwave radiance data from the Nimbus-7 Scanning Multifrequency Microwave Radiometer. The algorithm is intended for short-term climate studies over the ocean at low latitudes. To find a set of functional relationships, rain ratios are regressed on brightness temperatures for each channel. Next, these functions are integrated over a class of rain-rate distributions to find relations between mean brightness temperatures and mean rain rates. This step accounts for beam filling. Finally, weights are obtained for combining the rain rates from the individual channels. The weights vary with the rain rates, so that the optimum combination of channels is always used. Results are stored in a database grid 1° latitude × 1° longitude by one month. To test the algorithm, three years (1979–81) of data from the Indian Ocean are processed. These show spatial patterns very similar to previous climatological studies and to expected seasonal variations as determined at climatological observing stations. In addition, the results are compared with another microwave algorithm and with an infrared threshold method substantially the same as the GOES precipitation index.

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W-K. Tao, S. Lang, W. S. Olson, R. Meneghini, S. Yang, J. Simpson, C. Kummerow, E. Smith, and J. Halverson

Abstract

This paper represents the first attempt to use Tropical Rainfall Measuring Mission (TRMM) rainfall information to estimate the four-dimensional latent heating structure over the global Tropics for one month (February 1998). The mean latent heating profiles over six oceanic regions [Tropical Ocean and Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE) Intensive Flux Array (IFA), central Pacific, South Pacific Convergence Zone (SPCZ), east Pacific, Indian Ocean, and Atlantic Ocean] and three continental regions (South America, central Africa, and Australia) are estimated and studied. The heating profiles obtained from the results of diagnostic budget studies over a broad range of geographic locations are used to provide comparisons and indirect validation for the heating algorithm–estimated heating profiles. Three different latent heating algorithms, the Goddard Space Flight Center convective–stratiform heating (CSH), the Goddard profiling (GPROF) heating, and the hydrometeor heating (HH) algorithms are used and their results are intercompared. The horizontal distribution or patterns of latent heat release from the three different heating retrieval methods are very similar. They all can identify the areas of major convective activity [i.e., a well-defined Intertropical Convergence Zone (ITCZ) in the Pacific, a distinct SPCZ] in the global Tropics. The magnitudes of their estimated latent heating release are also in good agreement with each other and with those determined from diagnostic budget studies. However, the major difference among these three heating retrieval algorithms is the altitude of the maximum heating level. The CSH algorithm–estimated heating profiles only show one maximum heating level, and the level varies among convective activity from various geographic locations. These features are in good agreement with diagnostic budget studies. A broader maximum of heating, often with two embedded peaks, is generally derived from applications of the GPROF heating and HH algorithms, and the response of the heating profiles to convective activity is less pronounced. Also, GPROF and HH generally yield heating profiles with a maximum at somewhat lower altitudes than CSH. The impact of different TRMM Microwave Imager (TMI) and precipitation radar (PR) rainfall information on latent heating structures was also examined. The rainfall estimated from the PR is smaller than that estimated from the TMI in the Pacific (TOGA COARE IFA, central Pacific, SPCZ, and east Pacific) and Indian Oceans, causing weaker latent heat release in the CSH algorithm–estimated heating. In addition, the larger stratiform amounts derived from the PR over South America and Australia consequently lead to higher maximum heating levels. Sensitivity tests addressing the appropriate selection of latent heating profiles from the CSH lookup table were performed.

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