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Bette L. Otto-Bliesner and Donald R. Johnson

Abstract

A diagnostic approach to infer three-dimensional distribution of the thermally-forced, time-averaged horizontal mass and energy transport (Johnson and Townsend, 1981), which was previously applied in the Southern Hemisphere (Zillman, 1972), is used to determine the corresponding Northern Hemisphere circulations. The method is based on the steady form of the time-averaged isentropic continuity equation and allows calculation of the irrotational components of the mass circulations which are consistent with modeled diabatic heating fields determined from climatic information. The three-dimensional distributions of atmospheric heating in the Northern Hemisphere are estimated from summer and winter climatic data of precipitation, turbulent exchange of sensible heat and radiative fluxes.

The results highlight the large-scale coupling of the Northern Hemisphere heat source and heat sink regions by Hadley-type and Walker-type circulations. The zonally-averaged mass circulation exhibits a thermally direct, meridional cell spanning the entire hemisphere in winter. This circulation shifts northward and weakens in summer with the corresponding Southern Hemisphere winter Hadley-type circulation now extending to northern latitudes. The composite three-dimensional mass circulations also reveal prominent mass transports associated with the Asian monsoon in summer and winter. Zonal asymmetries in the heating lead to longitudinal variations of the meridional circulation and pronounced east-west overturnings. A complementary study (Johnson and Townsend, 1981) in which similar mass circulations were derived from the FGGE observational global data set validates the large-scale patterns established in this study.

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B. L. Davis, L. R. Johnson, and F. John Moeng

Abstract

The exceptional ice nucleating ability of aersols obtained from combustion of AgI-NH4I-acetone solution is now well known. The high nucleating ability has been determined to come from the existence of a complex compound having a better epitaxial fit with respect to ice than has silver iodide. The compound has a stability region which includes the temperature interval of −20 to +9°C at water saturation and has been observed to be present on AgI aerosol particles produced from standard aircraft seeding generators in a wind tunnel dilution system. Its presence is presumed to occur as a result of incomplete destruction of the NH4 + of the original solution. Although the unit cell of the phase is monoclinic, c-centered, it has a close packed structure nearly identical to silver iodide in the a0-b0 crystallographic plane. In this plane the phase has a misfit with respect to the basal plane of ice of 1.3% at −7°C as compared to 1.5% for silver iodide. The composition of the phase is 3AgI·NH4I·6H2O but with 25% of the silver positions being vacant on the average. Compositions of 2:1 mole ratio AgI:NH4I can also exist in an apparent metastable state. The threshold of ice nucleation for the pure Bn phase was found to be −1°C as contrasted to the recognized −4°C threshold for silver iodide.

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R. L. Johnson, D. E. Janota, and J. E. Hay

Abstract

During the spring-summer of 1979, six lightning warning devices were evaluated in a side-by-side comparison study at three test sites. Stock commercial devices were selected based upon distinct concepts of operation. The devices tested included a sferics counter, a corona point, a radioactive probe, a field mill, an azimuth/range locator and a triangulation locator. The test sites were chosen to provide varied thunderstorm conditions: 1) San Antonio, Texas (cold air advection), 2) Kennedy Space Center, Florida (localized surface heating) and 3) Langmuir Laboratory, New Mexico (orographic effects). The evaluation parameters were advance warning time, time to clear after hazard, alarm reliability, and false alarm and failure to alarm probabilities. The triangulation locator provided the best overall performance; however, all systems indicated a need for improvement in the failure to alarm rate.

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William L. Smith, Lyle H. Horn, and Donald R. Johnson

Abstract

TIROS II channel 2 and channel 4 measurements of infrared radiation are statistically correlated with corresponding radiometersonde measurements of radiation divergence within various layers of the atmosphere. Estimates of the correlation coefficient greater than 0.60 are obtained for all layers extending from the surface of the earth to levels beyond 600 mb.

The results of this study indicate that the satellite measured flux is most significantly related to the infrared cooling profile within the surface to 600-mb layer. It is shown that good estimates of the infrared cooling profile might be obtained from satellite measurements if the general sky condition is also known.

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L. A. Mysak, E. R. Johnson, and W. W. Hsiem

Abstract

The two-layer baroclinic instability model of the California Undercurrent from Mysak (1977) is modified to investigate the effects of the lateral boundary conditions on the stability properties of the system. As is common in baroclinic instability calculations, Mysak (1977) assumes the mean flow along the continental rise to be bounded laterally by vertical rigid walls, thus allowing the cross-stream structure of the perturbation flow to be decomposed into simple normal modes. Instability then occurs when waves of the same cross-stream structure interact. The dominant instability is that associated with the gravest mode.

In the first model presented here we consider the effect of replacing the rigid outer boundary with a quiescent, constant-depth ocean. Waves of short longshore wavelength are not greatly affected by the open seaward boundary. However, as consideration is turned to waves of longer longshore wavelength, the cross-stream wavenumber departs further from the integral values of the channel-flow problem and another class of baroclinic instabilities occurs due to interaction between waves of differing cross-stream structure. Nevertheless, the dominant baroclinic instability remains that associated with the gravest mode. A new barotropic instability is also present, drawing energy from the horizontal shear between the coastal current and the quiescent ocean.

In the second model the rigid outer boundary is retained but the inner boundary is replaced by a shallow sloping region, modeling the effects of a sloping shelf adjoining the coastal current which flows along the continental rise. Topographic waves are present above the sloping inshore region. These waves are coupled with the channel waves. Once again the cross-stream wavenumber departs from the integral values of the channel problem and instabilities are present due to interaction between waves of differing cross-stream structure. As in the previous model the dominant baroclinic instability is that of the gravest mode and a new barotropic instability is present due to the lateral shear in the mean flow at the shelf break.

For both models, a parameter study is presented in which we determine the effects of varying the shear, stratification and bottom slope.

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David R. Munday, Helen L. Johnson, and David P. Marshall

Abstract

This study uses a sector configuration of an ocean general circulation model to examine the sensitivity of circumpolar transport and meridional overturning to changes in Southern Ocean wind stress and global diapycnal mixing. At eddy-permitting, and finer, resolution, the sensitivity of circumpolar transport to forcing magnitude is drastically reduced. At sufficiently high resolution, there is little or no sensitivity of circumpolar transport to wind stress, even in the limit of no wind. In contrast, the meridional overturning circulation continues to vary with Southern Ocean wind stress, but with reduced sensitivity in the limit of high wind stress. Both the circumpolar transport and meridional overturning continue to vary with diapycnal diffusivity at all model resolutions. The circumpolar transport becomes less sensitive to changes in diapycnal diffusivity at higher resolution, although sensitivity always remains. In contrast, the overturning circulation is more sensitive to change in diapycnal diffusivity when the resolution is high enough to permit mesoscale eddies.

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H. L. Johnson Jr., R. D. Hart, M. A. Lind, R. E. Powell, and J. L. Stanford

Abstract

Thunderstorm radio noise measurements at several frequencies in the range 0.01–74 MHz have been made with specially designed remote recording stations in Iowa. The data were recorded during the spring and summer of 1974 when a series of severe storm systems produced a great number of large hail and tornado reports in Iowa. Computer analyses were made of nearly a billion bits of data, corresponding to 170 h of real-time recordings. Careful compilations of surface severe weather reports, hail damage information from insurance companies, and studies on the Des Moines WSR-57 radar echoes were compared with the analyzed radio noise data. The results include the following:

1) In agreement with earlier work, large‐amplitude radio noise impulse rates were found to he generally good indicators of thunderstorm severity. Although the majority of the radio energy radiated from major lightning strokes occurs in the 0.01 MHz range, this frequency was found to be a poor indicator of storm severity; the higher frequencies (megahertz range) were considerably better. The character of the noise appears similar at 2.5 and 74 MHz.

2) In at least five cases, tornadic events correlated in time with radio noise count rate peaks. One funnel cloud was reported equidistant at 60 km from two recording stations and coincident with count rate peaks at both stations, lending credence to the idea that the peak was associated with the storm occurrence, rather than with corona or other local effects.

3) No unusual radio noise was recorded during the lifetime of a small, verified tornado at 19 km range. In addition, the count rates for its parent thunderstorm would not have indicated severity.

In spite of inherent atmospheric variableness, the radio noise technique is a useful complementary indicator of storm severity.

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R. D. Harmel, C. W. Richardson, C. L. Hanson, and G. L. Johnson

Abstract

Weather simulation models are commonly used to generate synthetic daily weather for use in studies of crop growth, water quality, water availability, soil erosion, climate change, and so on. Synthetic weather sequences are needed if long-term measured data are not available, measured data contain missing records, collection of actual data is cost or time prohibitive, or when necessary to simulate impacts of future climate scenarios. Most weather generators are capable of producing one or more components of weather such as precipitation, temperature, solar radiation, humidity, and wind speed. This study focused on one generation component, the procedure commonly used by weather simulation models to generate daily maximum and minimum temperature. The normal distribution is used by most weather generators (including USCLIMATE, WXGEN, LARS-WG, CLIMGEN, and CLIGEN) to generate daily maximum and minimum temperature values. The objective of this study was to analyze the adequacy of generating temperature data from the normal distribution. To accomplish this objective, the assumption of normality in measured daily temperatures was evaluated by testing the hypothesis that daily minimum and maximum temperature are normally distributed for each month. In addition, synthetic temperature records generated with the normal distribution were compared with measured temperature records. Based on these analyses, it was determined that measured daily maximum and minimum temperature are generally not normally distributed in each month but often are slightly skewed, which contradicts the assumption of normality used by most weather generators. In addition, generating temperature from the normal distribution resulted in several physically improbable values.

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Helen R. Pillar, Helen L. Johnson, David P. Marshall, Patrick Heimbach, and So Takao

Abstract

Atmospheric reanalyses are commonly used to force numerical ocean models, but despite large discrepancies reported between different products, the impact of reanalysis uncertainty on the simulated ocean state is rarely assessed. In this study, the impact of uncertainty in surface fluxes of buoyancy and momentum on the modeled Atlantic meridional overturning at 25°N is quantified for the period January 1994–December 2011. By using an ocean-only climate model and its adjoint, the space and time origins of overturning uncertainty resulting from air–sea flux uncertainty are fully explored. Uncertainty in overturning induced by prior air–sea flux uncertainty can exceed 4 Sv (where 1 Sv ≡ 106 m3 s−1) within 15 yr, at times exceeding the amplitude of the ensemble-mean overturning anomaly. A key result is that, on average, uncertainty in the overturning at 25°N is dominated by uncertainty in the zonal wind at lags of up to 6.5 yr and by uncertainty in surface heat fluxes thereafter, with winter heat flux uncertainty over the Labrador Sea appearing to play a critically important role.

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AndréA. Doneaud, James R. Miller Jr., L. Ronald Johnson, Thomas H. Vonder Haar, and Patrick Laybe

Abstract

Early work attempting to apply GOES rapid scan satellite data to a recently developed simple radar technique used to estimate convective rain volumes over areas in a semiarid environment (the northern Great Plains) is described.

Called the Area-Time-Integral (ATI) technique, it provides a means of estimating total rain volumes over fixed and floating target areas. The basis of the method is the existence of a strong correlation between the radar echo area coverage integrated over the lifetime of the storm and the radar estimated rain volume. The technique does not require the consideration of the structure of the radar intensities to generate rain volumes. but only the area covered by radar echoes. This fact might reduce the source of errors generated by the structure differences between the radar and the satellite signatures above given thresholds.

Satellite and radar data from the 1981 Cooperative Convective Precipitation Experiment (CCOPE) and the North Dakota Cloud Modification Project (NDCMP) are used. Consecutive time steps with both radar reflectivities and satellite (VIS and IR) rapid wan data were considered during the evolution of six convective clusters: three on 12 June, and three on 2 July 1981. Radar echoes with reflectivity values ≥ 25 dBZ were used to define the area of rainfall and the respective digital unit thresholds within the satellite data delineating the rainy part of the cloud area. Correlation of the ATI versus IR digital count values was obtained for every time step and for the storm lifetime, respectively.

A comparison of the stepwise evolution of radar parameters such as echo areas maximum echo heights, maximum reflectivities and satellite parameters such as threshold count values and coldest cloud top temperature is presented graphically and reflects the multicell characteristics of the convective clusters. Also, a comparison of radar and satellite parameters for the cluster lifetime is made. Satellite parameters pertaining to the cluster lifetime were derived both dependently and independently of radar data.

The main purpose of this investigation is to compute convective rain volume of a convective cluster by application of the ATI technique based only on satellite data. As such, the key element is to determine the ATI from satellite data without consideration of radar data. This is possible if trends of satellite products generated independently are similar to those of satellite products based upon radar observations as done here.

A parallel with the two-step techniques generally used to estimate rain volume from satellite data is made. To delineate the rainy part of a cloud area, a regression analysis is used. The regression relate a satellite-independent product to a satellite-dependent product. For a given storm. the satellite-independent product is first computed; then the regression equation gives the ATI, Finally, the rain volume is obtained by using the ATI versus rain volume relationship.

By applying the ATI/rain volume relationship to satellite data, the errors generated by the complicated multiple area-volume transform relations am reduced, as similar errors were reduced when the technique was applied to radar data. In addition, a regression analysis gives more accurate estimates than a single threshold when used to delineate an area covered by rain events from an area covered by clouds. The advantages of the ATI technique are based on the fact that the technique operates on a storm lifetime integrated basis, while the previous techniques operate on a time-step basis. The new technique generates only total rain volume estimates (not rain rates). This indeed is a limitation.

The analyses of six convective clusters suggest that the extension of the ATI technique using satellite data holds promise.

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