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Richard D. Ray and Chester J. Koblinsky

Abstract

The sea-state bias in a satellite altimeter's range measurement is caused by the influence of ocean waves on the radar return pulse; it results in an estimate of sea level too low according to some function of the wave height. We have estimated this bias for Geosat by correlating collinear differences of altimetric sea-surface heights with collinear differences of significant wave heights (H 1/3). Corrections for satellite orbit error are estimated simultaneously with the sea-state bias. Based on twenty 17-day repeat cycles of the Geosat Exact Repeat Mission, the solution for the sea-state bias is 2.6±0.2% of H 1/3. The least-squares residuals, however, show a correlation with wind speed U, so we have supplemented the traditional model of the bias with a second term: α1 H 1/32 H 1/3 U. This second term produces a small, but statistically significant, reduction in variance of the residuals. Both systematic and random errors in H 1/3 and U tend to bias the estimates of α1 and α2, which complicates comparisons of our results with ground-based measurements of the sea-state bias.

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Chester Wisner, H. D. Orville, and Carol Myers

Abstract

A numerical model of a hail-bearing cumulus cloud is presented. The model is one-dimensional and time-dependent, and it employs extensive parameterization of the microphysical processes. The raindrop and hailstone size distributions are assumed to be exponential at all times. Cloud droplets are converted to raindrops according to Berry's parameterization of the autoconversion process and are accreted by the raindrops according to Kessler's formulation. Raindrops are frozen at a rate consistent with Bigg's freezing equation, and the hailstones so formed then accrete raindrops and cloud droplets. Ice crystals are not allowed by the model, and for consistency, then, it is assumed that the cloud droplets do not freeze except when accreted by hailstones at temperatures less than0C. The melting and evaporation processes are modeled, and their impact on the results is explored.

The results of five test cases are presented. The sequence of cases is designed to illustrate the effects of the presence of hail, the melting process, and the evaporation of rain on the model by eliminating them, one at a time, from the complete model. In addition, we examine briefly the effect of a lower limit on the cloud radius as it pertains to the entrainment process. The conclusions of this study are that 1) hail is a critical component of the precipitation process, 2) a steady-state assumption is appropriate until the formation of hail in the cloud, and 3) the downdraft begins at the melting level and propagates downward.

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D. LEE HARRIS and CHESTER P. JELESNIANSKI

Abstract

No Abstract Available.

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D. LEE HARRIS and CHESTER P. JELESNIANSKI

Abstract

The linearized two-dimensional hydrodynamic equations are presented in a manner which displays the principal assumptions involved. Several approximations are developed for the partial derivatives, and boundary conditions in finite difference form and the associated errors are discussed. The procedure for establishing a finite difference analog of the equations of motion and boundary conditions is illustrated, and computational stability for the solution of some simple problems is illustrated by means of examples.

The physical and computational problems associated with the introduction of friction in the computational model are discussed. It is concluded that friction should be neglected in many problems but that it must be considered in others.

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Robert W. Reeves, Chester F. Ropelewski, and Michael D. Hudlow

Abstract

Upper air and surface data from the GARP Atlantic Tropical Experiment (GATE) are used to examine the interrelationships between convective-scale precipitation and the larger scale wind field. The upper air winds from the inner (B) and outer (A/B) hexagonal observational arrays are fit with second-order polynomials to provide smooth estimates of the vorticity, divergence and vertical motion in the observational array. In these analyses we examined archived validated data from all three phases of the experiment and we formed averages based on the radar-estimated precipitation rates.

Mean profiles for 19-day periods during each of the three observational phases establish the basic similarity of the kinematics during each phase. Strong boundary-layer convergence balanced, for the most part, by upper tropospheric divergence, is common to all three phases.

Radar-estimated precipitation rates are used to define suppressed (precipitation rates <0.1 mm h−1) and highly disturbed (precipitation rates >0.5 mm h−1) states over the observational array. Mean profiles for the disturbed states in each phase show weaker easterly winds and much larger upward vertical velocities than do the mean profiles for the suppressed states. The mean vorticity profiles for each state do not show such clear-cut differences.

Time series of 12 h averages indicate that the precipitation events in Phase III corresponded very closely to the cyclonic maxima of the 700 mb relative vorticity, reflecting the influence of the easterly waves described by Reed et al. (1977). During Phases I and II, when easterly waves were poorly organized, the precipitation events did not correspond closely to the cyclonic vorticity maxima. On the other hand, precipitation events showed good correspondence with the large-scale (A/B) 700 mb upward vertical velocity maxima and surface meridional convergence ∂v/∂y during all three phases. This shows that the precipitation is clearly related to events on a larger scale.

The effects of convective activity on the large-scale flow are examined through the vorticity budget. The vorticity budget residual profiles were similar from phase to phase with cyclonic production maxima in the mid and upper troposphere. The upper tropospheric residual maximum is as strong during the suppressed state as it is during the highly disturbed side. At the surface, individual values of the residual are almost always opposite in sign to the vorticity. The mean vorticity budget for the A/B array shows the tipping term to have magnitudes comparable to other terms in the vorticity budget.

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Dennis Chesters, Wayne D. Robinson, and Louis W. Uccellini

Abstract

Precipitable water fields have been retrieved from the VISSR Atmospheric Sounder (VAS) using a radiation transfer model for the differential water vapor absorption between the 11 and 12 μm “split window” channels. Previous moisture retrievals using only the split window channels provided very good space-time continuity but poor absolute accuracy. This note describes how retrieval errors can be significantly reduced from ±0.9 to ±0.6 gm cm−2 by empirically optimizing the effective air temperature and absorption coefficients used in the two-channel model. The differential absorption between the VAS 11 and 12 μm channels, empirically estimated from 135 colocated VAS-RAOB observations, is found to be approximately 50% smaller than the theoretical estimates. Similar discrepancies have been noted previously between theoretical and empirical absorption coefficients applied to the retrieval of sea surface temperatures using radiances observed by VAS and polar-orbiting satellites. These discrepancies indicate that radiation transfer models for the 11 μm window appear to be less accurate than the satellite observations.

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Dennis Chesters, Louis W. Uccellini, and Wayne D. Robinson

Abstract

A simple physical algorithm is developed which calculates the water vapor content of the lower troposphere from the 11 and 12 μm (split window) channels on the VISSR Atmospheric Sounder (VAS) on the Geostationary Operational Environmental Satellites (GOES). The algorithm is applied to a time series of VAS split window radiances observed at 15 km horizontal resolution over eastern North America during a twelve hour period on 13 July 1981. Color coded images of the derived precipitable water (g cm−2) fields show vivid water vapor features whose broad structure and evolution are verified by the radiosonde and surface networks. The satellite moisture fields also reveal significant mesoscale features and rapid developments which are not resolved by the conventional networks. The VAS split window clearly differentiates those areas in which water vapor extends over a deep layer and is more able to support convective cells from those arms in which water vapor is confined to a shallow layer and is therefore less able to support convection. The spatial and temporal continuity of the water vapor features indicates very good relative accuracy, and point verification at radiosonde sites indicates fair absolute accuracy. Surface temperature variations are very effectively removed by the algorithm. Consequently, the VAS split window could be used operationally to monitor mesoscale developments in the low-level moisture fields over relatively cloud-free areas of the United States.

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Michael S. Halpert, Gerald D. Bell, Vernon E. Kousky, and Chester F. Ropelewski

The El Niño-Southern Oscillation (ENSO) phenomenon is a major contributor to the observed year-to-year variability in the Pacific Ocean and in the global atmospheric circulation. The short-term climate system witnessed the return to the mature phase of warm ENSO conditions (commonly referred to as the El Nino) during early 1995 for the third time in four years. This frequency of occurrence is unprecedented in the last 50 years and is comparable to that observed during the prolonged 1911–15 ENSO episode.

These warm ENSO conditions contributed to a large-scale disruption of the normal patterns of wind, rainfall, and temperature over much of the tropics and middle latitudes, particularly during the December 1994–February 1995 period. This period was followed by a dramatic decrease in sea surface temperatures in the tropical Pacific, resulting in a complete disappearance of all warm episode conditions during June–August and in the development of weak coldepisode conditions during September–November.

Changes in the tropical Pacific were accompanied by pronounced, large-scale changes in the atmospheric circulation patterns from those that had prevailed during much of the early 1990s. Particular examples of these changes include 1) a dramatic return to a very active hurricane season over the North Atlantic, following four consecutive years of significantly below-normal hurricane activity; 2) the return to above-normal rainfall throughout Indonesia, northern Australia, and southern Africa, following a prolonged period of below-normal rainfall and periodic drought; and 3) a northward shift of the jet stream and storm track position over the eastern half of the North Pacific during the latter part of the year, following several winter seasons (three in the last four) characterized by a significant strengthening, southward shift, and eastward extension of these features toward the southwestern United States.

Other regional climate anomalies during 1995 included extreme warmth throughout western and central Asia during January–May and colder than normal conditions in this region during November–December, severe flooding in the midwestern United States (April–May), abnormally wet conditions in California and the southwestern United States (December–February) combined with near-record warmth over eastern North America, deadly heat waves in the central United States (mid-July) and India (first three weeks of June), drought in the northeastern United States (August), a drier-than-normal rainy season in central Brazil (September–December), and an intensification of drier-than-normal conditions over southern Brazil, Uruguay, and northeastern Argentina at the end of the year.

The global annual mean surface temperature for land and marine areas during 1995 averaged 0.40°C above the 1961–90 mean. This value exceeds the previous warmest year in the record (1990) by 0.04°C. The Northern Hemisphere also recorded its warmest year on record during 1995, with a mean departure from normal of 0.55°C. The global annual mean surface temperature for land areas only during 1995 was the second warmest since 1951.

The year also witnessed near-record low ozone amounts in the Southern Hemisphere stratosphere, with minimum values only slightly higher than the record low values observed in 1993. The areal extent of very low ozone values during 1995 was as widespread over Antarctica as in the record low year of 1993.

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A. F. Hasler, K. Palaniappan, C. Kambhammetu, P. Black, E. Uhlhorn, and D. Chesters

Mesoscale wind fields have been determined for a mature hurricane with high spatial and temporal resolution, continuity, and coherency. These wind fields, near the tropopause in the inner core and at low levels inside the eye, allow the evolution of mesoscale storm features to be observed. Previously, satellite-derived winds near hurricanes have been determined only at some distance from the eye over a typical time period of 1–2 h. Hurricane reconnaissance aircraft take 30 min to 1 h to complete an inner-core pattern. With the long observation periods of these previous methods, steady-state conditions must be assumed to give a complete description of the observed region.

With the advent of 1-min interval imagery, and fourfold improvement of image dynamic range from NOAA's current generation of GOES satellites, there is a new capability to measure inner-core tropical cyclone wind fields near the tropopause and within the eye, enabling mesoscale dynamical processes to be inferred. These measurements give insights into the general magnitude and structure of the hurricane vortex, along with very detailed measurements of the cloud-top wind's variations in response to convective outbursts. This paper describes the new techniques used to take advantage of the GOES satellite improvements that, in turn, allowed the above innovations to occur.

The source of data for this study is a nearly continuous 12-h sequence of 1-min visible images from NOAA GOES-9 on 6 September 1995. These images are centered on Hurricane Luis with maximum winds of 120 kt (CAT4) when it was 250 km northeast of Puerto Rico. A uniform distribution of long-lived cirrus debris with detailed structure is observed in the central dense overcast (CDO), which has been tracked using the 1-min images. The derived wind field near the tropopause at approximately 15 km in the CDO region has a strong closed circulation with speeds up to 25 m s−1, which pulses in response to the convective outbursts in the eye wall. Cloud displacements are computed at every pixel in every image, resulting in a quarter-million uv winds in each of 488 hurricane images observed at 1- to 4-min intervals over 12 h. For analysis and presentation, these ultradense wind fields are reduced to 8- or 16-km grids using a 7-min time base by smoothing displacement vectors in space and time.

Cloud structures were tracked automatically on a massively parallel processing computer, but with manual spot-checking. Manual tracking has been used to follow CDO structure over long time periods, up to 90 min for a small test sample. Cloud tracking for the wind fields presented here is accomplished using a Massively Parallel Semi-Fluid Motion Analysis (MPSMA) automatic technique. This robust deformable surface-matching algorithm has been implemented on the massively parallel Maspar supercomputer. MPSMA automatic tracking typically follows a feature for 7 min. For this time base the error of these winds is estimated to be 1.5 m s−1. However, systematic navigation and height assignment errors in the moderately sheared hurricane environment must still be considered. Spatial and temporal smoothing of the wind field have been performed to reduce systematic navigation errors and small-scale turbulent noise. The synthesis used here to compute the wind fields gives an order of magnitude reduction in the amount of data presented compared to the amount of data processed. Longer tracking could give higher accuracy but would smooth out the smaller-scale spatial and temporal features that appear dynamically significant.

The authors believe that the techniques described in this paper have great potential for further research on tropical cyclones and severe weather as well as in operational use for nowcasting and forecasting. United States and foreign policymakers are urged to augment the GOES, GMS, FY2, and Meteosat geostationary satellite systems with dual imaging systems such that 1-min observations are routinely taken.

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Chester W. Newton, Roy Lee, Edwin B. Fawcett, William T. Chapman, Donald E. Martin, Frederick F. Sanders, Robert J. Renard, Robert D. Fletcher, and Maurice E. Pautz
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