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Kamal Puri and M. J. Miller

Abstract

The sensitivity of the ECMWF analysis-forecast system for four tropical cyclones during the period spanned by the Australian Monsoon Experiment to cumulus parameterization and model resolution is presented. Two parameterization schemes were compared: namely, the Kuo cumulus parameterization and the Betts-Miller adjustment scheme. Both analyses and forecasts show considerable sensitivity with the Betts-Miller scheme generating more intense cyclonic systems as indicated by maps of sea-level pressure, low-level winds, vorticities, and cross sections in the neighborhood of the cyclones. The Betts-Miller scheme also results in better consistency of the fields in the vertical between the divergent circulation and vertical velocity. Of the two cases considered, one (for tropical cyclone Jason) showed a marked sensitivity to increased model resolution with the higher resolution forecast showing considerable improvement.

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Kamal Puri and M. J. Miller

Abstract

Although diabatic processes play an important role in the tropical circulation, current analysis schemes and numerical weather prediction models are unable to adequately include diabatic heating information. In this paper, procedures for using outgoing longwave radiation data as proxy data for diabatic heating rates and to initialize the moisture field are proposed. The feasibility of using this data in the ECMWF data assimilation system is demonstrated, and it is further shown that using appropriate diabatic heating together with moisture initialization has the potential to substantially reduce the spinup time in numerical weather prediction models.

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David Miller and J. M. Fritsch

Abstract

A climatological study of mesoscale convective complexes (MCCs) during 1983–1985 over the western Pacific region (WPR), using full-disc, enhanced infrared satellite imagery from the Japanese Geostationary Meteorological Satellite is presented.

The results indicate that MCCs are common in the WPR and display many of the same characteristics as those found in the Americas. The systems are nocturnal and tend to form over or in the immediate vicinity of land. Cold-cloud shields in the Americas last for about 10 h while WPR shields last about 11 h. The cold-cloud-shield size distribution is similar to that of the Americas, with most systems exhibiting areas between 2 × 105 and 3 × 105 km2. Seasonal distributions of WPR systems are also similar to that in the Americas. Specifically, the frequency of midlatitude systems peaks in late spring and early summer while low-latitude MCCs are distributed uniformly throughout the warm season.

As with western systems, WPR MCCs occur in preferred zones. Climatologically, low-level jets of high-θe, air and upper-level diffluence are present in these zones. Tracks of WPR MCCs show that, like American systems, they typically move to the right (left in the Southern Hemisphere) of the climatological mean 700–500-mb flow. The deviation from the mean flow is in the direction of the source region of higher-θe air. A few MCCs that moved over water formed tropical storms. Likewise, a few tropical systems moved over land and formed MCCs.

It is concluded that the strong similarity of the properties and environment of WPR MCCs to that in the Americas indicates that they are essentially the same phenomenon. Their high frequency in the Americas and the WPR makes them potentially important contributors to the global hydrologic cycle.

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Alvin J. Miller and Christopher M. Hayden

Abstract

Standard atmospheric energy budget computations are made for three distinct sets of Northern Hemisphere synoptic analyses prepared from data gathered during the August 1975 Data Systems Test. The first analysis set (System 1) included all data, the second (System 2) all but the satellite temperature retrievals (excepting some retained in the Southern Hemisphere for analysis model stability), and the third (System 3) all but the rawinsondes. Our results indicate that significant differences occur in the energetics of the analyses. In particular, there is a significant loss of longitudinal variance in an analysis based mainly on satellite retrievals as compared to that based mainly on rawinsonde data. In addition, forecasts by the NMC 6-layer numerical model initiated from System 1 and 2 analyses were evaluated for forecast periods from 00 to 72 h. It appears that this forecast model is sensitive to variations supplied by the initial data sets, but only to 12 h. Thereafter the forecast energetics are controlled by the model physics, and energy differences evolving from the different data sets remain constant in time.

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M. J. Miller and A. K. Betts

Abstract

The low-level atmospheric transformation associated with a class of traveling convective storms observed. over Venezuela is described. A strong low-level cooling is observed, confined mostly to the subcloud layer, and associated with a deeper layer of drying and acceleration of the easterly flow. A density current model is used to stratify the storm travel speeds, peak surface gusts and the accelerated flow at low levels behind the storm, and to relate these to the low-level flow ahead of the storm. There is reasonable agreement between these atmospheric data and laboratory observations of density currents. The updraft and down-draft structure is discussed using an interesting sounding cross section and trajectories from a three-dimensional numerical simulation. It appears that two distinct downdrafts exist: one driven by precipitation within the cumulonimbus cell, and a second mesoscale feature which is dynamically driven, and associated with descent over the spreading cold outflow.

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Andrew J. Heymsfield and Karen M. Miller

Abstract

The transport of water substance (ice and vapor) into the anvils of midlatitude continental thunderstorms is examined. Doppler radar reflectivity fields and horizontal and vertical windfields, in situ anvil measurements, and environmental soundings were used to estimate the anvil water mass flux for approximately five-minute intervals over one hour periods in six moderate to severe storms.

Vapor and ice mass fluxes into the anvil time-averaged for the study periods are about equal. Ratios of the time-averaged sums of these fluxes (A¯) to aircraft-derived cloud base influx (from Fankhauser) range from 18% to greater than 100%. Estimated accuracies are ±30 to 40%. Anvil fluxes exceed rainout at cloud base level as derived from radar reflectivity data by Fankhauser for half the storms.

It is shown that influx values alone are not reliable predictors of total storm condensation rates. The water mass storage term is evaluated and is found to be unimportant in relation to influx for all but one storm studied. Both A¯/influx and A¯/ rainout are highly correlated with the vertical shear of the horizontal winds.

Changes in the ice mass flux in the anvil with respect to altitude and distance from the updraft imply the absence of mesoscale ascent in the anvil.

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RAYMOND M. McINTURFF and ALVIN J. MILLER

Abstract

Significant temporal variations in the “quasi-biennial” oscillation (QBO) of the equatorial stratosphere raise questions concerning relationships between the various characteristics of the oscillation. A comparison of observations made before 1962 with those made after 1962 suggests the following relationships: β ≈ PU/4 in the 10- to 30-mb layer; PU/8≤β≤PU/4 in the 30- to 50-mb layer; and cUPU ≈ constant from 10 to 50 mb (where β is the phase difference between the zonal wind-QBO and temperature-QBO, PU is the period of the zonal wind-QBO, and cU is the speed of vertical propagation of the zonal wind-QBO).

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A. J. Miller and L. M. Leslie

Abstract

Forecast probabilities of rain were calculated up to 12 hours in advance using a Markov chain model applied to three-hourly observations from five major Australian cities. The four weather states chosen in this first study were three cloudiness states (0–2 oktas, 3–5 oktas and 6–8 oktas) and a rain state. Second-order Markov models with time-of-day dependent transition probabilities were fitted after appropriate statistical testing.

Forecasts were made using transition probabilities for summer and winter seasons. The skill of the Markov chain forecast probabilities of rain was evaluated in terms of Brier scores using to years of independent data, and compared with forecasts based upon persistence and climatology. The skill of the Markov model forecasts appreciably exceeded that of persistence and climatology and a real time trial of the procedure is being planned.

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A. J. Thorpe, M. J. Miller, and M. W. Moncriefe

Abstract

Abstract not available.

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C. M. Naud, I. Rangwala, M. Xu, and J. R. Miller

Abstract

Using 13 yr of satellite observations for the Tibetan Plateau, the sensitivities (or partial derivatives) of daytime surface downward shortwave and longwave fluxes with respect to changes in cloud cover and cloud optical thickness are investigated and quantified. Coincident cloud and surface flux retrievals from the NASA Moderate Resolution Imaging Spectroradiometer and the Clouds and the Earth’s Radiant Energy System, respectively, as well as ground-based observations at 11 stations across the plateau are used to examine the spatial and seasonal variability of this sensitivity over the entire plateau. The downward shortwave flux is found to be modulated primarily by changes in cloud cover, but changes in optical thickness also have an impact, as revealed by a multiple regression fit. The coefficient of determination of the regression increases by more than 15% when optical thickness is added. There is significant seasonal and regional variability in the cloud radiative impact. On average, at all stations, the sensitivity of surface shortwave flux to changes in cloud cover is about −0.5 ± 0.1 W m−2 %−1 in winter according to both ground-based and satellite observations but in summer reaches −1.5 ± 0.3 and −1.8 ± 0.2 W m−2 %−1 according to ground-based and satellite observations, respectively. Cloud cover itself has little impact on the sensitivity when clouds are optically thin, but above an optical thickness of 12, sensitivities increase with both cloud cover and cloud optical thickness. The daytime longwave flux response to changes in cloud properties is also examined. The radiative impact of a decrease in cloud cover on the surface net flux can be offset or even canceled if cloud opacity increases by 5%–10%.

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