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Jordan C. Alpert

Abstract

An analysis of the kinetic energy budget is made for two intensely developing cyclones over North America. The principal kinetic energy source for the first cyclone is the net horizontal transport of kinetic energy across the boundaries of the region enclosing the cyclone. For the second cyclone, it is the local kinetic energy generation. By investigating the kinetic energy budget of the vertically averaged flow (barotropic part) and the vertical shear flow (baroclinic part) it is found that the horizontal transport contribution to the kinetic energy budget of the first cyclone is evenly divided between the barotropic and baroclinic components. However, the kinetic energy generation is the dominant energy source of the second cyclone, and the horizontal transport is an energy sink. The vertical shear kinetic energy reservoir did not act as a “catalyst” as in hemispheric studies but varied during cyclone development.

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Tsing-Chang Chen and Jordan C. Alpert

Abstract

The systematic error in the National Meterological Center's (NMC) medium-range operational forecasts of the global divergence during 1987 and 1988 is examined in this study. As in other operational NWP models the NMC model has too weak an annual mean, annual cycle, and 30–60 day oscillation. This weakness shows up after only a few days, especially over the monsoon region and tropical areas. When the intraseasonal oscillation amplitude is large the model predictive skill is improved. The eastward propagation of the low-frequency mode is well predicted throughout the 10-day forecast period. The north-south migration of the tropical Hadley circulation, depicted primarily by the annual-cycle mode, is also weaker although the phase pattern predicted by the MRF is consistent with analyses.

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Marvin A. Geller and Jordan C. Alpert

Abstract

The possibility of planetary wave coupling between the troposphere and solar-induced alterations in the upper atmosphere providing a viable mechanism for giving rise to sun-weather relationships is investigated. Some of the observational evidence for solar-activity-induced effects on levels of the upper atmosphere ranging from the thermosphere down to the lower stratosphere are reviewed. It is concluded that there is evidence for such effects extending down to the middle stratosphere and below. Evidence is also reviewed that these effects are due to changes in solar ultraviolet emission during disturbed solar conditions. A theoretical planetary wave model is then used to see at what levels in the upper atmosphere moderate changes in the mean zonal wind state would result in tropospheric changes. It is concluded that changes in the mean zonal flow of ∼20% at levels in the vicinity of 35 km or below would give rise to changes in the tropospheric planetary wave pattern that are less than but on the same order as the observed interannual variability in the tropospheric wave pattern at middle and high latitudes. Thus, planetary wave coupling between the troposphere and the upper atmosphere appears to be a plausible mechanism to give a tropospheric response to solar activity. This mechanism is not viable, however, to provide for short-period changes such as the suggested solar sector boundary vorticity index relation, but rather is applicable to changes of longer period such as the 11- or 22-year solar cycles.

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Jordan C. Alpert and V. Krishna Kumar

Abstract

The spatial and temporal densities of Weather Surveillance Radar-1988 Doppler (WSR-88D) raw radar radial wind represent a rich source of high-resolution observations for initializing numerical weather prediction models. A characteristic of these observations is the presence of a significant degree of redundant information imposing a burden on an operational assimilation system. Potential improvement in data assimilation efficiency can be achieved by constructing averages, called super-obs. In the past, transmission of the radar radial wind from each radar site to a central site was confined to data feeds that filter the resolution and degrade the precision. At the central site, super-obs were constructed from this data feed and called level-3 super-obs. However, the precision and information content of the radial wind can be improved if data at each radar site are directly utilized at the highest resolution and precision found at the WSR-88D radar and then transmitted to a central site for processing in assimilation systems. In addition, with data compression from using super-obs, the volume of data is reduced, allowing quality control information to be included in the data transmission. The super-ob product from each WSR-88D radar site is called level-2.5 super-obs. Parallel, operational runs and case studies of the impact of the level-2.5 radar radial wind super-ob on the NCEP operational 12-km Eta Data Assimilation System (EDAS) and forecast system are compared with Next-Generation Weather Radar level-3 radial wind super-obs, which are spatially filtered and delivered at reduced precision. From the cases studied, it is shown that the level-3 super-obs make little or no impact on the Eta data analysis and subsequent forecasts. The assimilation of the level-2.5 super-ob product in the EDAS and forecast system shows improved precipitation threat scores as well as reduction in RMS and bias height errors, particularly in the upper troposphere. In the few cases studied, the predicted mesoscale precipitation patterns benefit from the level-2.5 super-obs, and more so when greater weight is given to these high-resolution/precision observations. Direct transmission of raw (designated as level 2) radar data to a central site and its use are now imminent, but this study shows that the level-2.5 super-ob product can be used as an operational benchmark to compare with new quality control and assimilation schemes.

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Glenn K. Rutledge, Jordan Alpert, and Wesley Ebisuzaki

An online archive of real-time and historical weather and climate model output and observational data is now available from the National Oceanic and Atmospheric Administration (NOAA). This archive, known as the NOAA National Operational Model Archive and Distribution System (NOMADS), was jointly initiated in 2001 by the National Climatic Data Center (NCDC), the National Centers for Environmental Prediction (NCEP), and the Geophysical Fluid Dynamics Laboratory (GFDL). At present, NOMADS provides access to real-time and historical 1) numerical weather prediction (NWP) model input and output, 2) GFDL's Coupled Global Climate Models (CGCM) output, 3) global and regional reanalysis from NCEP and the National Center for Atmospheric Research (NCAR), and 4) limited surface, upper-air, and satellite observational datasets from NCDC and NOAA's National Ocean Data Center (NODC) and Earth System Research Laboratory [formerly the Forecast System Laboratory (FSL)]. NOMADS is but one of many similar data services across the United States and abroad that are embracing and leveraging various pilot efforts toward distributed data access using agreed-to data transport and data format conventions. This allows for the interoperable access of various subsets of data across the Internet. These underlying community-driven agreements and the Open Source Project for a Network Data Access Protocol (OPeNDAP) data transport protocol form the core of NOMADS services as well as have the capability for “format neutral” access to data in many different formats and locations.

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Shyh-Chin Chen, John O. Roads, and Jordan C. Alpert

Abstract

Climatological and predictability features of a simplified moist general circulation model are described herein. The simplified model is driven by empirical forcings designed to eliminate systematic errors and maintain computational efficiency. Resulting perpetual January climatological features of forced variables such as the tropospheric heights and rotational winds, as well as unforced variables such as the velocity potential, compare well with the observations. Unforced temporal variations in the midlatitude 500-mb geopotential, as well as the tropical circulation's intraseasonal oscillation, are also simulated reasonably well.

Short-range persistence and predictability features of this model replicate geographical persistence and predictability features from simpler models and from numerical weather prediction. The streamfunction is highly persistent in the extratropics, less so in the tropical regions; similarly, the streamfunction is predicted better in midlatitude regions than in the tropics. By contrast, the velocity potential is more persistent in tropical regions but, like the streamfunction, is still predicted best in extratropical regions for short-range forecasts. At longer forecast ranges, the velocity potential is better predicted in the tropics than in midlatitudes. Interestingly, during prominent tropical intraseasonal oscillations, the model consistently demonstrates lower tropical forecasting skill. Predictions are more skillful during stagnant tropical periods.

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Tsing-Chang Chen, Jenq-Dar Tsay, Jun Matsumoto, and Jordan Alpert

Abstract

Examination of the development of cold season heavy rainfall/flood (HRF) events around the South China Sea (SCS) from their parent cold surge vortices (CSVs) shows three new development processes. First, the formation mechanism of the parent CSV of an HRF event [CSV(HRF)] has a preference as to geographic location, flow type of the cold surge inside the SCS, and time of day. The surface trough east of the Philippines, Taiwan, and southern Japan island chain in late fall and the near-equator trough across Borneo in winter facilitate the CSV(HRF) formation in two regions—the vicinity of the Philippines and Borneo. The formation of the Philippine (Borneo) CSV(HRF) occurs at 0600 UTC (0000 UTC) with involvement from the Philippine Sea (PHS)-type (SCS type) of cold surge flow. Second, the flow type of the cold surge determines the CSV(HRF) propagation across the South China Sea. The PHS-type (SCS type) facilitates (hinders) the CSV(HRF) westward propagation. This occurs because the easterly (northerly) flow is greater than (less than) the northerly (easterly) flow at the maximum isotach location of the cold surge flow associated with CSV(HRF) and is centered east of the demarcation line for propagation. This flow-type contrast is substantiated by the vorticity budget analysis for CSV(HRF). The positive 925-hPa vorticity tendency is located west of (coincident with) the 925-hPa vorticity center for the PHS-type (SCS type) of cold surge. Third, the CSV(HRF) development into a HRF event is achieved through multiple interactions of former vortices with sequential cold surges across the South China Sea. The first two CSV(HRF) development processes are reported herein; the last process is presented in Part II.

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John O. Roads, T. Norman Maisal, and Jordan Alpert

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Precipitation forecasts made by the National Meteorological Center's medium-range forecast (MRF) model are evaluated for the period, 1 March 1987 to 31 March 1989. As shown by Roads and Maisel, the MRF model wet bias was substantially alleviated during this period. As is shown here, the MRF model forecast skill in predicting individual wet and dry events has also increased. We show that there is substantial skill in the model forecasts of precipitation occurrences beyond 2.5 days. These MRF model forecasts have not yet been fully exploited by the forecasting community, in part, because they have not been readily available.

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Tsing-Chang Chen, Jenq-Dar Tsay, Jun Matsumoto, and Jordan Alpert

Abstract

The peak intensity occurrence frequency over the life cycles of parent cold-surge vortices (CSVs) for heavy rainfall/flood (HRF) events is classified into two types depending on their life cycles having two or three peak intensities, denoted as HRF2 or HRF3, respectively. The formation of an HRF2 event from its parent CSV(HRF2) formation is ≤5 days, while the formation of an HRF3 event is ≥6 days. The latter group contributes ~57% of the total number of HRF events. As a result of some model constraints, the formation and development of HRF3 events are not well forecasted by the Global Forecast System (GFS) and regional forecast models. The life cycle and second peak intensity for CSV(HRF3) allow for the introduction of a forecast advisory for HRF3 events. Identification of CSVs and two sufficient requirements for the formation and occurrence of HRF events were developed by previous studies. Nevertheless, two new necessary steps are now included in the proposed forecast advisory. The population ratio for CSV(HRF3) and the regular CSV is only about 15%. The occurrence optimum time t o for the CSV(HRF3) second peak intensity from this vortex formation is about 3 days 6 h. The GFS forecast over t o is utilized to identify CSV(HRF3). Then, the relay of the GFS forecast from the occurrence time of the CSV(HRF3) second peak is used to predict the formation/occurrence of HRF3 events. Six HRF3 events during cold seasons for 2013–16 are used to test the feasibility of this forecast advisory. Results clearly demonstrate this advisory is a success for the forecast of HRF3 events over the entire life cycles of their parent CSV(HRF3)s.

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Tsing-Chang Chen, Jenq-Dar Tsay, Jun Matsumoto, and Jordan Alpert

Abstract

After the onset of the Southeast Asian summer monsoon in mid-May, the South China Sea (SCS) trough is deepened by the intensified monsoon westerlies to facilitate the development of a synoptic cyclonic shear flow. This shear flow forms an environment favorable for the SCS tropical storm (TS)/typhoon (TY) genesis triggered by the surge of this monsoon circulation. This genesis mechanism has not been well documented. Seventeen named SCS TS/TY geneses in May over 1979–2016 occurred under the following environmental conditions/processes: 1) with its maximum located south of 15°N, the intensified monsoon westerlies are extended eastward beyond 120°E, 2) the synoptic SCS cyclonic shear flow is developed by the tropical easterlies fed by a northeast Asian cold surge (or a North Pacific cold-air outbreak) and the intensified monsoon westerlies, and 3) SCS TS/TY genesis is triggered by the surge of monsoon flow. The accuracy of the monthly mean forecasts is limited. However, it is found that SCS TS/TY genesis only occurs after the existence of persistent, strong, monsoon westerlies lasting for at least 5 days. Forecasts from the National Centers for Environmental Prediction Global Forecast System (2004–16) and the Global Ensemble Forecast System (1985–2003) cover these 15 SCS TS/TY geneses. The requirements for SCS TS/TY genesis in May described above are met by the 5-day-mean Southeast Asian summer monsoon circulation. Based on a statistical analysis of 5-day forecasts for these TS/TY geneses, a four-step forecast advisory is introduced. The forecasts for SCS TS/TY genesis can be made 3 days prior to occurrence.

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