Daytime Precipitation Research Articles

Precipitation Forecast of MM5 in the Taiwan Area during the 1998 Mei-yu Season

This study presents precipitation verification, in the Taiwan area, for a real-time Pennsylvania State University‐ National Center for Atmospheric Research Fifth-Generation Mesoscale Model (MM5) system during the 1998 Mei-yu season. The highest equitable threat score (ETS) of precipitation forecasts, verified against observed precipitation, was about 0.2 at the 2.5-mm threshold for this nearly 2-month period. The complex and steep terrain in this region presented great challenges to the 15-km model in predicting realistic rainfall because the precipitation was driven by local forcings such as thermal effects and orographic lifting. In addition, the lack of observational data over the surrounding ocean greatly limited the quality of the model’s initial data. It was found that the model system more accurately simulated nighttime rainfall than daytime precipitation. This was caused by the model underforecasting the rainfall events that resulted from solar heating and orographic lifting over the mountain slopes during the daytime hours. Precipitation, however, was overforecast over the high mountain regions (.1200 m). Further, the analysis of ETS with regard to the terrain height indicated that the model performed better over the lowlands than over the mountainous areas (slopes and highlands). It was discovered that the ETSs were much higher for precipitation forecasts after the onset of the east Asia summer monsoon than prior to the onset. Overall, the model more accurately predicted precipitation for the rainfall events associated with the Mei-yu front and the accompanying mesoscale convective systems than it predicted precipitation associated with the local forcings.

An attempt to derive electron energy spectra for auroral daytime precipitation events

A set of quiet daytime electron density profiles is established on the basis of EISCAT measurements. This set is used as a background correction in the CARD program [ Brekke A., Hall C. and Hansen T.L. (1989) Ann. Geophysicae 7, 269] in order to derive the energy spectra of precipitating electrons at daytime. For disturbed daytime events on 25–26 June 1985, we find that the particle precipitation typically consists of small fluxes (10 5 el/cm 2 sster keV) of high energetic particles (20–30 keV). The Hall: Pedersen conductance ratios for such events are found to be meaningless as an indicator of the energy of the particles, in contrast to nighttime precipitation.

Analysis of Recording Raingage Data for the Israeli II Experiment. Part II: Differential Day and Night Effects of Cloud Seeding

This paper reports separate analyses of daytime and nighttime precipitation based on data from recording raingages of the second Israeli randomized experiment. These analyses seemed important because there are a number of hypotheses on the differential effects of AgI seeding during the day and at night, and because about half of the seeding of this experiment was done at night. Our findings are unfortunately equivocal because of the large variability of the date which had to be broken down by categories of modal cloud-top temperatures and 12-b (day/night) periods. The increase of precipitation efficiency that had been noticed (on 24-h data) to occur in the −12° to −21°C window appears to have been larger at night, but the difference is not significant. The increase in the number of rain events for warmer clouds may have been only a daytime effect, but again, the present data are not conclusive.

The L shell region of importance for waves emitted at ground level as a loss mechanism for trapped electrons > 68 keV

The objective of this paper is to identify the L shell region(s) where VLF waves emitted at ground level may play a significant role as a loss mechanism for radiation belt electrons with energies above 68 keV. This assessment is made possible by studying the day‐night differences in inner belt electron precipitation and comparing with the much stronger transmission of waves at frequencies of 15–25 kHz through the ionosphere at night and the known higher daytime intensities for waves of natural origin other than lightning. Intensities of electrons trapped on drift shells which dip below sea level in the anomaly are used to study the rates of loss from the radiation belts. In 1979 the inner radiation belt was shown to have had the following unique features for electrons >68 keV: (1) the rates of precipitation from the radiation belts at longitudes west of 140°E were significantly greater near midnight than near noontime for L ≈ (1.6–2.2); (2) at L ≈ 1.75 the nighttime rates of injection showed little correlation with the precipitation on other L shells, in contrast with the daytime precipitation in which a significant correlation existed with the precipitation rates throughout the range L = 1.2–5.0. From these empirical findings one concludes that in the heart of the inner radiation belt at longitudes west of 140°E, lightning and/or transmitter‐generated VLF waves emitted from ground level probably played a significant role in precipitating electrons >68 keV at nighttime. Below the region L = 1.6 and above L = 2.2 the observed diurnal variations were much less pronounced, indicating that VLF waves emitted at ground level in this frequency range were not the major source of precipitation.

High-resolution auroral zoneEregion neutral wind and current measurements by incoherent scatter radar

In this paper we present the first high-resolution E region neutral wind measurements made at the Chatanika, Alaska, incoherent scatter radar facility. The high-resolution (∼10 km) measurements are made possible by a new digital correlator system that is now fully operational. Daytime measurements show that the neutral transport in the low E region follows the general pattern of the global pressure-driven winds. However, above 110 km there is evidence that the neutral winds are being influenced by electric fields associated with daytime precipitation events. Data taken during a moderately disturbed nighttime period show an equatorward neutral wind component above ∼115 km during the evening eastward electrojet and a poleward component through the morning westward electrojet. In our analysis we have emphasized the critical dependence of the neutral wind estimates on the collision rate parameter, particularly above ∼120 km, where ion-neutral collisions rapidly diminish.

Climate of Hidden Valley, Mukut Himal during the Monsoon in 1974

For the purpose of understanding climate of the glacier area of the northern side of the Himalayan range, a temporary station was set up at a height of 5055 m in Hidden Valley, Mukut Himal during the monsoon season of 1974. The temperature pattern, especially the trend of minimum temperature and the diurnal temperature range, seems to reflect the monsoonal character and to represent its effect better than the precipitation pattern. At the beginning and the end of the observation period, a rapid change of minimum temperature and large temperature range were recorded, as opposed to the stationary trend of mean temperature from the middle of July to the middle of August, which seems to be the monsoon season in Hidden Valley. At the station, most of the precipitation was in the form of rain and rain-snow mixed. Nocturnal precipitation was more than daytime precipitation, which was less than 5 mm. The daily mean of evaporation slightly exceeded the daily mean of precipitation even in the monsoon period. The diurnal variation of atmospheric pressure shows two maxima and two minima.

On the morphology of auroral absorption during substorms

Multistation riometer measurements of ionospheric radio-wave absorption have been used to infer the characteristics of the higher-energy component of electron precipitation during substorms. Two analyses are presented which deal separately with longitudinal and latitudinal variations. The first is based on data from four riometer stations located at L ≈ 5.2 and at about 90° longitude intervals, and the second on a chain of five riometer stations spaced at about 3° intervals across the auroral zone. Previous work has shown that substorms detected at night are accompanied by daytime precipitation. In the present study, the average substorm absorption is found to have two broad maxima, one at night within 15 min of the absorption onset, and another during the morning about an hour after this onset. There is very little effect in the evening.From the latitude study, a transition is observed during the nighttime phase of substorms between the abrupt, irregular, intense absorption at higher latitudes and the smoother, less intense absorption at lower latitudes. This transition, designated as Λa, is observed at a median latitude of Λa = 65 ± 1.5°. For the majority of a sample of 181 events selected from 1964 and 1967 records, the absorption was observed to start in the vicinity of Λa. With increased magnetic disturbance (measured by Kp), Λa is observed to shift to lower latitudes. The Kp variation is compared with that of energetic-particle boundaries.

Auroras and polar substorms: Observations and theory

The results of observations of auroras and polar magnetic substorms are compared with magnetospheric models. In general, present qualitative or semi‐quantitative magnetospheric models describe fairly well the basic features of auroras and magnetic substorms, or they can probably be adapted to the observations by minor modifications. However, some observations cannot currently be easily interpreted in terms of the models. Of these, the existence of two daytime precipitation zones and some detailed features of the observed particle energy spectrum are among the most important. The stationary models cannot account for the high‐energy part of the auroral particle spectrum seen also in quiescent auroras. Some peaks in the observed spectrum also seem to pose problems. On the whole, the quantitative problems of entry and energization of the auroral particles are still unsolved.

Some quantitative aspects of electron precipitation in and near the auroral zone

Observations made with the International Quiet Sun Years (IQSY) forward‐scatter paths, supplemented with a special program of ground‐based observations at the midpoint of one of the two forward‐scatter paths in Alaska, have been used as a basic body of experience against which to discuss some of the quantitative aspects of electron precipitation into the ionosphere and mesosphere. Because most of the electron precipitation events from which we can extract quantitative spectral information occur in or near the auroral zone, the results obtained in Alaska are the main subject of our study and review.The limitations of the forward‐scatter technique as now in use are discussed in some detail. They stem from uncertainties about the actual height of scattering at any moment and from uncertainties in estimating the radio‐wave absorption during an electron precipitation event, that result from the competing effects of simultaneous enhancement of the intensity of the scattered signals. In spite of these inherent handicaps progress can be made. Unmistakable daytime decreases in the intensity of the scattered signals can be identified. They result from intense and abnormal ionization below the scattering stratum. We can identify and eliminate from consideration SID and PCA effects, which were rare during the observing program. The remaining effects have been shown to be due to the precipitation of electrons and to result from electrons in the high‐energy, tail of the spectrum describing the precipitation. We also show that the spectra describing daytime precipitation are usually considerably harder than the spectra describing the nighttime precipitation associated with the aurora and with auroral absorption observed by riometers, a result confirmed by a number of other investigations. The Es propagation that occurs from time to time, mostly at night, masks the scatter signals but provides information about the comparatively soft nighttime electron precipitation.We conclude that exponential energy spectra for electron energies greater than about 10 kev will always describe adequately the electron precipitation responsible for the riometer absorption, the behavior of the scattered signal, and the r‐type Es (or night E) that is found to be responsible for the Es propagation observed with the scatter‐path. The results from the forward‐scatter program are related quantitatively to other observations concerned with electron precipitation, such as riometer absorption, auroral luminosity, fluxes of precipitating electrons observed by satellite and rocket, and auroral bremsstrahlung X rays observed by balloon; and in a more limited way the results are related to magnetometer behavior. The electron precipitation results are also related, but only qualitatively, to such recently developed concepts as the auroral oval, the auroral substorm, and the magnetosphere.Suggestions are made for improving the ground‐based observing techniques, and for investigations that are required to allow a more exact interpretation of the ground‐based observations.