Summertime Precipitation Research Articles

中国とモンゴルの乾燥・半乾燥地域における夏季降水量の経年変動 : その地域性とアジアの夏季モンスーンとの関係

In this study, interannual variations of summertime precipitation over arid and semi-arid regions in China and Mongolia are investigated. To clarify the regionality of the interannual variability in summer precipitation, an analysis technique of rotated empirical orthogonal functions is applied for a recent 40-year (1951-1990) period of summer precipitation. As a result of the REOF, five regions have been determined : I) Taklimakan Desert, II) Loess Plateau, III) North China to central and the southeastern part of Mongolia, IV) the north of Tianshan Mountains, and V) the northern part of Mongolia. Summertime precipitation over Region III) shows a significant decreasing trend after 1955. Next, to examine how the variations in precipitation in these regions are influenced by the Asian (Indian)summer monsoon activity in the mid-latitudes, correlations with all-India monthly and seasonal rainfall (IMR) are investigated. Further, the change of atmospheric circulation patterns with the interannual variation of summer precipitation of Regions I and II are also examined. The results are summarized as follows : 1) The interannual variation of summer precipitation of Region I (Taklimakan Desert) is mainly related to the windward mid-latitude circulation and eastward (westward) shift of the Tibetan High in a wet (dry)year. This region shows a clear negative correlation with IMR in June and July, and the relationships are caused by a rather local circulation change with IMR variation over Central Asia. 2) In Region II (Loess Plateau, the middle reaches of Yellow River), interannual variation of summer precipitation shows a positive correlation with IMR through the summer monsoon season. It shows a clear 2-3 year periodic oscillation, and seems to be closely related to the atmosphere/ocean interaction in the equatorial Pacific.

The Response to Orbital Perturbations in an Atmospheric Model Coupled to a Slab Ocean

The sensitivity of an atmospheric GCM coupled to a mixed-layer ocean to changes in orbital parameters is investigated. Three experiments are compared. One has perihelion at summer solstice and a large obliquity; another has perihelion at winter solstice and a low obliquity. The first of these is favorable for warm summers; the second for cool summers. A third experiment, with perihelion at summer solstice and the lower value of obliquity, is used to examine the relative importance of the changes in perihelion and obliquity. The eccentricity is set at 0.04 in all cases. Surface temperature responses are as large as 15°C, with the largest response over North America in summer. Changes in monsoons and Arctic sea ice are consistent with previous GCM studies. A perpetual summer version of the atmospheric model is used to investigate the positive feedback due to soil moisture. Drying of the soil over North America is found to increase the temperature response by approximately 50% and is also essential to the decrease in summertime precipitation in that region. Soil moisture changes also enhance the precipitation response over central Africa, but have little effect on the model's Asian monsoon. The orbital parameters most favorable for expansion of the Northern Hemisphere glaciers, that is, minimal seasonality, do not produce permanent snow cover. Several model deficiencies that act to accelerate the melting of snow in spring may be responsible.

Validation of a regional atmospheric model over Europe: Sensitivity of wintertime and summertime simulations to selected physics parametrizations and lower boundary conditions

AbstractThis paper analyses wintertime and summertime simulations over Europe using the National Center for Atmospheric Research (NCAR) regional atmospheric model (MM4). The sensitivity of the model to selected physics parametrizations (explicit moisture scheme; slower release of condensation heat; horizontal diffusion on s̀‐surfaces), and lower boundary conditions (sea surface temperature; initial soil moisture; snow cover) is examined. The simulation periods are January 1979 and June 1979; initial and lateral meteorological boundary conditions are provided by analyses of observations. The main focus of the analysis is on prediction by the model of surface air temperature and precipitation, but also sea‐level pressure and upper‐air variables are considered. In general, the model reproduces the main features of the synoptic events which prevailed during the two simulation periods. Mid‐tropospheric and upper‐tropospheric biases in the model are generally small. Surface maximum and minimum air temperature biases do not exceed a few degrees K over various regions of Europe. The largest model bias (2‐3 K) is found for summertime minimum temperature. Wintertime surface temperatures are sensitive to the precipitation parametrizations tested, as these affect cloud formation and, in turn, the surface radiative fluxes. Also, temperature biases in the driving large‐scale fields are partially transmitted to the surface air temperature calculations. Summertime temperatures are sensitive to the soil moisture content. Sea surface temperature variations influence land temperatures more strongly for wintertime than for summertime. Precipitation is sensitive to the parametrizations used. The explicit moisture scheme generally induces underprediction of precipitation. When this is not used, the biases are of the order of −10 to −20% of the observations. Summertime precipitation over the highest Alpine regions is overpredicted, except when diffusion of moisture and temperature along terrain‐following s̀‐surfaces is strongly reduced. Sea surface temperature variations substantially affect precipitation, especially over coastal areas, both in January and June. Summertime precipitation is significantly modified by the initialization of soil water content.

Sensitivity of Simulated Summertime Precipitation over the Western United States to Different Physics Parameterizations

Abstract This paper investigates the effect of different physics parameterizations on summertime precipitation as simulated by the Pennsylvania State University-National Center for Atmospheric Research (NCAR) Mesoscale Model (MM4). The period of simulation is July 1979. The control simulation is carried out with a standard version of the MM4 including a simplified Kuo scheme, a bulk boundary-layer representation, and a force-restore scheme for ground temperature calculation. The parameterizations tested are a modification of the standard MM4 Kuo scheme, which imposes storage and slow release of condensation heat, an explicit moisture scheme, a version of the Arakawa-Schubert scheme, and a relatively sophisticated hydrology package. The standard MM4 strongly overestimates precipitation over mountainous terrain and, in particular, it produces an excessively large number of gridpoint precipitation events in excess of several centimeters (“numerical point storms” or NPS's). These are due to intense vertical m...

A comparison of a GCM simulation of the seasonal cycle of the atmosphere with observations part II: Stationary waves and transient fluctuations

Abstract This paper presents the seasonal dependence of the stationary and transient eddies of the GLAS/UMD GCM from a two‐year annual cycle integration. The simulated Northern Hemisphere stationary waves are realistic in winter (below 250 mb) and in spring and fall; in winter a large anomalous ridge over the date‐line is noted above 250 mb. The model does not simulate the winter barotropic trough over eastern Canada. In summer the mid‐latitude stationary waves are poorly simulated (possibly owing to anomalous summer rainfall), but the monsoonal structure in the tropics is captured. The stationary wave field at 500 mb in the Southern Hemisphere is not well simulated, with the range of season‐to‐season variability being much larger than observed. The zonally averaged stationary wave rms is realistic below 200 mb in winter and spring, but is less so in summer and autumn, possibly due to erroneous summertime precipitation. The geographical distributions of 500‐mb transient and band‐pass height rms, of transient 850‐mb heat flux and of 200‐mb momentum flux in the Northern Hemisphere are well simulated except for summer. The latitude‐height dependence of height rms and low‐level transient heat flux is realistic in both summer and winter, but the transient momentum flux is not well simulated in summer. The mid‐level transient heat flux is too strong. The overall pattern of transient activity at 500 mb in the Southern Hemisphere is reasonable in the GCM, although there is too much variability in the eastern Pacific, while the observed peak in rms in the New Zealand sector is displaced eastwards in the GCM. The latitude‐height dependence of transient height rms and transient fluxes of heat and momentum looks quite realistic, and is similar in accuracy to the Northern Hemispheric results.

Forcing Mechanisms and Other Characteristics of Significant Summertime Precipitation

Satellite, radar, surface, and upper-air data from the June–August periods of 1982–83 are examined to determine the mechanisms, and their relative contributions, for producing warm-season precipitation in the United States. Only areas where rainfall equaled or exceeded 12.7 mm during the 24-h period ending at 1200 UTC were considered. Rainfall associated with extratropical cyclones accounted for about half of the warm-season precipitation. Most of the remaining precipitation was produced by mesoscale forcing mechanisms acting independently from the traveling extratropical cyclones. Nearly all the warm-season precipitation was convective; over 80% of the total was directly or indirectly associated with thunderstorms. Nearly three-fourths of the precipitation that occurred between the Rockies and the Mississippi Valley was nocturnal. Conversely, about three-fourths of the precipitation east of the Mississippi Valley fell during the daylight hours. During the 1983 July-August drought, the area of precipitation from extratropical cyclones was significantly smaller than the precipitation area during the corresponding period in 1982. The decrease was the result of a sharp drop in the frequency of cyclone events. Conversely, the frequency of mesoscale events and associated precipitation increased during the drought year. This difference in precipitation mode from one year to the next may help explain the large differences in numerical model forecasting skill from summer to summer. The large proportion of warm-season precipitation produced by mesescale forcing mechanisms suggests that further improvement in forecasting cyclonic storms and fronts will likely result in only limited increases in summertime quantitative precipitation forecast (QPF) skill. Moreover, the historically smaller values of warm-season QPF skill, relative to the cool-season levels, are likely to continue until operational numerical models can predict mesoscale convective systems as well as they predict extratropical cyclones and attendant fronts.

ANALYSIS OF SUMMERTIME DAILY PRECIPITATION FOR THE OKEFENOKEE SWAMP REGION

An assessment of the summertime precipitation climate for the Okefenokee Swamp region was undertaken utilizing daily precipitation data from meteorological sites near the swamp for the period 1955-1980. “Runs of dry days” were investigated in relation to total seasonal precipitation and with regard to the distribution of runs by length. Utilizing Markov chain analysis and regression techniques, relationships between the length of runs of dry days and total seasonal precipitation as well as cumulative number of runs of dry days and total seasonal precipitation were established. However, further analysis of the intensty distribution of daily precipitation suggested that the summertime precipitation climate could best be described utilizing an assessment of days with a significant amount of precipitation rather than by studying runs of dry days. An analytical definition of a “significant rain day” was attempted utilizing the correlation of the number of rain days exceeding a given daily precipitation thresho...