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Brant Liebmann
and
Arnold Gruber

Abstract

The annual variation of the diurnal cycle of outgoing longwave radiation (OLR) is examined. Our results are based on the climatological amplitude and phase of the first diurnal harmonic for each month. The diurnal harmonic was extracted from a composite daily cycle from several polar orbiting satellites that flew in different years with ten different equator crossing times. We compute a “diurnal vector standard deviation” which is the square root of the sum of the variances of both components of the 12 climatological monthly diurnal vectors. This allows contributions from both phase and amplitude changes of the diurnal vector.

A map of the diurnal vector standard deviation is presented. The values over land are an order of magnitude larger than over the ocean. The maxima are located over the seasonally migrating monsoons and over the midlatitude semi-arid zones. In midlatitudes the large standard deviation results from an increased daily cycle of insolation during summer and from clouds associated with midlatitude storms which reduce the diurnal cycle during winter. In the tropical monsoon regions a large variability of the diurnal cycle results from a larger daily cycle of cloudiness during the wet season than in the dry season. At some locations over the monsoons, however, the diurnal amplitude is actually a minimum during the wet summer season. We believe the minimum is caused by the pervasive cloudiness in the most convective regions. In the midlatitudes and during the dry season in the tropics, the maximum emission generally occurs between 1200 and 1400 local time. During the rainy season it occurs between 0600 and 0900.

We hypothesize that there should be a spatial relationship between the diurnal cycle variability and the standard deviation of the 12 climatological monthly means of OLR, and we compare maps of the two quantities The large-scale features are in broad agreement and the correlation between the two maps is marginally statistically significant. A detailed comparison, however, reveals that the diurnal vector standard deviation is of much smaller scale than the standard deviation of OLR. We attribute the regional structure of the diurnal cycle variability to varying geography, vegetation, and available moisture. Some of the small-scale structure, however, undoubtedly arises because the diurnal cycle involves day-night differences which are inherently more noisy than the OLR field itself.

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S. Bibiana Cerne
,
Carolina S. Vera
, and
Brant Liebmann

Abstract

This note describes the physical processes associated with the occurrence of a heat wave over central Argentina during the austral summer of 2002/03, during which the South American Low-Level Jet Experiment (SALLJEX) was carried out. The SALLJEX heat wave that lasted between 25 January and 2 February 2003 was punctuated by extreme conditions during its last 3 days, with the highest temperature recorded over the last 35 yr at several stations of the region. It was found that not only the activity of synoptic-scale waves, but also the intraseasonal oscillation variability, had a strong impact on the temperature evolution during this summer. During the weeks previous to the heat wave development, an intensified South Atlantic convergence zone (SACZ) dominated the atmospheric conditions over tropical South America. Temperatures started to increase in the subtropics due to the subsidence and diabatic warming associated with the SACZ, as depicted by SALLJEX upper-air observations. An extratropical anticyclone that evolved along southern South America further intensified subsidence conditions. By the end of January the warming processes associated with SACZ activity weakened, while horizontal temperature advection began to dominate over central Argentina due to the intensification of the South American low-level jet. This mechanism led to temperature extremes by 2 February with temperature anomalies at least two standard deviations larger than the climatological mean values. Intense solar heating favored by strong subsidence was responsible for the heat wave until 31 January, after which horizontal temperature advection was the primary process associated with the temperature peak.

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Brant Liebmann
,
M. Chelliah
, and
H. M. van den Dool

Abstract

We examine the persistence of outgoing longwave radiation (OLR) anomalies in the tropics on many different time scales during 1974–86. We calculate “one-lag autocorrelations” by constructing nonoverlapping 1-, 15-, and 60-day averages and calculating the correlation at every grid point between every time average and the following average for the entire dataset. One-day averages produce the 1argest local autocorrelations everywhere except over the equatorial Pacific. Large autocorrelations using 15-day averages are confined to the equatorial Pacific, but large autocorrelations based on 60-day averages extend eastward from the eastern Indian Ocean through South America We attribute the increase in autocorrelation in some areas as the averaging period increases to the presence of the 30–60 day oscillation in those areas The spatial match between the autocorrelation and the standard deviation of OLR is best for 60-day averages and worst for 15-day averages.

We then calculate pattern correlations over a domain that extends along the equator from the eastern Indian Ocean through the central Pacific. When plotted as a time series the one-lag pattern correlations for two-month means are seen to vary wildly, although they are generally positive. There are some extended periods, however, during which the pattern correlation remains 1arge, most notably during the 1982–83 ENSO event.

The average one-lag pattern correlation is plotted for many different time averages. They decrease until a minimum at 20-day averages, beyond which they slowly increase as the averaging 1ength is increased.

The average one-lag pattern correlations using one-day averages are smallest during the mid-year months, but using 60-day averages they are largest during these months. The seasonality, however, is not large.

Finally, we identify eastward propagation of OLR anomalies with at least two distinct phase-speeds in addition to a quasi-persistent signal. It is suggested that forecasts of OLR anomalies might be improved over simple, local persistence by a multiple regression technique.

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Sara A. Rauscher
,
Anji Seth
,
Brant Liebmann
,
Jian-Hua Qian
, and
Suzana J. Camargo

Abstract

The potential of an experimental nested prediction system to improve the simulation of subseasonal rainfall statistics including daily precipitation intensity, rainy season onset and withdrawal, and the frequency and duration of dry spells is evaluated by examining a four-member ensemble of regional climate model simulations performed for the period 1982–2002 over South America. The study employs the International Centre for Theoretical Physics (ICTP) regional climate model, version 3 (RegCM3), driven with the NCEP–NCAR reanalysis and the European Centre–Hamburg GCM, version 4.5. Statistics were examined for five regions: the northern Amazon, southern Amazon, the monsoon region, Northeast Brazil, and southeastern South America. RegCM3 and the GCM are able to replicate the distribution of daily rainfall intensity in most regions. The analysis of the rainy season timing shows the observed onset occurring first over the monsoon region and then spreading northward into the southern Amazon, in contrast to some previous studies. Correlations between the onset and withdrawal date and SSTs reveal a strong relationship between the withdrawal date in the monsoon region and SSTs in the equatorial Pacific, with above-average SSTs associated with late withdrawal. Over Northeast Brazil, the regional model errors are smaller than those shown by the GCM, and the strong interannual variability in the timing of the rainy season is better simulated by RegCM3. However, the regional model displays an early bias in onset and withdrawal over the southern Amazon and the monsoon regions. Both RegCM3 and the GCM tend to underestimate (overestimate) the frequency of shorter (longer) dry spells, although the differences in dry spell frequency during warm and cold ENSO events are well simulated. The results presented here show that there is potential for added value from the regional model in simulating subseasonal statistics; however, improvements in the physical parameterizations are needed for this tropical region.

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Harry H. Hendon
,
Brant Liebmann
,
Matthew Newman
,
John D. Glick
, and
J. E. Schemm

Abstract

Systematic forecast errors associated with active episodes of the tropical Madden–Julian oscillation (MJO) are examined using five winters of dynamical extended range forecasts from the National Centers for Environmental Prediction reanalysis model. Active episodes of the MJO are identified as those periods when the amplitude of either of the first two empirical orthogonal functions of intraseasonally filtered outgoing longwave radiation, which efficiently capture the MJO, is large. Forecasts initialized during active episodes of the MJO are found not to capture the eastward propagation of the tropical precipitation and circulation anomalies associated with the MJO. Rather, the MJO-induced anomalies of precipitation and winds are systematically forecast to weaken and even retrograde. By about day 7 of the forecast the convectively coupled, tropical circulation anomalies produced by the MJO are largely gone. Systematic errors in the extratropical 200-mb streamfunction also fully develop by day 10. The initial development of these errors is argued to result from the collapse of the tropical divergence forcing produced by the MJO and, thus, the lack of correct Rossby wave source. Forecast skill in the Tropics and Northern Hemisphere extratropics is found to be systematically reduced during active periods of the MJO as compared to quiescent times. This reduced skill is suggested to result because the MJO is the dominant mode of convective variability and not because the model is better able to forecast intraseasonal convection unrelated to the MJO.

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Jia-Lin Lin
,
Toshiaki Shinoda
,
Brant Liebmann
,
Taotao Qian
,
Weiqing Han
,
Paul Roundy
,
Jiayu Zhou
, and
Yangxing Zheng

Abstract

This study evaluates the intraseasonal variability associated with summer precipitation over South America in 14 coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Eight years of each model’s twentieth-century climate simulation are analyzed. Two dominant intraseasonal bands associated with summer precipitation over South America are focused on: the 40- and the 22-day band. The results show that in the southern summer (November–April), most of the models underestimate seasonal mean precipitation over central-east Brazil, northeast Brazil, and the South Atlantic convergence zone (SACZ), while the Atlantic intertropical convergence zone (ITCZ) is shifted southward of its observed position. Most of the models capture both the 40- and 22-day band around Uruguay, but with less frequent active episodes than observed. The models also tend to underestimate the total intraseasonal (10–90 day), the 40-, and the 22-day band variances. For the 40-day band, 10 of the 14 models simulate to some extent the 3-cell pattern around South America, and 6 models reproduce its teleconnection with precipitation in the south-central Pacific, but only 1 model simulates the teleconnection with the MJO in the equatorial Pacific, and only 3 models capture its northward propagation from 50° to 32°S. For the 7 models with three-dimensional data available, only 1 model reproduces well the deep baroclinic vertical structure of the 40-day band. For the 22-day band, only 6 of the 14 models capture its northward propagation from the SACZ to the Atlantic ITCZ. It is found that models with some form of moisture convective trigger tend to produce large variances for the intraseasonal bands.

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