No abstract

This study investigates the structural and evolutionary characteristics of the eastward-and northward-propagating intraseasonal oscillation (ISO) in the Indian Ocean and western Pacific during the boreal summer. Along the equator, the near-surface moisture convergence located to the east of the deep convection region appears to result in the eastward propagation of the ISO, consistent with the frictional wave-CISK (conditional instability of the second kind) mechanism proposed in previous studies. The eastward propagation is characterized by sequentially downstream development of deep convection occuring mainly in certain regions such as 60Њ, 95Њ, 120Њ, and 145ЊE, and the date line. The northward propagation of deep convection can be attributed to the low-level moisture convergence located to the north. This convergence is a deep structure extending from the surface to the middle troposphere. Nearsurface convergence appears only after the systems approach the landmass in the north. It is suggested that both the deep convergence in the lower free atmosphere and in the boundary layer contribute to the northward propagation. The lifting effect of the sloping terrain and the stronger surface frictional effect over the land in South Asia contribute to the near-surface convergence north of the deep convection. The northward propagation occurs sequentially from west to east in the following order: the Arabian Sea, the Bay of Bengal, and the South China Sea. A mechanism is proposed to explain this downstream occurrence of northward propagation. It was also found that surface sensible heating might contribute to the northward propagation, especially in the Arabian Sea, by making the lower troposphere less stable. Latent heat flux is released to the atmosphere in the region located to the southwest of the deep convection and does not directly contribute to the destablization in the lower troposphere ahead of the deep convection. In contrast, during the eastward propagation the surface heating does not seem to precondition the lower troposphere to the east of the deep convection. Frictional convergence is seemingly the dominant factor in the eastward propagation.

[1] The specific humidity retrieved from the FORMOSAT-3/Constellation Observation System for the Meteorology, Ionosphere, and Climate (COSMIC) (FC) occultation observations of the GPS radiowaves were analyzed and compared with radiosonde observations, aircraft dropsonde observations, Atmospheric Infrared Sounder (AIRS) retrievals, and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalyses. For the limited number of radio soundings at the sites of small islands near Taiwan coincident in space and time with FC observations, the difference of humidity in individual cases is <40% below $7 km without obvious bias. Above this level, the humidity of radiosonde observations is systematically lower than that of FC observations, which is consistent with previous studies showing the dry bias of certain radiosonde observations. The humidity of FC observations differs significantly throughout the troposphere with dropsonde observations surrounding a strong typhoon. The difference reaches 100% in the middle and upper troposphere. The cause of the large discrepancy is likely to be the large horizontal variation of humidity in regions of an active weather system as shown in closely spaced dropsonde observations, where FC observations represent humidity averaged over humid and dry regions in the long path of GPS radiowaves, whereas dropsonde observations are a point measurement. Global distributions of specific humidity were compared between FC retrievals and the AIRS retrievals and NCEP/NCAR reanalyses. There is a good agreement between AIRS and FC humidity retrievals. The discrepancy is generally within 15%. The discrepancy between the NCEP/NCAR and FC humidity is significant, especially in the upper troposphere. For the layer 300-400 hPa, the NCEP/ NCAR humidity is higher than the FC humidity by >30% over most of oceanic regions.

During the THORPEX (The Observing System Research and Predictability Experiment) Pacific Asian Regional Campaign (T‐PARC), from 1 August to 30 September 2008, ∼1900 high‐quality, high vertical resolution soundings were collected over the Pacific Ocean. These include dropsondes deployed from four aircrafts and zero‐pressure balloons in the stratosphere (NCAR's Driftsonde system). The water vapor probability distribution and spatial variability in the northern subtropical Pacific (14°–20°N, 140°E–155°W) are studied using Driftsonde and COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) data and four global reanalysis products. Driftsonde data analysis shows distinct differences of relative humidity (RH) distributions in the free troposphere between the Eastern and Western Pacific (EP and WP, defined as east and west of 180°, respectively), very dry with a single peak of ∼1% RH in the EP and bi‐modal distributions in the WP with one peak near ice saturation and one varying with altitude. The frequent occurrences of extreme dry air are found in the driftsonde data with 59% and 19% of RHs less than or equal to 5% and at 1% at 500 hPa in the EP, respectively. RH with respect to ice in the free troposphere exhibits considerable longitudinal variations, very low (<20%) in the EP, but varying from 20% to 100% in the WP. Inter‐comparisons of Driftsonde, COSMIC and reanalysis data show generally good agreement among the Driftsonde, COSMIC, ECMWF Reanalysis‐Interim (ERA‐Interim) and Japanese Reanalysis (JRA) below 200 hPa. The ERA‐Interim and JRA are approved to be successful on describing RH frequency distributions and spatial variations in the region. The comparisons also reveal problems in Driftsonde, two National Center for Environmental Prediction (NCEP) reanalyses and COSMIC data. The moist layer at 200–100 hPa in the WP shown in the ERA‐Interim, JRA and COSMIC is missing in Driftsonde data. Major problems are found in the RH means and variability over the study region for both NCEP reanalyses. Although the higher‐moisture layer at 200–100 hPa in the WP in the COSMIC data agrees well with the ERA‐Interim and JRA, it is primarily attributed to the first guess of the 1‐Dimensional (1D) variational analysis used in the COSMIC retrieval rather than the refractivity measurements. The limited soundings (total 268) of Driftsonde data are capable of portraying RH probability distributions and longitudinal variability. This implies that Driftsonde system has the potential to become a valuable operational system for upper air observations over the ocean.

This study reveals that the atmosphere‐ocean coupled intraseasonal oscillation (ISO) enhances the occurrence frequency of Category 5 (C5) typhoons in the western North Pacific (WNP). Climatologically, the major region of C5 typhoon occurrence in the WNP is collocated with the intraseasonal variance of outgoing longwave radiation and tropical cyclone heat potential. The active convection and large ocean heat content associated with ISO create an environment conducive to the occurrence of C5 typhoons. Between 1980 and 2009, approximately 82% of C5 typhoons occurred when one or both of the two conditions were fulfilled. Our results suggest that compared with the thermodynamic factor of ocean heat content, dynamic factors (i.e., convection and near‐surface moisture convergence) within the favorable intraseasonal background state likely play a more influential role in inducing C5 typhoons.