[1] Using hourly station rain-gauge data during 1991–2004, we studied the relation between rainfall duration and diurnal variation during the warm season (May to September) over the central eastern China (105–120°E, 26–36°N). Our result shows that more than 60% of the total rainfall in the central eastern China comes from long-duration rainfall events (an event that lasts longer than 6 hours). The long-duration rainfall events tend to have their maximum hourly rainfall around early morning, while short-duration rainfall events (an event of 1–3 hours in duration) have their peak rainfall around late afternoon. This contrast explains the two peaks in the diurnal variation of averaged precipitation over the central eastern China shown in previous studies.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007GL030315

Relation between rainfall duration and diurnal variation in the warm season precipitation over central eastern China

The space-time evolution of the ocean and atmosphere associated with 1998-2000 monsoon intraseasonal oscillations (ISO) in the Indian Ocean and west Pacific is studied using validated sea surface temperature (SST) and surface wind speed from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager, and satellite outgoing longwave radiation. Monsoon ISO consist of alternating episodes of active and suppressed atmospheric convection moving northward in the eastern Indian Ocean and the South China Sea. Negative/positive SST anomalies generated by fluctuations of net heat flux at the ocean surface move northward following regions of active/suppressed convection. Such coherent evolution of SST, surface heat flux and convection suggests that air-sea interaction might be important in monsoon ISO.

Coherent Intraseasonal Oscillations of Ocean and Atmosphere During the Asian Summer Monsoon

In this paper, we present TropFlux wind stres- ses and evaluate them against observations along with other widely used daily air-sea momentum flux products (NCEP, NCEP2, ERA-I and QuikSCAT). TropFlux wind stresses are computed from the COARE v3.0 algorithm, using bias and amplitude corrected ERA-I input data and an additional climatological gustiness correction. The wind stress products are evaluated against dependent data from the TAO/TRITON, PIRATA and RAMA arrays and inde- pendent data from the OceanSITES mooring networks. Wind stress products are more consistent amongst each other than surface heat fluxes, suggesting that 10 m-winds are better constrained than near-surface thermodynamical parameters (2 m-humidity and temperature) and surface downward radiative fluxes. QuikSCAT overestimates wind stresses away from the equator, while NCEP and NCEP2 underestimate wind stresses, especially in the equatorial Pacific. QuikSCAT wind stress quality is strongly affected by rain under the Inter Tropical Convergence Zones. ERA- I and TropFlux display the best agreement with in situ data, with correlations(0.93 and rms-differences\0.012 Nm -2 . TropFlux wind stresses exhibit a small, but consistent improvement (at all timescales and most locations) over ERA-I, with an overall 17 % reduction in root mean square error. ERA-I and TropFlux agree best with long-term mean zonal wind stress observations at equatorial latitudes. All products tend to underestimate the zonal wind stress sea- sonal cycle by *20 % in the western and central equatorial Pacific. TropFlux and ERA-I equatorial zonal wind stresses have clearly the best phase agreement with mooring data at intraseasonal and interannual timescales (correlation of *0.9 versus *0.8 at best for any other product), with TropFlux correcting the *13 % underestimation of ERA-I variance at both timescales. For example, TropFlux was the best at reproducing westerly wind bursts that played a key role in the 1997-1998 El Nino onset. Hence, we recom- mend the use of TropFlux for studies of equatorial ocean dynamics.

/pdf/tropflux-wind-stresses-over-the-tropical-oceans-evaluation-1gxmkar637.pdf

TropFlux wind stresses over the tropical oceans: evaluation and comparison with other products

The Bay of Bengal, the northeastern limb of the tropical Indian Ocean is a region strongly coupled with summer and winter monsoons and tropical cyclones. The Bay is also a region of strong vertical stratification near the surface due to large inputs of freshwater through rainfall and river run-off. In situ subsurface ocean measurements are quite sparse both in space and time in this region. The National Institute of Ocean Technology (NIOT), Chennai deployed instrumented moored buoys in the Bay since 1997 to provide continuous time-series measurements of surface meteorological and oceanographic parameters at selected locations. In the recent years several studies have shown the important role of variability of heat storage in the near-surface layers on the intraseasonal and interannual evolution of monsoons and cyclones. Hence a strong need was felt to augment some of these buoys with subsurface temperature, salinity and current sensors to continuously record the temporal evolution of their vertical structures. Under a new initiative, NIOT has deployed six moored buoys attached with sensors to collect subsurface oceanographic parameters on real-time basis in the Bay. These are coded as the OMNI (Ocean Moored buoy Network for Northern Indian Ocean) buoy system. The time-series of vertical profiles of temperature and salinity in 500 m water column from the surface and currents in the topmost 100 m water column are monitored at discrete depths in the Bay. The OMNI buoy programme addresses a long-standing need to understand the observed variability of upper ocean thermohaline and current structures on several timescales that has important bearing on the evolution of seasonal monsoons and cyclones. This article presents an account on the evolution, status and usefulness of the OMNI buoy programme.

OMNI buoy network in the Bay of Bengal

Comparison of reanalyzed, analyzed, satellite-retrieved and NWP modelled winds with buoy data along the Iberian Peninsula coast

The performance of QuikSCAT-derived wind vectors is evaluated using in-situ data from moored buoys over the Indian Ocean. The results show that the mean differences for wind speed and wind direction are 0.37 ms-1 and 5.8°, root mean square deviations are 1.57 ms-1 and 44.1° and corresponding coefficients of correlation are 0.87 and 0.75, respectively. The matching between in-situ and satellite estimates seems to be better in the North Indian Ocean than in the Equatorial Indian Ocean. The effects of sea surface temperature and air-sea temperature difference on wind residuals were also investigated. In general, QuikSCAT is found to overestimate the winds. It is speculated that low wind speed during rain-free conditions and high wind speed, normally associated with rain, may be the reason for the less accurate estimation of the wind vector from QuikSCAT over the Indian Ocean.

Comparison of wind data from QuikSCAT and buoys in the Indian Ocean

The diurnal cycle of rainfall over the eastern equatorial Indian Ocean was studied for the period 23rd October 2001 to 31st October 2003 using hourly data from the Triton buoy positioned at 1.5° S and 90° E. An analysis of the active and weak spells of rainfall for different seasons revealed peaks in the late evening hours in Winter, Summer and Fall and in early morning hours (in Spring) in 2002. The active spell of rainfall peaked in the afternoon hours, during Winter, Spring and Summer in 2003, which agrees with the previous results of Janowiak et al. (1994). An analysis of rainfall events showed that Fall 2002 had a maximum number of rainfall events (90) and minimum (60) were observed in Spring 2003. Further it was found that the majority of rain events (>60%) were less than 3 hours in duration throughout the study period. The longer duration rainfall events (i.e. rain events greater than 6 hour duration) contributed significantly to Spring 2002 (20% of the total rainfall) and Winter 2003 (21% of the total rainfall). Harmonic analysis of the hourly rainfall data for different seasons revealed that diurnal harmonic explains more than 80% of the variance for all seasons. Furthermore, the diurnal harmonic has a maximum amplitude for all seasons except summer, where the semidiurnal and six hourly harmonics are significant.

/pdf/seasonal-variation-of-the-diurnal-cycle-of-rainfall-in-the-4k5x7yauo4.pdf

Y. Kuroda

Papers

Comparison of wind data from QuikSCAT and buoys in the Indian Ocean

Seasonal variation of the diurnal cycle of rainfall in the eastern equatorial Indian Ocean