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Kevin Hamilton

Abstract

A search was conducted for the principal lunar diurnal tide (O1) in an 18½ year time series of twice-daily digitized sea level pressure analyses covering the region 20–90°N. At 20, 25, 30 and possibly at 35°N there is evidence for a systematic variation of the zonal wavenumber one harmonic of the pressure as a function of the phase of the O1 tidal potential. This variation is clearly dominated by a westward traveling component (i.e., one that follows the tidal potential around the earth each day). The computed amplitudes are very small (less than 0.01 mb), and north of 35°N the random meteorological noise cbscures the O1 tidal oscillation to the point where it cannot be detected from analysis of the present data.

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Kevin Hamilton

Abstract

The consequences of the hypothesis of Lindzen (1978) that latent heat release may be a significant excitation mechanism for the semidiurnal atmospheric tide are examined in some detail. Harmonic analysis of hourly rainfall data from 79 tropical stations shows that the semidiurnal variation of rainfall in the tropics is ∼1 mm day−1 and has a phase near 0300 LST, just as Lindzen's theory requires. Analysis of data at 85 midlatitude stations shows that the sermidiurnal rainfall oscillation there has its phase rather later (about 0600). The results of simple classical tidal theory calculations which indicate that the geographical distribution of the surface pressure response to latent heat forcing largely follows that of the forcing itself are presented. This result is then used to suggest a plausible explanation for the observed seasonal cycle of the semidiurnal pressure oscillation in midlatitudes. Further calculations show that the magnitude of the non-migrating components of the semidiurnal barometric oscillation produced by latent heat excitation is not likely to be unrealistically large. These calculations also suggest that Lindzen's hypothesis might be verified by observing the phase of the semidiurnal pressure oscillation in particularly and regions.

The rainfall observations also show a strong diurnal (24 h) component in the rainfall both in the tropics and in midiatitudes. The effects of latent heat release on the 24 h tide are briefly discussed.

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Yuqing Wang, Li Zhou, and Kevin Hamilton

Abstract

A regional atmospheric model (RegCM) developed at the International Pacific Research Center (IPRC) is used to investigate the effect of assumed fractional convective entrainment/detrainment rates in the Tiedtke mass flux convective parameterization scheme on the simulated diurnal cycle of precipitation over the Maritime Continent region. Results are compared with observations based on 7 yr of the Tropical Rainfall Measuring Mission (TRMM) satellite measurements. In a control experiment with the default fractional convective entrainment/detrainment rates, the model produces results typical of most other current regional and global atmospheric models, namely a diurnal cycle with precipitation rates over land that peak too early in the day and with an unrealistically large diurnal range. Two sensitivity experiments were conducted in which the fractional entrainment/detrainment rates were increased in the deep and shallow convection parameterizations, respectively. Both of these modifications slightly delay the time of the rainfall-rate peak during the day and reduce the diurnal amplitude of precipitation, thus improving the simulation of precipitation diurnal cycle to some degree, but better results are obtained when the assumed entrainment/detrainment rates for shallow convection are increased to the value consistent with the published results from a large eddy simulation (LES) study. It is shown that increasing the entrainment/detrainment rates would prolong the development and reduce the strength of deep convection, thus delaying the mature phase and reducing the amplitude of the convective precipitation diurnal cycle over the land. In addition to the improvement in the simulation of the precipitation diurnal cycle, convective entrainment/detrainment rates also affect the simulation of temporal variability of daily mean precipitation and the partitioning of stratiform and convective rainfall in the model. The simulation of the observed offshore migration of the diurnal signal is realistic in some regions but is poor in some other regions. This discrepancy seems not to be related to the convective lateral entrainment/detrainment rate but could be due to the insufficient model resolution used in this study that is too coarse to resolve the complex land–sea contrast.

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Chunxi Zhang, Yuqing Wang, and Kevin Hamilton

Abstract

A modified Tiedtke cumulus parameterization (CP) scheme has been implemented into the Advanced Research Weather Research and Forecasting model (ARW-WRF) to improve the representation of marine boundary layer (MBL) clouds over the southeast Pacific (SEP). A full month simulation for October 2006 was performed by using the National Centers for Environmental Prediction (NCEP) final analysis (FNL) as both the initial and lateral boundary conditions and the observed sea surface temperature (SST). The model simulation was compared with satellite observations and with results from an intense ship-based campaign of balloon soundings during 16–20 October 2006 at 20°S, 85°W.

The model with the modified Tiedtke scheme successfully captured the main features of the MBL structure and low clouds over the SEP, including the geographical distribution of MBL clouds, the cloud regime transition, and the vertical structure of the MBL. The model simulation was repeated with the various CP schemes currently provided as standard options in ARW-WRF. The simulations with other CP schemes failed to reproduce the geographical distribution of cloud fraction and the observed cloud regime transition, and displayed an MBL too shallow compared to observations. The improved simulation with the modified Tiedtke scheme can be attributed to a more active parameterized shallow convection with the modified Tiedtke scheme than with the other CP schemes tested, which played a critical role in lifting the inversion base and the low cloud layer. Results from additional sensitivity experiments employing different planetary boundary layer (PBL) parameterization schemes demonstrated that the basic feature of the MBL structure and low clouds over the SEP were not particularly sensitive to the choice of the PBL scheme.

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Chunxi Zhang, Yuqing Wang, Axel Lauer, and Kevin Hamilton

Abstract

The Weather Research and Forecasting (WRF) model V3.3 has been configured for the Hawaiian Islands as a regional climate model for the region (HRCM). This paper documents the model configuration and presents a preliminary evaluation based on a continuous 1-yr simulation forced by observed boundary conditions with 3-km horizontal grid spacing in the inner nested domain. The simulated vertical structure of the temperature and humidity are compared with twice-daily radiosonde observations at two stations. Generally the trade wind inversion (TWI) height and occurrence days are well represented. The simulation over the islands is compared with observations from nine surface climatological stations and a dense network of precipitation stations. The model simulation has generally small biases in the simulated surface temperature, relative humidity, and wind speed. The model realistically simulated the magnitude and geographical distribution of the mean rainfall over the Hawaiian Islands. In addition, the model simulation reproduced reasonably well the individual heavy rainfall events as seen from the time series of pentad mean rainfall averaged over island scales. Also the model reproduced the geographical variation of the mean diurnal rainfall cycle even though the observed diurnal cycle displays quite different features over different islands. Comparison with results obtained using the land surface dataset from the official release of the WRF model confirmed that the newly implemented land surface dataset generally improved the simulation of surface variables. These results demonstrate that the WRF can be a useful tool for dynamical downscaling of regional climate over the Hawaiian Islands.

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