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This paper investigates the process behind climate model biases, especially in rainfall, during the southwest (June to August; JJA) and northeast monsoon (December to February; DJF) seasons over the Philippines using the downscaled Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations of the Southeast Asia Regional Climate Downscaling/Coordinated Regional Climate Downscaling Experiment-Southeast Asia (SEACLID/CORDEX-SEA). Biases in the surface and near-surface climate are assessed over land in eastern (EastPH) and western (WestPH) Philippines using observation and reanalysis datasets. Results show that models with well-represented sea surface temperature (SST) provided better added values after downscaling. Although the CMIP5-driven models tend to overestimate both surface energy and water partitioning, simulations with well-represented SST have better surface energy partitions. In contrast to the reference simulation driven by ERA-Interim reanalysis, the CMIP5-driven simulations tend to underestimate (overestimate) moisture flux convergence during the DJF (JJA) season, which might have contributed to the overall underestimation (overestimation) of rainfall in this season. Further investigation along a vertical transect shows the differences in governing mechanisms for each season. During DJF (JJA) season, the simulation with underestimated (overestimated) SST, near-surface air temperature, and mixing ratio results in underestimated (overestimated) rainfall over EastPH (WestPH). In addition, the CMIP5-driven models were found to overestimate low-level winds, which may have induced the deep convection regardless of season, suggesting the role of low-level winds in the convection scheme used in RegCM4 aside from topography and land cover. [ABSTRACT FROM AUTHOR]

The influence of planetary boundary layer (PBL) parameterization on tropical cyclones (TCs) over the Philippine region is examined using the Holtslag and the University of Washington (UW) schemes in RegCM4.7 at 25-km resolution. Comparisons made between the model-simulated TCs and best track data indicate that more TCs were reproduced in both the RegCM4.7 simulations than the ERA-Interim reanalysis. It is further revealed that while only small biases were obtained in the number of TCs detected from the two RegCM4.7 simulations, only the UW scheme was able to simulate strong (Category 4–Category 5) TCs. A composite analysis on the radial cross section of azimuthally averaged wind fields shows that the UW scheme generates stronger wind velocities with narrower and elevated maximum tangential wind, and enhances low-level momentum convergence, compared with the Holtslag scheme. In addition, the radial positioning of the strong diabatic heating of UW simulation within the radius of maximum wind supported the needed conditions for warmer core formation and contributed to the enhancement of the secondary circulation by vertical advection of high absolute angular momentum, hence the higher intensity of TCs. The resulting higher surface maximum winds with increased diabatic heating within the eyewall and stronger warm-core structures in the UW simulation suggest that the induced convergence has offset the dissipative effects of momentum loss more than that in the Holtslag run. [ABSTRACT FROM AUTHOR]

Regional climate simulations from the Southeast Asia Regional Climate Downscaling/Coordinated Regional Climate Downscaling Experiment – Southeast Asia (SEA) indicated model biases in temperature and rainfall over SEA. Given the influence of sea surface temperature (SST) variability on SEA climate, this study examines SST representation in climate models to investigate its potential contribution to the resulting model biases over the Philippines. Observed SST over SEA is first characterized by its spatial patterns and temporal variability. An analysis of the SST representation over SEA and its potential influence on modelled climate over the Philippines in Coupled Model Intercomparison Project Phase 5 (CMIP5) global climate models (GCMs) is then conducted, followed by an assessment of the potential influence of SST representation in CMIP5 GCMs on downscaled regional climate output. Our results show that GCMs with well represented SSTs (i.e., low bias, well captured variability, and pattern) can produce climate simulations well over the Philippines. Whether or not the GCMs with poor SST representation can perform well is inconclusive. During boreal winter (summer), climate variables with high (low) spatial correlation with model SST get poor (better) spatial correlation with observed climate. Over west of the Philippines, where model SST seasonal variability is captured well, models also adequately simulate climate variables. Results suggest that the negative temperature biases, and positive precipitation and wind speed biases, in both GCMs and downscaled simulations, are associated with negative model SST biases. These findings give a better understanding on how SST potentially influences modelled climatology over the Philippines. [ABSTRACT FROM AUTHOR]

In this study, a subset of the downscaled simulations of the Southeast Asia Regional Climate Downscaling/Coordinated Regional Climate Downscaling Experiment—Southeast Asia (SEACLID/CORDEX‐SEA) was analysed to examine the representation of tropical cyclone (TC) climatology over Southeast Asia, in terms of pattern, intensity, frequency, and lifetime. A modified vortex tracking algorithm is used to detect TCs over the SEACLID/CORDEX‐SEA domain in the historical simulations from 1986 to 2005. Sensitivity tests for the detection method criteria, including vorticity, outer core wind strength, sea level pressure anomaly, and temperature anomaly at 300, 500, 700, and 850 hPa, were conducted to determine the optimum threshold configuration for each SEACLID/CORDEX‐SEA simulation used in the study. Comparison with the best track data of the Joint Typhoon Warning Center showed that model simulations underestimated the total number of TCs east of the Philippines for the 1986–2005 period but captured the annual cycle of the total number of TCs. This underestimation of TCs is possibly due to the domain used, which does not extend further east to cover most of the TC genesis area in the Western North Pacific. The structure of a typical TC from the regional climate model simulation is comparable to observed TC structure. However, results indicate that the resolution of the simulations is still not sufficient to capture the fine details of the observed TC structure, which could explain in part the lower intensification rate of TCs in the model output. [ABSTRACT FROM AUTHOR]

This paper examines the projected changes in rainfall in Southeast Asia (SEA) in the twenty-first century based on the multi-model simulations of the Southeast Asia Regional Climate Downscaling/Coordinated Regional Climate Downscaling Experiment–Southeast Asia (SEACLID/CORDEX–SEA). A total of 11 General Circulation Models (GCMs) have been downscaled using 7 Regional Climate Models (RCMs) to a resolution of 25 km × 25 km over the SEA domain (89.5° E–146.5° E, 14.8° S–27.0° N) for two different representative concentration pathways (RCP) scenarios, RCP4.5 and RCP8.5. The 1976–2005 period is considered as the historical period for evaluating the changes in seasonal precipitation of December–January–February (DJF) and June–July–August (JJA) over future periods of the early (2011–2040), mid (2041–2070) and late twenty-first century (2071–2099). The ensemble mean shows a good reproduction of the SEA climatological mean spatial precipitation pattern with systematic wet biases, which originated largely from simulations using the RegCM4 model. Increases in mean rainfall (10–20%) are projected throughout the twenty-first century over Indochina and eastern Philippines during DJF while a drying tendency prevails over the Maritime Continent. For JJA, projections of both RCPs indicate reductions in mean rainfall (10–30%) over the Maritime Continent, particularly over the Indonesian region by mid and late twenty-first century. However, examination of individual member responses shows prominent inter-model variations, reflecting uncertainty in the projections. [ABSTRACT FROM AUTHOR]

Gridded rainfall products could augment the shortage of available rainfall data in archipelagic countries like the Philippines, where weather stations are still sparsely distributed especially over its remote and lessdeveloped islands. However, these products need to be validated first using ground measurements to determine their ability to represent properties of local rainfall. This study compares four high-resolution, gridded datasets--APHRODITEv1101, CHIRPSv2, TRMM 3B42v7, and PERSIANN-CDR--with respect to 49 synoptic weather stations over the Philippines from 1998 to 2005. The performance of these datasets was assessed in terms of bias, distribution, and different statistical error metrics and skill scores across time scales and climate types. Results show that all the datasets were able to capture the basic climatology and to varying extents, spatial patterns of Philippine rainfall. TRMM 3B42v7 has the least overall average monthly bias and most closely resembles the rainfall distribution observed at weather stations, especially dry days and torrential rain days for the whole Philippines.APHRODITEv1101 performs best in terms of errormetrics and skill scores but displays consistent underestimates. CHIRPSv2, on the other hand, best captures the seasonal rainfall peaks in the different climate types in the Philippines but is prone to larger errors. Last, PERSIANN-CDR shows generally poormetrics and rainfall distributions, in comparison to the other datasets. These key findings are used to identify possible research applications in the Philippines that are best suited for each dataset. [ABSTRACT FROM AUTHOR]

The seasonal and diurnal characteristics of localized convective rain events in Metro Manila, Philippines were examined using observations from 16 automated weather stations in the city during the years 2013–2014. After partitioning total rainfall to determine the proportion that is mainly due to localized convection, the diurnal patterns of frequency and intensity of convective rain were then investigated during the northeast monsoon (from November to March), summer (from April to May), and southwest monsoon (from June to October) seasons. Maximum changes in meteorological variables were used to further verify and characterize localized convective rain events over the city. Results show higher rainfall (by ~500 mm) over northern Metro Manila compared to the southern area throughout the period of study. Although total rainfall was highest during the southwest monsoon season, it was during the summer season, when the total rainfall was least, that the largest proportion of rainfall, i.e. 55%, was attributed to convective rain. Almost 45% of the total convective rain events were identified as localized rain events. During the summer season, localized convective rain events in Metro Manila were prominent and vigorous (i.e. greater change in meteorological variables) in the afternoon, and were noted to move along the direction of the prevailing wind, i.e. from the northeastern side of the city towards the west. Increased understanding of the meteorological characteristics of these localized rain events can help improve rainfall forecasting over Metro Manila, leading to better disaster risk reduction and resilience in the city. • Automated weather stations network used to characterize localized convective rain. • 55% of rainfall in Metro Manila during summer season are convective rain. • Peaks of convective rainfall observed around early to late afternoon over the city. • Spatial progression of meteorological variables examined for an intense rain event. [ABSTRACT FROM AUTHOR]