A two-dimensional numerical study on a daytime sea-breeze front was carried out in order to investigate the interaction of the sea-breeze front with Benard-Rayleigh-type convective cells within the wellmixed boundary layer ahead of the sea-breeze front. By using a nonhydrostatic compressible dry model with a very fine spatial resolution, it was found that either a frontogenesis (FG), or a frontolysis (FL) phenomenon, occurs alternately at the foremost part of the sea-breeze head. Through these interactions, the propagation speed of the sea-breeze front varied discontinuously. The frontal structure such as the shape, vertical velocity, temperature field and so on showed periodic variations as it was affected by upward and downward motions associated with the prefrontal convective cells. Based on horizontal distribution of the vertical velocity, the periodic variation was classified into three stages: FG, transition, and FL stages. For each stage, the detailed processes of FG and FL phenomena were examined by estimating each term of the frontogenesis equation. It is found that the con-fluence, and tilting terms, play an important role in the FG and FL processes.
The surface heat flux over the Sea of Okhotsk has been calculated from 1987 to 2001 by bulk parameterizations using ECMWF, ISCCP, and GISST data, corrected by COADS data. Sea ice concentration and ice type are incorporated by using an SSM/I open water and thin-ice algorithm to better represent the sea ice conditions. The mean seasonal variation of the net heat flux, averaged over the entire Okhotsk Sea, ranges from a summer maximum of 158 W m-2 in June (a positive value indicates that the sea or sea ice gains heat form the air), to a winter minimum of -219 W m-2 in December. The seasonal geographic distribution of the net heat flux is determined mostly by turbulent (sensible and latent) heat flux. Because of the heat insulating effect of sea ice, the turbulent heat flux and accordingly the net heat flux has the largest variation in winter, particularly in the north region, reflecting the yearly difference of sea ice extent. The geographic distribution of the annual net heat flux shows a distinct contrast, significant cooling of the ocean in the north and net heating of the ocean in the south. This contrast is a result of heat transport by both sea ice and the southward East Sakhalin Current; sea ice formed by obtaining negative latent heat in the north is transported to the south, then it releases the negative latent heat by melting. The annual net heat budget is estimated to be -22 W m-2 averaged over the entire sea. The Okhotsk Sea loses heat to the atmosphere on an annual basis, although at least the order of 20 W m-2 error exists in the estimation. A trial was performed to estimate sea ice production using the heat budget calculation. The results suggest that most of the production occurs in open water and thin ice areas. The highest production area is located over the northwest shelf, within 100 km of the coast.
This study explores the impacts of three different ensemble configurations on the predictability of typhoon-related precipitation. These ensemble configurations are constructed by; (1) using a single model starting with physically initialized ECMWF analyses assimilating five different precipitation estimates and with an ECMWF analysis itself (multi-analysis, MA), (2) using a single model with six different convection schemes and initialized with ECMWF analyses (multi-convection, MC), and (3) six operational forecast models initialized with the centers’ own analyses (multi-model, MM). Ensemble precipitation forecasts are verified with help of satellite-derived precipitation estimates. Typhoon-related precipitation systems are best predicted by the MM configuration, from both deterministic and probabilistic viewpoints. The single model ensemble setups (MA and MC) contain more biases than the MM setup. A weighed ensemble, the so-called superensemble (SE), technique shows a slight increase in forecast skills, compared to the bias-corrected ensemble.
This report shows genesis of a polar low by an AGCM without specialized initial condition. A case of polar low genesis over the eastern coast of the Asian Continent, simulated in the seasonally varying climatological SST run by an AGCM (T42L52, primitive equation spectral model with 42 wave-number and 52 layers), is presented. A polar low simulated in January 22-23 of the fourth year (Y04) integration after the 10-year period of the spin-up is studied in comparison with polar lows described in several observational studies. In January Y04, large-scale circulation systems, such as Asian winter monsoon, extratropical cyclones and upper cold lows are reasonably simulated. A typical polar low is formed in January 22 over the coastal sea area ∼1500 km west of the major extratropical cyclone that developed over the Northwestern Pacific Ocean, under the influence of a short wave trough, which propagates along the rim of an upper cold low. The polar low genesis takes place first in connection with a deepening of the surface trough, which extends westward from the major cyclone. The deepening of the surface trough in the zone of strong low-level thermal gradient over the coastal sea area suggests the important role of the low-level baroclinicity for the polar low genesis. The strong heating due to the energy supply from the sea surface contributes for the genesis of the polar low through the decreasing of the vertical stability, and the sustaining thermal gradient. Meanwhile, the heating around 700 hPa associated with the precipitation concentrated within the polar low indicates the influence of the condensation heating for the development of the polar low. Aforementioned various processes contribute together to generate and develop the polar low. The structure and the evolution process of the simulated polar low are consistent with those of the observed polar lows. It is concluded that the realistic polar low genesis takes place in the model, when the large-scale phenomena such as the upper cold low, the short-wave trough, parent major cyclone and polar air outbreak are reasonably simulated. The present study is significant in presenting an AGCM simulation of a polar low for the first time.
Observed or simulated mesoscale convective systems (MCSs) often show pulsations with time periods of a few hours. In this paper, results from a two-dimensional cloud model are used to determine the dynamical processes responsible for such self-modulating oscillations (meso-β-scale cycle, MBC) in squallline type convective systems. The simulated storms have weak cold pools and include notable pulsations with time periods of 3∼4 hours. These pulsations are manifested as periodic organizations of convective-scale cells into upsheartilted meso-β-scale cells (MCs). A rearward-propagating, dissipating MC and disturbances on the downshear side have the properties of vertically propagating gravity waves, with storm-relative time periods that are about 1.5 hour less than those of the MBC. Dry model simulations driven by prescribed thermal forcings show that the waves arise in response to the reduction of latent heating in the MC in its stationary stage. The evolution of storm structure following a convective burst is related to the phasing of the uppertropospheric wave disturbances relative to the lower-tropospheric disturbances. The storm exhibits multicellular structure so long as the downward and frontward acceleration phase of gravity waves exerts suppressing effects on convections behind the gust front. As the phase propagates to the rear, it in turn enhances the convergence at the terminus of the steady lower-tropospheric front-to-rear flow, aiding the cell reinvigoration there. The time period of MBC increases as MCs tilt more horizontally in their stationary growth stage, and as the storm-relative rear-to-front upper-tropospheric wind increases. The former effect increases the intrinsic period of excited waves, and the latter increases the storm-relative period of the waves by Doppler-shifting the phase speed. These results support the argument that gravity waves generated by tilted MCs are prominent in the dynamics of MBCs.
The mixing state of polar stratospheric cloud (PSC) particles in the vertical sandwich structure observed simultaneously by lidar, and by Optical Particle Counter (OPC) above Ny Ålesund on 6th January 1996, is investigated. The vertical sandwich structure of PSC (observed by lidar) is characterized by one layer that exhibits large backscattering and low depolarization enclosed between two layers that exhibit low backscattering and high depolarization. The assumption that the observed optical properties of the cloud can be explained by an external mixture of solid and liquid particles is used as a basis for developing a new scheme for interpreting the lidar data. The backscatter and depolarization ratios observed by lidar are employed in equations, that allow for the separate calculation of the backscattering coefficients of the liquid and the solid particles, and for clear distinction of the mixing state of the cloud. Utilization of the developed scheme to analyze the vertical profile of the investigated PSC observed by lidar reveals that no purely liquid or solid layers exist, and that the entire altitude of the cloud can be represented by an external mixture of solid and liquid particles. Comparison of the vertical profiles of the calculated backscattering coefficients of the solid and liquid particles observed by lidar, and the number concentrations of the particles in different size categories observed by the Optical Particle Counter (OPC) implies radiuses of greater than 1.8 mm, and number concentrations of around 10-4-10-3 cm-3 for the solid particles.
A microphysical box model is used to simulate the optical properties of sandwich structure polar stratospheric cloud (PSC) observed by lidar, and interpreted as an external mixture of liquid and solid particles in the companion paper (part 1). The liquid particles are assumed to consist of a supercooled ternary solution (STS). For the composition of the solid particles, nitric acid tryhidrate (NAT) is employed. The simulations indicate that the observed backscattering coefficients of the liquid and solid particles can be explained by the evolution of an ensemble of externally mixed STS and NAT particles along the air parcels trajectory, if initial mixing ratio of HNO3 in the range 13-16 ppbv is assumed for the lower depolarizing layer of the sandwich structure. In the range 15-19 ppbv for the nondepolarizing layer, and the mixing ratio of H2O is assumed to be 5 ppmv. The total number of NAT particles derived from the simulations agrees with the Optical Particle Counter (OPC) observations for the number of particles larger than 1.8 mm, and is on the order of 10-3-10-4 cm-3 along the cloud’s altitude. The solid particles at the altitude of the upper depolarizing layer could not be explained in terms of NAT particles with the narrow size distributions expected from the simulations, while the liquid particles in that layer are probably background stratospheric sulfuric aerosol.
The present study is an assessment of a two-member ensemble of transient climate change simulations, with a focus on the Indian summer monsoon and ENSO-monsoon teleconnection. The CNRM ocean-atmosphere coupled model is integrated from 1950 to 2099 and driven by changes in concentrations of greenhouse gases and sulfate aerosols. The simulated monsoon climate is first validated against available observations and NCEP/NCAR reanalyses over the second half of the 20th century. The model captures the main features of the Indian monsoon climate and the main mode of variability found in the tropical regions, namely the El Niño Southern Oscillation, reasonably well. During the second half of the 21st century, both scenarios indicate a significant increase in the annual mean surface air temperature (about 2°C) and in monsoon precipitation (less than 10%) over India, relative to the 1950-1999 climatology. However, the model does not show a clear strengthening of the monsoon circulation, but rather a northward shift of the westerly monsoon flow. The increase in monsoon precipitation is therefore partly due to a ‘non-dynamical’ response to global warming, namely a large increase in precipitable water over India. While the transient response of the model shows a qualitative agreement with the surface warming observed over recent decades, neither the observations nor the model indicate significant trends in All India monsoon rainfall in the late 20th century. A long-term increase in simulated monsoon precipitation does appear from 1950 to 2099, but is superimposed onto relatively large multi-decadal fluctuations. The simulated ENSO-monsoon teleconnection also shows a strong modulation on multi-decadal time scales, but no systematic change with increasing amounts of greenhouse gases.
The features of ENSO-like phenomena simulated in a coupled ocean-atmosphere model (CGCM) incorporated with two different convection schemes, the Kuo scheme (KUO) and the prognostic Arakawa-Schubert scheme (PAS), are compared. There are the significant differences in the model ENSO properties between KUO and PAS. The sea surface temperature (SST) anomaly in KUO shows a standing oscillation pattern in the eastern equatorial Pacific, while it propagates westward in the western equatorial Pacific. In contrast, the SST anomaly propagates eastward slowly across the entire equatorial Pacific basin in PAS. The amplitude of Niño3 SST anomaly is larger in PAS and smaller in KUO than observations. The strongest equatorial westerly wind anomalies during the mature phase of the ENSO are displaced slightly eastward in PAS but westward in KUO compared to observations, associated with the difference in the mean equatorial SST. The examination of heat balance shows that the SST anomaly is strongly influenced by the thermocline depth anomaly in PAS, which is favorable for the eastward phase propagation of SST anomalies. In KUO, on the other hand, the westward phase propagation of SST anomalies in the western equatorial Pacific is related to the anomalous horizontal advection. Comparing PAS with KUO, the mean thermocline depth in the western equatorial Pacific is shallower in PAS than KUO, while the mean zonal SST contrast in the western equatorial Pacific is stronger in KUO than PAS. They are responsible for the difference in the phase propagation of SST anomalies between KUO and PAS. In KUO, the Kelvin waves reflected at the western boundary seem to contribute to the reversal of the model ENSO. In PAS, on the other hand, the western equatorial wind anomalies appear during the mature phase of the model ENSO, and then induce the forced-Kelvin waves playing an important role in the termination of the model ENSO. These Kelvin waves cross the equatorial Pacific as an air-sea coupled mode with a slow phase speed, compared to the reflected Kelvin waves in KUO. As a result, the time needed for Kelvin waves acting as a negative feedback to propagate in the eastern equatorial Pacific is longer in PAS than KUO, leading to the amplification of Niño3 SST anomalies. These differences in ENSO properties in turn are related to the difference in the mean states that result from the two different convection schemes. A comparison of the mean wind fields suggests that the difference in ENSO characteristics can be attributed to the difference in the mean wind fields over the western Pacific.
Seasonal variations of wind in the troposphere are analyzed based on operational rawinsondes at 11 stations over Indonesia for 1992-1999 and the results are compared with NCEP reanalysis. For meridional wind, an annual oscillation caused by north-south shift of a meridional (Hadley-type) circulation is clearly found. Winter-poleward flow of the upper-tropospheric meridional circulation is stronger than summer-poleward flow. The winter (southern) hemispheric cell in northern summer has a larger invasion across the equator (as suggested in the zonal-mean Hadley cell), and weaker meridional flows than the winter (northern) hemispheric cell in northern winter. Along the boundary of twin meridional circulation cells, a zonal (Walker-type) circulation seems to exist, and has easterly (westerly) wind in the upper (lower) troposphere in the Indonesian region. The zonal circulation is also shifted from the equator associated with the meridional circulation cell boundary. However, a semiannual oscillation, which has easterly maxima in January—February and July—August is more clearly observed in the upper-tropospheric zonal wind, which is consistent with a consideration on conservation of absolute angular momentum for the upper-tropospheric winter-poleward (equatorward) air mass transport associated with the winter-hemispheric meridional circulation cell, invading the summer hemisphere across the equator. In comparison with the seasonal-vertical variations based on the operational rawinsonde data, the NCEP reanalysis seems to overestimate the upper-tropospheric northerly and easterly maxima during northern summer. In other words, the NCEP reanalysis produces a winter hemispheric side meridional circulation cell with similar intensities between both solsticial seasons, in spite that in the operational rawinsonde data the winter hemispheric side meridional circulation cell in northern summer is weaker than that in northern winter. The overestimation of easterly may also be related with an insufficient reproduction of the winter-hemispheric meridional circulation cell—such as too large northward invasion.
In order to describe the connection from an event of MJO to the next in the boreal winter, the eastward propagation of MJO is studied, focusing on that over the western hemisphere. Propagation signal is identified by EEOF analysis, performed on the bandpass filtered OLR for the period of 1979-2000. Besides NOAA OLR, total precipitable water (TPW), and surface winds from Special Sensor Microwave/ Imager (SSM/I), precipitation observed from Microwave Sounding Unit (MSU), and reanalysis and operational analysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF), are utilized for the composite. Positive TPW anomalies are found, synchronizing with tropospheric and surface zonal wind anomalies. They propagate eastward all around the equator in the boreal winter. They propagate at a speed of about 6 ms-1, with a Kelvin-Rossby coupled mode structure in the eastern hemisphere, and at about 20 ms-1 as an envelope of a radiating response in the western hemisphere. Within the envelope in the western hemisphere, faster propagating signals corresponding to 30-40 ms-1 exist in the fields of TPW, zonal wind at 200 and 700 hPa, surface zonal wind. It is especially clear in the geopotential anomalies at 1000 hPa. This fast propagation speed of 30-40 ms-1 is consistent with a first-baroclinic dry Kelvin wave mode recently rediscovered by Milliff and Madden (1996), and Bantzer and Wallace (1996). TPW increases under surface easterly anomalies along the equator. After the preceding TPW accumulation for 5-7.5 days, convective anomalies begin to occur as a part of the next cycle of the MJO from the eastern Atlantic to the western Indian Ocean. These results suggest a following conceptual model for propagations and event-to-event connections of MJO. Equatorial Kelvin wave generated by convection of the MJO propagates eastward emanating from a warm pool region at a faster speed (30-40 ms-1) in the western hemisphere. Elevated topography of the South American and African continent, blocks the wave propagation. After being blocked several days by topography, they continue to proceed. As a result, the signal propagates at 20 ms-1 on average. Frictional convergence with lower easterlies of the dry Kelvin wave results in the associated propagation of TPW positive anomaly. Although it does not induce deep convections over large-scale subsidence regions, once it enters over the warm water in the western Indian Ocean, it helps to induce active convections for the next cycle of MJO.
Observational data of RSD (Raindrop Size Distribution) in the Baiu season were analyzed by using Principal Component Analysis in order to detect objectively the characteristics of variation of RSD. The results indicated that 88.4% of the variation of RSD was represented by the first and second principal components. The primary component of variation was to increase the number concentration for all sizes of drops; in other words, a parallel movement in a semi-logarithmic plot (linear diameter-logarithmic number concentration). It was found that this variation has a good correlation with that in the rainfall rate. This is different from the Marshall-Palmer-type RSD, which assumes only the slope parameter λ depending on the rainfall rate. The secondary component of variation was to increase (decrease) number concentration for larger (smaller) drops, in other words, the rotational movement in a semi-logarithmic plot. This component was considered to be connected with physical factors other than rainfall rate. With respect to small rainfall rate, comparison between RSDs in convective, and stratiform precipitations, showed that the slope of RSD was smaller in stratiform than in convective precipitation for equal rainfall rate. This means that the difference between convective and stratiform precipitation is one of the possible factors relating to the rotational variation.
The mechanisms driving the annual cycle of the South China Sea surface temperature are examined using the 1979-1999 NCEP/NCAR reanalysis data. The cycle is not symmetric. The summer half-year from May to October is the warm phase with the sea surface temperature decreasing slowly. This slow decrease is due to the effect of oceanic processes that overcome the net surface heating. The winter halfyear is the cold phase with a rapid decrease in sea surface temperature from October to January followed by a rapid rise from February to May. These rapid temperature changes are dominated by net surface heating with negligible effects from oceanic processes. The net surface heating is mainly a result of the offsetting between heating due to shortwave radiation and cooling due to latent heat flux and longwave radiation, whose magnitudes have an approximate ratio of 3:2:1. The rapid warming and cooling periods can both be accounted for by the opposite variations of shortwave radiation heating and wind speedregulated evaporation cooling.
A radiative-dynamical cloud feedback heating (CFH) is incorporated into a two-dimensional mechanistic model in an equatorial longitude-height plane of Venus. For a typical basic field, the CFH triggered by random disturbance results in the formation of convection cells with various scales in the lowstability layer (∼ 55 km) sandwiched between stable layers, and the convection cells generate vertically propagating gravity waves. The wave-generation mechanism by convection cells resulting from CFH is different from previous mechanisms, in which waves were directly forced by CFH. If the cloud feedback process works in real atmosphere, the convection resulting from the perturbed CFH should be considered as a possible formation mechanism of gravity waves in Venus’ cloud layer.