A high-resolution (9-km) diabatic data assimilation system—the Local Analysis and Prediction System (LAPS), has been developed and used to initialize the real-time fifth-generation Pennsylvania State University—National Center for Atmospheric Research Mesoscale Model (MM5) at the Central Weather Bureau in Taiwan. During 2003, the more extensive network of four high quality Doppler radars and the access to satellite data from the Geostationary Operational Environmental Satellite (GOES-9) provided an excellent opportunity for advancing the short-range precipitation forecasts over the Taiwan area. The parallel forecasts of four tropical cyclones (Tropical Storm Morakot and Vamco, Typhoon Dujuan, and Tropical Storm Melor) that affected Taiwan in 2003 are performed, both with and without the inclusion of the LAPS cloud analysis scheme. Except for the inclusion of the LAPS cloud field, the model integrations are identical in all other respects. Forecast results demonstrate that using LAPS to diabatically initialize MM5 leads to an improved prediction of tropical cyclones in terms of the storm’s track, intensity, cloud pattern, and movement of rainbands in the early portion ofmodel prediction. During the first 6-h of the forecast, the heavy rainfall prediction associated with the cases studied was improved when the LAPS cloud analysis scheme was included. The assimilation of data from Doppler radars, and GOES-9 satellite, played an important role in the improvement of storm hydrometeorological features in the model initial condition and thus had a beneficial impact on reducing the model spin-up time. However, further studies are needed to clarify the reasons for the poor performance in simulating the typhoon eyewall. This paper represents a major step toward building a short-range mesoscale modeling system that predicts more realistic storm structures and rainfall distribution over the Taiwan area in real time. The overall results suggest that the impact ofthe LAPS/MM5 system can be significant for short-range, high spatial-resolution, rainfall prediction associated with a tropical cyclone, especially for the heavy rainfall occurring during the early hours of the model integration.
An investigation has been made to understand the development mechanisms for the heavy rainfalls of 6-7 August 2002 over the middle of the Korean peninsula, using both observations and a numerical model. This paper describes 1) the observed heavy precipitation systems, 2) the environment and its relation to the heavy rainfalls, 3) simulated heavy rainfalls and the role of latent heat release, and 4) themechanisms for the heavy rainfall development. Heavy rainfalls occurred over the mid-peninsula with a maximum precipitation amount exceeding 300 mm during the 18-h period of 12 UTC 6-06 UTC 7 August. Observations showed that convection bands continuously developed over the Yellow Sea and coastal area near the mid-peninsula. They moved eastward, and embedded convective cells developed into deep convections, producing heavy rainfalls over the coastal and inland areas. A large meso-β-scale rainfall area with several heavy-rain cells formed in land, ahead of the low-level jet (LLJ) maximum area where strong convergence occurred. A close relationship was found among the LLJ, upper-level jet streak (ULJS), and heavy rainfalls. In this event, large-scale conditions not only provided a favorable environment for the development of the heavy rainfalls, but also actively interacted with mesoscale systems. Simulation using MM5 has well reproduced the convection bands over the coastal area, and the large meso-β-scale rainfall area with heavy-rain cells over the inland area. Simulated results suggest two types of band formation: one initiated by an elongated convergence of large-scale flows, and another initiated by convective cells existing in a strong southwesterly air stream. Simulations also indicate that both the LLJ and ULJS are intensified and maintained by convective heating. A favorable, large-scale environment, and its interaction with convective systems, may be the primary reason for the persistence of the heavy rainfalls.
This study investigates the northward, and northwestward propagation of 30-60 day oscillation over the western North Pacific (WNP) at upper, and low levels with a three-dimensional streamfunction tendency equation. In the tropical WNP, the surface frictional effect associated with the cyclonic circulation enhances the low-level convergence at the cyclonic vorticity center to the northwest of the convection, causing the 30-60 day convection to develop northwestward. The vorticity advection induces the 30-60 day circulation at upper and low levels to propagate northwestward, with a baroclinic structure. The combined effect of surface frictional-diabatic heating, and vorticity advection, causes the 30-60 day convection and circulation to develop and propagate simultaneously northwestward. After the convection fully develops, increased static stability, associated with adiabatic cooling, reduced solar radiation due to the cloud-radiation effect, and negative land-surface feedbacks on moisture availability, restrict any further development of the 30-60 day convection. A wave train emanating from the South China Sealwestern North Pacific (SCS/WNP) into the extratropical North Pacific, is well established 15-days after the convection reached maximum intensity over the SCS/WNP. The main process and mechanism responsible for the northwestward propagation of this 30-60 day oscillation in the extratropical WNP is similar to the process proposed for the tropical WNP, except that in the mid-latitudes where the coriolis parameter becomes large, the influence of upper-level vorticity advection extends down to the low levels.
The impact of the El Niño-Southern Oscillation (ENSO) on the Madden-Julian Oscillation (MJO) is studied, based on reanalysis data and output from an ensemble general circulation model (GCM) experiment. Observed monthly sea surface temperature variations over the period of 1950-99 are imposed in the deep tropical eastern/central Pacific in the course of the SST experiment. Both GCM, and reanalysis data, indicate that intraseasonal activity of the low-level zonal wind is enhanced (reduced) over the central (western) Pacific during El Niño events. The propagation and growthldecay characterisitcis of the MJO in different phases of ENSO is also examined, based on a lag correlation technique. During warm events there is an eastward shift in the locations of strong growth and decay, and the propagation of the MJO becomes slower in the warm ENSO phase. These changes are reversed during La Niña epsiodes. Using output from the GCM experiment, the effects of ENSO on the circulation and convection during the MJO lifecycle are studied in detail. Further eastward penetration of MJO-related convection is simulated during warm events over the central Pacific. An instability index related to the vertical gradient of the moist static energy is found to be useful for depicting the onset of MJO convection along the equator. During warm events, the stronger magnitudes of this index over the central Pacific are conducive to more eastward penetration of convective anomalies in the region. These changes are mainly due to the intensified moisture accumulation at low levels. Analysis of the moisture budget suggests that the stronger moisture accumulation can be related to the increased low-level humidity over the central Pacific during warm events.
To investigate the vertical profiles of fractional entrainment rate as to cumulus convection, numerical simulations of a tropical cyclone rainband are conducted, using a high-resolution three-dimensional cloud-resolving model (CRM), with the 200-m horizontal resolution. On the basis of the results of CRM simulations, venically variable entrainment rate is applied to the Arakawa-Schubert (AS) cumulus parameterization. Fractional entrainment rate, derived from the calculation based on the vertical gradient of cloud mass flux, clearly shows larger near cloud base and top. Between the heights of cloud base and top, entrainment rate is smaller, and even negative in many cases, suggesting laterally detrained air from a cumulus into the environment. From the analyses where entrainment rate is divided into three terms, it is found that the contributions of updraft is relatively large near the cloud base and top, compared to that in between. The cloud amount contribution depends on whether cloudy areas expand or shrink accompanying cloud growth or decay, respectively. On the basis of the result of the CRM simulations, vertically variable entrainment rate is applied to the AS scheme. For investigating the effect of the modifications of the scheme, simulations of typhoon Saomai (2000) are conducted. The simulations show significant improvements: underestimates of moisture in the mid- to upper troposphere are reduced. The result is predominantly attributed to cloud mass flux, greatly influenced by lateral detrainment. The peak height of the mass flux corresponds to that in the moisture tendency.
We have developed a sequential algorithm to retrieve vertical profiles of cloud and rainfall microphysical information from radar reflectivities of a 14 GHz precipitation radar, and a 95 GHz cloud profiling radar. This sequential algorithm retrieves the number density NT, the mean volume diameter Dm and the liquid water content LWC from the nearest to the farthest ranges considering the significant attenuation and Mie scattering. To avoid the accumulation of numerical errors, the retrieval is computed in adjacent ranges assuming NT to be constant in each pair. We demonstrate that our method retrieves NT and Dm stably, even if the raindrop size distribution is different from our assumption, or even if some bias errors are appended to the radar constant. The method is applied to collocated ground-based observation by a 14 and a 95 GHz radar, and the retrieved results are evaluated by comparing them against disdrometer-derived size distributions, where the rainfall rate was from 0.1 to 4.6 mm/hr. The retrieved Dm, and the disdrometer-derived Dm, agree to within 30%. The retrieved log10LWC, and the disdrometer-derived log10LWC, also agree to within 30% when the rainfall rate is greater than 0.5 mm/hr.
Snow surfaces have unique energy-exchange characteristics, which need to be correctly represented in hydrological and climate models. An intensive field study was conducted in an open farm field in Tokachi, Hokkaido, Japan in which all energy exchange fluxes were monitored during the snowmelt period in 2004. Energy inputs to the snow surface was dominated by net radiation, which provided 75% of total input, while sensible heat contributed significant input on days with strong wind. Snowmelt consumed 80% of energy and evaporation consumed 20%. Sensible and latent heat had similar magnitude but opposite direction, meaning that sensible heat input was nearly cancelled by latent heat loss (i.e., evaporation). Therefore, the snowmelt rate was strongly controlled by net radiation. Compared to previous studies in northern Japan, very high daily evaporation rates, up to 2.2 mm d−1, were observed during episodic events. High evaporation was caused by the foehn (Tokachi-kaze), characterized by warm, dry, north-westerly wind descending the eastern slope of the mountains. The foehn events were associated with major extratropical cyclones over the sea east, or northeast of Hokkaido. Historical analysis of daily climate data showed that similar high-evaporation events associated with the foehn are common in Tokachi, although the magnitude of the event in 2004 was exceptional. Estimated evaporation rates during the melt periods in 1997-2004 had an average of 0.24 mm d−1, indicating that evaporation plays a relatively minor role in the overall water balance of the snowpack. However, latent heat flux plays a significant role in energy balance.
Several previous studies indicate that an increase in atmospheric CO2 could increase summer dryness on the land surface over the continental region in mid- to high-latitudes. However, the numerical models used in these studies did not sufliciently reproduce permafrost at high latitudes in the Northern Hemisphere, because of their simplified representation of ground hydrological processes; most of them consist of only one layer, and/or exclude freezing and thawing of soil moisture. In order to investigate the mechanisms associated with permafrost and the changes it undergoes, four experiments are performed using a coupled atmosphere-ocean GCM (MRI-GCM1), which includes a four-layer ground model, with a bottom at 10 m depth: the runs with freezing/thawing of soil moisture under normal and doubling concentration of atmospheric CO2 (F1 and F2) and their counterparts without the freezing/thawing (NF1 and NF2). The result with soil freezing (F2-F1) predicts a substantial increase of surface soil moisture over northern high latitudes during summer, while the result without soil freezing (NF2-NF1) indicates enhanced summer dryness in the same area. It is suggested, by means of a comparison study, that the CO2-induced wet summer due to the inclusion of soil freezing is mostly attributable to the thawing of a part of the permafrost in deep layers caused by the warming, augmenting liquid water available to upper layers, which moderates summer dryness at the surface. The thawing effect may be reduced to some extent if a gravitational term, which contributes to reducing the upward diffusion, is included in the formulation of soil water migration.
Near the tropopause over the North Pacific in summer an isolated low pressure system (Upper Cold Low, UCL) is often generated by the deepening and cutting off of a trough in the mid-latitude westerlies. The tracks and structures of these UCLs have been investigated in previous studies, but understanding of the cut off and weakening processes remains poor. In this paper, the tracks of UCLs generated in the 1999 summer are analyzed using ECMWF data. The physical processes occurring in one ofthese systems are investigated in detail using the ECMWF data, and the meso-scale model MM5. We focus particularly on cut off and weakening processes, and on the structure of the vertical velocity in the UCL. The summer of 1999 was hot over Japan, and part of the Tibetan high pressure, around 200 hPa, was shifted northward. This allowed some UCLs to approach Japan in July and August. A UCL on August 19th is selected for detailed analysis, and was generated in the following process. Positive vorticity in a westerly wave at 200 hPa was extended by a northeast wind in the upper layer only. The positive vorticity was cut off by non-liner effects and upper level divergence, associated with convective clouds, generating the isolated UCL. The structure of the cyclonic circulation and the warm and cold cores were similar to those in previous studies. The structure of the vertical motion of the moving UCL was explained by dry dynamics and there was upward motion on the front side of the UCL, in the direction of movement. Upper level clouds in the UCL strengtJhened this upward motion. Convective clouds were seen in the system. The latent heat of these convective clouds played an important role in weakening the cold core of the UCL.
This paper describes a diurnal cycle in systematic cloud system migration observed with the GMS IR1 sensor over Sumatera (approximately 1,500 km in length) from May 2001 to April 2002. Convective clouds developed over mountainous areas in the afternoon, and migrated westward and/or eastward for several hundred kilometers (∼ 500 km) from midnight to morning. Westward migration occurred in almost every month except August over southernmost Sumatera Island. Eastward migration occurred when lower-tropospheric winds were westerly and/or when super cloud clusters moved eastward along the Intertropical Convergence Zone (ITCZ), which moves northward and southward with an annual cycle.
We have investigated the accuracy ofthe semi-implicit semi-Lagrangian (SISL) method in simulating internal gravity wave (IGW) motion. We have focused on the relative accuracy of the hydrostatic, and nonhydrostatic IGW solutions. The analysis is based on a linearized model and a Global Circulation Model-Dynamic Core (GCM-DC) with a stretched grid. The nonhydrostatic version of the GCM-DC model produces the familiar IGW train disturbance anchored to an isolated hypothetical mountain. The wave has a distinct tilt away from the vertical direction, which is consistent with classical theory. For the hydrostatic version of the model, the axis of the resulting IGW train rests nearly perpendicular to the mountain top, thus again consistent with classical theory. Increasing the time step from 10 s; Courant number (Cn) = 0.5; to 60 s (Cn = 3.0), results in stable solutions for both the hydrostatic and nonhydrostatic versions of the model. The nonhydrostatic solution is in close agreement with the control run however, the hydrostatic solution exhibits large phase truncation errors. The solutions for the one-dimensional linearized SISL model confirm the GCM-DC results that the nonhydrostatic IGW train is less damped and shifted by the SISL scheme than the corresponding hydrostatic IGW motion. The linear solutions indicate very high accuracy of the physical mode of the solution, but it rapidly deteriorates when Cn exceeds unity. As Δt → 0 the amplitude of the computational mode tends to zero and its frequency to infinity. However, as Δt → ∞, the frequency of the computational SISL mode asymptotically approaches the value of the frequency of the corresponding SISL physical mode. Furthermore, the amplitude of the SISL computational mode is directly proportional to the size of the time step. Therefore, at large time steps, the amplification of the computational mode could offset some of the numerical damping of the physical mode by the SISL scheme.
Statistical errors of rain rate estimators due to natural variations in raindrop size distribution (DSD) are studied for 3-cm wavelength polarimetric radar. Four types of estimators are examined: A classical estimator R(ZH), and three types of polarimetric radar estimators R(KDP), R(ZH, ZDR), and R(KDP, ZDR), where R is the rain rate, ZH is the reflectivity factor at horizontal polarization, KDP is the specific differential phase, and ZDR is the differential reflectivity. The T-matrix method is employed for the scattering calculations, and a total of 7,664 one-minute raindrop size spectra, measured with a Joss-Waldvogel type disdrometer are used. According to simulation results, the normalized errors (NEs) of R(ZH), R(KDP), R(KDP,ZDR), and R(ZH,ZDR) for all DSD samples are 25%, 14%, 9%, and 10%, respectively. The NEs of all estimators, except R(ZH), tend to decrease with increasing rain rate. For rain rates larger than 10 mmh−1, e.g., the average NEs of R(ZH), R(KDP), R(KDP, ZDR), and R(ZH,ZDR) are 25%, 9%, 5%, and 7%, respectively. The simulation results show that the classical estimator R(ZH) is the most sensitive to variations in DSD and the estimator R(KDP, ZDR) is the least sensitive. The lowest sensitivity of the rain estimator R(KDP, ZDR) to variations in DSD can be explained by the following facts. The difference in the forward-scattering amplitudes at horizontal and vertical polarizations, which contributes KDP, is proportional to the 4.78th power of the drop diameter. On the other hand, the exponent of the backscatter cross section, which contributes to ZH, is proportional to the 6.38th power of the drop diameter. Because the rain rate R is proportional to the 3.67th power of the drop diameter, KDP is less sensitive to DSD variations than ZH. However, DSD spectra with unusually large median volume diameter D0 can increase the estimation error of R(KDP). The differential reflectivity ZDR reduces the effect of unusual D0 and is useful for further improvement of the estimator R(KDP). This is due to the fact that ZDR itself is a good measure of D0.
A radar remote-sensing technique of humidity profiles, which was originally developed for the middle and upper atmosphere radar (the MU radar), is applied to the lower troposphere radar (LTR) with Radio Acoustic Sounding System (RASS) operated on L-band (1357.5 MHz) in order to expand the height range into the atmospheric boundary layer. Radar volume reflectivity and turbulence energy dissipation rate are derived from LTR observations of turbulence echoes. Virtual temperature profiles are simultaneously monitored by using the RASS technique. The mixing ratio of water vapor (q) is then calculated by solving a first-order differential equation of q versus height. An observational campaign of the LTR with RASS was conducted on May 28-29, 2002 at a meteorological station in Feixi, China as a part of the Coordinated Enhanced Observing Period (CEOP) project. From the observed clear air echo intensity we successfully determined a humidity profile at the height of 0.2-2.2 km. Precipitable water vapor (PWV) derived from the propagation delay of GPS radio signals was incorporated in the estimation algorithm as a constraint of the q profiles. A kytoon was moored at about 160 m during the radar operation, and monitored temperature, pressure and humidity, though not continuously, which are used for defining the lower boundary values in retrieving q profiles. By using the RASS temperature, and kytoon data, we could successfully estimate continuous q profiles. The radar-derived q profiles generally agreed with radiosonde data. This study successfully demonstrated a potential of the humidity estimation with the LTR in the atmospheric boundary layer. Future long-term observation is expected to investigate the precision under various meteorological conditions.
To reanalyze ozone field with the MRI Chemical Transport Model (CTM) driven by the nudged General Circulation Model (GCM), we investigated the impact of changing meteorological variables of the ERA40 data to be nudged into a GCM. Two experiments were performed: one nudges only horizontal wind (mechanical nudging), and the other nudges both horizontal wind and temperature (mechanical and thermal nudging). The reanalyzed ozone field produced by the two experiments is different, due to the difference in the meridional circulation of the nudged GCM simulation. This may be related to the cold bias of MRI-GCM, which is commonly found in many GCMs. Mechanical nudging causes weaker meridional circulation in the lower stratosphere and a stronger circulation in the troposphere, compared to ERA40 due to the temperature bias of the GCM. Thermal nudging systematically creates a spurious heat source in the GCM, which makes the meridional circulation stronger in the lower stratosphere and weaker in the troposphere. As a result, thermal nudging decreases ozone in the tropical lower stratosphere and increases it in the mid-latitudes upper troposphere and lower stratosphere. The meridional circulation of the nudged GCM depends on the relaxation time for the thermal nudging. It may be possible to optimize the relaxation time to obtain a realistic meridional circulation.
This study investigates the difference of the predictability ofseasonal mean precipitation with Japan Meteorological Agency (JMA) Atmospheric General Circulation Model (AGCM), using two types of prescribed sea surface temperature (SST). One set of seasonal prediction simulations is called “SMIP” (which stands for “Seasonal prediction Model Intercomparison Project”). In the SMIP, observed SSTs are prescribed. The other set is called “HINDCAST”. In the HINDCAST, SST anomalies at initial time are assumed to persist during the forecast period. The December-January-February (DJF) averaged precipitation predictability over the tropical Pacific in SMIP is higher than that in HINDCAST, as was expected. However, it was found that the June-July-August (JJA) averaged precipitation predictability over the western tropical Pacific in SMIP was lower than that in HINDCAST. In the western tropical Pacific, there is a negative correlation between the observed precipitation anomalies and SST anomalies in JJA. The observed precipitation anomalies in early summer (May-June-July) are well correlated with the SST anomalies in spring (March-April-May). The simulated precipitation anomalies are strongly influenced by local SST anomalies in the same period. Because of this observed lag-correlation between precipitations and SSTs, and the property ofsimulated precipitation by the model, JJA averaged precipitation predictability over the western tropical Pacific in HINDCAST is higher than that in SMIP.