Monthly Weather Review

Davis, Christopher A.; Trier, Stanley B.

doi: 10.1175/MWR3398.1pmid: N/A

Five cases of mesoscale convective vortices (MCVs) are described from observations collected during the Bow Echo and MCV Experiment (BAMEX) over the central United States during the period from 20 May to 6 July 2003. In the present paper, the kinematic and thermodynamic structure of each vortex and its environment are emphasized. Data consist of BAMEX dropsondes, the National Oceanic and Atmospheric Administration profiler network, and National Weather Service soundings. In addition, Weather Surveillance Radar-1988 Doppler observations documented the signatures of nascent MCVs within nocturnal convection systems as well as the spatial pattern of convection within MCVs during the following day. The vertical structure of each vortex was highly dependent on the vertical shear and the presence of upper-tropospheric cyclonic vorticity anomalies. In strong shear, a pronounced downshear tilt of the vortex was evident, but with the presence of an upper-tropospheric trough, the tilt was upshear in the upper troposphere. In only one case did the tangential velocity of the vortex greatly exceed the vertical shear across its depth, and thus the vortex could maintain itself against the shear. The vortices were generally deep structures, extending through 5–8 km in all cases and maximizing their tangential winds between 550 and 600 hPa. In one of the five cases, vertical penetration into the boundary layer was unambiguous. Lower-tropospheric virtual potential temperature anomalies were generally 1–2 K, greatest when not directly beneath the midtropospheric MCV center but rather on its upshear and downshear flanks. Upper-tropospheric warm anomalies were found above and downshear from the midtropospheric MCV center, with a cool anomaly upshear, the latter being stronger in cases with an upshear tropopause-based trough. A diagnostic balance calculation was performed and indicated that the temperature anomalies were approximately balanced on the scale of the vortex.

Trier, Stanley B.; Davis, Christopher A.

doi: 10.1175/MWR3399.1pmid: N/A

Observations from the Bow Echo and Mesoscale Convective Vortex (MCV) Experiment are used to examine the role of the five mesoscale convective vortices described in Part I on heavy precipitation during the daytime heating cycle. Persistent widespread stratiform rain without deep convection occurs for two strong MCVs in conditionally stable environments with strong vertical shear. Two other MCVs in moderate-to-strong vertical shear have localized redevelopment of deep convection (termed secondary convection) on their downshear side, where conditional instability exists. The strongest of the five MCVs occurs in weak vertical shear and has widespread secondary convection, which is most intense on its conditionally unstable southeast periphery. The two MCVs with only localized secondary convection have well-defined mesoscale vertical motion couplets with downshear ascent and upshear descent above the planetary boundary layer (PBL). Although the amplitude is significantly greater, the kinematically derived vertical motion dipole resembles that implied by steady, vortex-relative isentropic flow, consistent with previous idealized (dry) simulations and diagnoses based on operational model analyses. In the other three cases with either widespread precipitation or weak environmental vertical shear, the kinematic and isentropic vertical motion patterns are poorly correlated. Vertical motions above the PBL provide a focus for secondary convection through adiabatic cooling downshear and adiabatic warming upshear of the MCV center. The MCVs occur within surface frontal zones with large temperature and moisture gradients across the environmental vertical shear vector (Part I). Thus, the effect of vertical motions on conditional instability is reinforced by horizontal advections of high equivalent potential temperature air downshear, and low equivalent potential temperature air upshear within the PBL. On average, the quadrant immediately right of downshear (typically southeast of the MCV center) best supports deep convection because of the juxtaposition of greatest mesoscale ascent, high equivalent potential temperature PBL air, and MCV-induced enhancement of the vertical shear.

Schultz, David M.; Knox, John A.

doi: 10.1175/MWR3400.1pmid: N/A

Several east–west-oriented bands of clouds and light rain formed on 20 July 2005 over eastern Montana and the Dakotas. The cloud bands were spaced about 150 km apart, and the most intense band was about 20 km wide and 300 km long, featuring areas of maximum radar reflectivity factor of about 50 dB Z . The cloud bands formed poleward of an area of lower-tropospheric frontogenesis, where air of modest convective available potential energy was being lifted. During initiation and maintenance of the bands, mesoscale regions of dry symmetric and inertial instability were present in the region of the bands, suggesting a possible mechanism for the banding. Interpretation of the extant instabilities in the region of the bands was sensitive to the methodology to assess the instability. The release of these instabilities produced circulations with enough vertical motion to lift parcels to their lifting condensation level, resulting in the observed cloud bands. A high-resolution, numerical weather prediction model demonstrated that forecasting these types of events in such real-time models is possible, although the timing, evolution, and spacing of the bands were not faithfully reproduced. This case is compared to two previous cases in the literature where banded convection was associated with a combination of conditional, symmetric, and inertial instability.

Sipprell, Benjamin D.; Geerts, Bart

doi: 10.1175/MWR3354.1pmid: N/A

High-resolution airborne cloud radar data and other International H 2 O Project datasets are used to describe the vertical structure of an unusual prefrontal dryline. This dryline, observed in northwestern Kansas on 19 June 2002, first progressed eastward and tilted toward the west, and later became more stationary and reversed its tilt, toward the moist side. The convective boundary layer (CBL) depth difference also reversed: only in the later phase did the dry-side CBL become deeper than on the moist side. Echo and single/dual-Doppler velocity data in a vertical transect across the dryline suggest a solenoidal circulation dynamically consistent with the observed horizontal buoyancy gradient. Both this gradient and the solenoidal circulation reversed in the later phase. Simultaneously, confluence toward the dryline increased, resulting in an increasing moisture gradient as well as a deepening CBL in the dryline convergence zone. It is speculated that the baroclinically generated horizontal vorticity contributed to this CBL deepening, as the sign of this vorticity was opposite to that of the low-level wind shear on the opposite side of the dryline in both phases. Deep-convective initiation appears to have resulted from this local CBL deepening, leading to a total elimination of convective inhibition near the dryline.

Byun, Young-Hwa; Hong, Song-You

doi: 10.1175/MWR3397.1pmid: N/A

This study describes a revised approach for the subgrid-scale convective properties of a moist convection scheme in a global model and evaluates its effects on a simulated model climate. The subgrid-scale convective processes tested in this study comprise three components: 1) the random selection of cloud top, 2) the inclusion of convective momentum transport, and 3) a revised large-scale destabilization effect considering synoptic-scale forcing in the cumulus convection scheme of the National Centers for Environmental Prediction medium-range forecast model. Each component in the scheme has been evaluated within a single-column model (SCM) framework forced by the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment data. The impact of the changes in the scheme on seasonal predictions has been examined for the boreal summers of 1996, 1997, and 1999. In the SCM simulations, an experiment that includes all the modifications reproduces the typical convective heating and drying feature. The simulated surface rainfall is in good agreement with the observed precipitation. Random selection of the cloud top effectively moistens and cools the upper troposphere, and it induces drying and warming below the cloud-top level due to the cloud–radiation feedback. However, the two other components in the revised scheme do not play a significant role in the SCM simulations. On the other hand, the role of each modification component in the scheme is significant in the ensemble seasonal simulations. The random selection process of the cloud top preferentially plays an important role in the adjustment of the thermodynamic profile in a manner similar to that in the SCM framework. The inclusion of convective momentum transport in the scheme weakens the meridional circulation. The revised large-scale destabilization process plays an important role in the modulation of the meridional circulation when this process is combined with other processes; on the other hand, this process does not induce significant changes in large-scale fields by itself. Consequently, the experiment that involves all the modifications shows a significant improvement in the seasonal precipitation, thereby highlighting the importance of nonlinear interaction between the physical processes in the model and the simulated climate.

Basu, B. K.

doi: 10.1175/MWR3355.1pmid: N/A

Satellite-derived hourly precipitation values over India and neighboring areas are examined during the summer monsoon season of 2004 to determine the observed patterns of diurnal variations. These are compared with the patterns found in the forecasts from the global spectral model in operation at the National Centre for Medium Range Weather Forecasting in India. The observed hourly precipitation shows that maximum amounts are recorded over most areas of India during the afternoon hours, coinciding with the maximum in surface temperature. This pattern is modified in areas where local mesoscale events like katabatic winds or land–sea breezes produce strong convergence patterns and associated convection. The model forecasts weaken the mesoscale effects on precipitation and the convection due to ground heating seems to start in the model 2–3 h before the time it is observed by the satellites. The frequency and amount of precipitation increases with the forecast length but the hour of maximum precipitation remains almost the same. Harmonic analysis of the frequency of observed precipitation shows that the diurnal cycle predominates in both magnitude and the amount of variance explained. The semidiurnal cycle is considerably smaller in magnitude and explains significant variance only over a small area. Other cycles of smaller periodicity are unimportant in the diurnal variation of precipitation. A similar result is also obtained for the model forecasts except that the spatial distributions of amplitude and variance explained are different from that obtained from the observed data. The spatial distribution and values remain almost the same with forecast length.

West, Gregory L.; Steenburgh, W. James; Cheng, William Y. Y.

doi: 10.1175/MWR3375.1pmid: N/A

Spurious grid-scale precipitation (SGSP) occurs in many mesoscale numerical weather prediction models when the simulated atmosphere becomes convectively unstable and the convective parameterization fails to relieve the instability. Case studies presented in this paper illustrate that SGSP events are also found in the North American Regional Reanalysis (NARR) and are accompanied by excessive maxima in grid-scale precipitation, vertical velocity, moisture variables (e.g., relative humidity and precipitable water), mid- and upper-level equivalent potential temperature, and mid- and upper-level absolute vorticity. SGSP events in environments favorable for high-based convection can also feature low-level cold pools and sea level pressure maxima. Prior to 2003, retrospectively generated NARR analyses feature an average of approximately 370 SGSP events annually. Beginning in 2003, however, NARR analyses are generated in near–real time by the Regional Climate Data Assimilation System (R-CDAS), which is identical to the retrospective NARR analysis system except for the input precipitation and ice cover datasets. Analyses produced by the R-CDAS feature a substantially larger number of SGSP events with more than 4000 occurring in the original 2003 analyses. An oceanic precipitation data processing error, which resulted in a reprocessing of NARR analyses from 2003 to 2005, only partially explains this increase since the reprocessed analyses still produce approximately 2000 SGSP events annually. These results suggest that many NARR SGSP events are not produced by shortcomings in the underlying Eta Model, but by the specification of anomalous latent heating when there is a strong mismatch between modeled and assimilated precipitation. NARR users should ensure that they are using the reprocessed NARR analyses from 2003 to 2005 and consider the possible influence of SGSP on their findings, particularly after the transition to the R-CDAS.

Jacob, S. Daniel; Koblinsky, Chester J.

doi: 10.1175/MWR3366.1pmid: N/A

The effect of precipitation on the upper-ocean response during a tropical cyclone passage is investigated using a numerical model in this paper. For realistic wind forcing and empirical rain rates based on satellite climatology, numerical simulations are performed with and without precipitation forcing to delineate the effects of freshwater forcing on the upper-ocean heat and salt budgets. Additionally, the performance of five mixing parameterizations is also examined for the two forcing conditions to understand the sensitivity of simulated ocean response. Overall, results from 15 numerical experiments are analyzed to quantify the precipitation effects on the oceanic mixed layer and the upper ocean. Simulated fields for the same mixing scheme with and without precipitation indicate a decrease in the upper-ocean cooling of about 0.2°–0.5°C. This is mainly due to reduced mixing of colder water from below induced by the increased stability of the added freshwater. The cooler rainwater contributes a maximum of approximately 10% to the total surface heat loss from the ocean. The rate of freshening due to precipitation exceeds the rate of mixing of the more saline water from below, leading to a change in sign of the mixed layer salinity response. As seen in earlier studies, large uncertainty exists in the simulated upper-ocean response due to the choice of mixing parameterization. Although the nature of simulated response remains similar for all the mixing schemes, the magnitude of freshening and cooling varies by as much as 0.5 psu and 1°C between the schemes to the right of the storm track. While changes in the mixed layer and in the top 100 m of heat and salt budgets are strongly influenced by the choice of mixing scheme, integrated budgets in the top 200 m are seen to be affected more by advection and surface fluxes. However, since the estimated surface fluxes depend upon the simulated sea surface temperature, the choice of mixing scheme is crucial for realistic coupled predictive models.

Sun, Chaojiao; Rienecker, Michele M.; Rosati, Anthony; Harrison, Matthew; Wittenberg, Andrew; Keppenne, Christian L.; Jacob, Jossy P.; Kovach, Robin M.

doi: 10.1175/MWR3405.1pmid: N/A

Two global ocean analyses from 1993 to 2001 have been generated by the Global Modeling and Assimilation Office (GMAO) and Geophysical Fluid Dynamics Laboratory (GFDL), as part of the Ocean Data Assimilation for Seasonal-to-Interannual Prediction (ODASI) consortium efforts. The ocean general circulation models (OGCM) and assimilation methods in the analyses are different, but the forcing and observations are the same as designed for ODASI experiments. Global expendable bathythermograph and Tropical Atmosphere Ocean (TAO) temperature profile observations are assimilated. The GMAO analysis also assimilates synthetic salinity profiles based on climatological T – S relationships from observations (denoted “TS scheme”). The quality of the two ocean analyses in the tropical Pacific is examined here. Questions such as the following are addressed: How do different assimilation methods impact the analyses, including ancillary fields such as salinity and currents? Is there a significant difference in interpretation of the variability from different analyses? How does the treatment of salinity impact the analyses? Both GMAO and GFDL analyses reproduce the time mean and variability of the temperature field compared with assimilated TAO temperature data, taking into account the natural variability and representation errors of the assimilated temperature observations. Surface zonal currents at 15 m from the two analyses generally agree with observed climatology. Zonal current profiles from the analyses capture the intensity and variability of the Equatorial Undercurrent (EUC) displayed in the independent acoustic Doppler current profiler data at three TAO moorings across the equatorial Pacific basin. Compared with independent data from TAO servicing cruises, the results show that 1) temperature errors are reduced below the thermocline in both analyses; 2) salinity errors are considerably reduced below the thermocline in the GMAO analysis; and 3) errors in zonal currents from both analyses are comparable. To discern the impact of the forcing and salinity treatment, a sensitivity study is undertaken with the GMAO assimilation system. Additional analyses are produced with a different forcing dataset, and another scheme to modify the salinity field is tested. This second scheme updates salinity at the time of temperature assimilation based on model T – S relationships (denoted “T scheme”). The results show that both assimilated field (i.e., temperature) and fields that are not directly observed (i.e., salinity and currents) are impacted. Forcing appears to have more impact near the surface (above the core of the EUC), while the salinity treatment is more important below the surface that is directly influenced by forcing. Overall, the TS scheme is more effective than the T scheme in correcting model bias in salinity and improving the current structure. Zonal currents from the GMAO control run where no data are assimilated are as good as the best analysis.

Zehnder, Joseph A.; Hu, Jiuxiang; Razdan, Anshuman

doi: 10.1175/MWR3401.1pmid: N/A

This paper describes a technique for photogrammetric analysis of stereo pairs of images that is applied to the study of orographic convection. The technique is designed for use with digital images and assumes detailed knowledge of the camera properties (focal length and imaging chip) and that the position and orientation are known as a first guess. An iterative scheme using known landmarks on the frame is used to determine the camera orientation. The scheme is accurate to 10–100 m at a distance of 15 km from the camera pair. The transition from shallow to deep convection over the Santa Catalina Mountains in southern Arizona on 26 July 2005 is presented. The three-dimensional structure of the visible portion of the cloud is determined and compared with the composite reflectivity from the National Weather Service Weather Surveillance Radar-1988 Doppler radar and the tropopause height from the 1200 UTC sounding in Tucson, Arizona, providing additional validation of the scheme. The shallow to deep transition is characterized by tracking individual turrets and determining the maximum height of the cloud top. The cloud tops were limited to beneath 6000 m MSL for the first 1.5 h followed by the development of deep convection. The motion of the turrets and location of the eventual deep convection were consistent with the idea that moistening by shallow convection conditions the atmosphere for further development.