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Yanping Li and R. E. Carbone

Abstract

The authors have examined 4 years of satellite-derived SST and rainfall data in anticipation of a relationship between SST structure and the excitation of convective rainfall. The results exhibit a strong excitation signal consistent with the presence of mesoscale SST gradients in about 75% of approximately 10 000 rainfall onset events. Rainfall onset events occur at locations with enhanced horizontal convergence, as inferred by the Laplacian of SST on scales of order 100 km. The daily SST field exhibits multiscale patchiness, spanning a 2+°C range. The signal is disproportionately large at SSTs that are 0.25°C above the mean, near 29.5°C; disproportionately weak for SST ≤ 28.8°C; and proportionately neutral for SST ≥ 30.3°C. The calculations suggest that a characteristic strength of this lower-boundary forcing (~3 × 10−5 s−1) is approximately one order of magnitude stronger than the mean regional background forcing (~3 × 10−6 s−1). The periphery of warm oceanic patches exhibits both convergent and divergent Laplacian values of similar frequency and magnitude; however, rainfall onset favors the locally convergent locations by a 3:1 ratio.

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Yanping Li and Ronald B. Smith

Abstract

Harmonic analysis of pressure, temperature, and precipitation data from 1000 Automated Surface Observing System (ASOS) stations reveals a mix of stationary and east–west moving disturbances east of the Rockies. Optimization of the pressure data using a “temperature-based tide assumption” separates a strong sun-following continentally enhanced tide from a smaller eastward-propagating wave (EPW). The latter signal moves at a similar speed to the previously discovered eastward-moving precipitation systems. Analysis of ASOS summer precipitation data confirms eastward propagation, but east of 90°W it shows nonpropagating diurnal convection at a fixed local time (i.e., 1800 LST). Analysis of winter days still finds the EPW, suggesting that it is the cause and not the result of the propagating precipitation.

A possible mechanism for the EPW is developed from the linear Bousinesq equations with heating and wind shear. Solutions show eastward-moving diurnal pulses of potential vorticity (PV) generated by imposed heating over the Rockies. Because of the background shear, these pulses produce vertical motion in the lower troposphere.

The PV hypothesis for precipitation propagation was tested with North American Regional Reanalysis (NARR) data. Diurnal drifting thermal and PV anomalies are clearly found near the 500- and 600-hPa levels in both winter and summer. In winter, the PV signal is weaker, moves faster, and does not influence precipitation. The existence of the winter PV signal again suggests that it is the cause, not the effect, of summer propagating precipitation.

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Yanping Li and Ronald B. Smith

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Harmonic analysis of summer Automated Surface Observing System (ASOS) data over North America shows sun-following diurnal temperature and pressure oscillations with amplitudes increasing in the western United States (i.e., 5–8 K and 60–120 hPa, respectively) due to larger sensible heating in the dryer western terrains. The phases of temperature and pressure (i.e., 220° and 110°) are constant with longitude after an interfering eastward propagating wave is subtracted. Tidal amplitudes and phases shift significantly with season.

A linear Boussinesq model with thermal forcing can reproduce these observed oscillations with properly selected parameters. The model neglects global effects to focus on a single transect across a single ideal continent. A damping parameter α ranging from 5 × 10−5 to 9 × 10−5 s−1, comparable to the inertia and Coriolis parameters, is needed to explain the temperature phase lag relative to local solar noon (40°–50°C). The phase lag between surface pressure minimum and temperature maximum (45°–70°C) requires a 3–5-h time delay between surface and elevated heating. The ratio of pressure and temperature amplitude requires a heating depth varying between 550 (winter) and 1250 m (summer). Both the heating delay and depth are consistent with a vertical heat diffusivity of about K = 10 m2 s−1 in winter, but K theory gives inconsistent summer K values. The observed tide amplitude requires diurnal heating amplitudes in the range of 100–250 W m−2.

When the model is applied to an inhomogeneous continent, it is possible to obtain a clearer idea of how wide a region must be to approach the tidal (i.e., long-wave) limit. Traveling diurnal heating generates gentle tides over the large uniform interior regions but causes vigorous sea breezes and mountain–plain circulations in regions of heating gradient. These gradient regions have significant vertical motions and are moderately sensitive to the Coriolis force and the mean wind speed. Surprisingly, these local circulations do not alter the phases of the temperature and pressure oscillations, in agreement with observations.

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R. E. Carbone and Yanping Li

Abstract

Based upon on the findings of Y. Li and R. E. Carbone, the association of tropical rainfall with SST structure is further explored, with emphasis on the MJO passband. Analyses include the tropical Indian Ocean, Maritime Continent, and tropical western Pacific regions. The authors examine the anomalies of and correlations between SST structure, the frequency of rainfall events, and rainfall amount. Based on detailed examination of a 49-month time series, all findings are statistical inferences and interpretations consistent with established theory.

The statistical inferences are broadly consistent with a pivotal role played by the convergent Laplacian of SST together with an expected, but somewhat indirect, role of SST itself. The main role of SST in the MJO passband appears limited to production of moist static energy, which is highly correlated with cumulative precipitation, yet bears a decidedly conditional relationship to the occurrence of rainfall. If rain occurs, then more rain is likely over warmer SST. The convergent Laplacian of SST is strongly associated with the onset of rainfall, apparently through its capacity to induce vertical air motion with sufficient kinetic energy to overcome convective inhibition in a conditionally unstable troposphere. The convergent Laplacian of SST is directly associated with the location and the variability of rainfall event frequency while having a less direct relationship to cumulative rainfall. These nuanced interpretations of rainfall forcing by the Laplacian of SST, and conditional modulation of cumulative rainfall by SST, may underlie systematic errors in highly parameterized models as a consequence of variable asymmetry in the field of Laplacian anomalies.

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Yanping Li and R. E. Carbone

Abstract

This work focuses on the seaward propagation of coastal precipitation with and without mountainous terrain nearby. Offshore of India, diurnal propagation of precipitation is observed over the Bay of Bengal. On the eastern side of the bay, a diurnal but nonpropagating signal is observed near the west coast of Burma. This asymmetry is consistent with the inertio-gravity wave mechanism. Perturbations generated by diurnal heating over the coastal mountains of India propagate offshore, amplify in the upwind direction, and dissipate in the downwind direction relative to the steering wind, owing to critical-level considerations. A linear model is applied to evaluate sensitivity to gravity waves, as these affect deep moist convection and propagation. Analyses are performed for various heating depths, mountain widths, stability, Coriolis effect, background mean wind, and friction. Calculations reveal how these factors affect the amplitude, dissipation, initiation phase, and propagation speed of the diurnal disturbance. The propagation of precipitation triggered by land–sea breezes is distinguishable from that triggered by a mountain–plains circulation. Convection resulting purely from mountain heating begins earlier, propagates slower, and damps faster than that of the land–sea breeze. For mountains near a coast, slower propagation and stronger earlier convection result from a resonance-like combination of two dynamical mechanisms. The propagation of precipitation is initially triggered by the mountain breeze near the coastal mountain. Over the open ocean, the dominant signal propagates as that of the land breeze but with stronger convection.

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Yanping Li, Ronald B. Smith, and Vanda Grubišić

Abstract

Harmonic analysis has been applied to data from nearly 1000 Automatic Surface Observation System (ASOS) stations over the United States to extract diurnal pressure signals. The largest diurnal pressure amplitudes (∼200 Pa) and the earliest phases (∼0600 LST for surface pressure maximum) were found for stations located within deep mountain valleys in the western United States. The origin of these unique characteristics of valley pressure signals is examined with a detailed study of Owens Valley, California. Analysis of observational data from the Terrain-Induced Rotor Experiment (T-REX) project shows that the ratio of the valley surface pressure to temperature amplitude can be used to estimate the daily maximum mixed-layer depth H. On days with strong westerly winds above the valley, the mixed layer is found to be shallower than on quiescent days because of a flushing effect in the upper parts of the valley. Idealized two-dimensional Weather Research and Forecasting Model simulations were used to explain the pressure signal. In agreement with observations, the simulations show a 3-h difference between the occurrence of a surface pressure minimum (1800 LST) and a surface temperature maximum (1500 LST). The resolved energy budget analysis reveals that this time lag is caused by the persistence of subsidence warming in the upper part of the valley after the surface begins to cool. Sensitivity tests for different valley depths and seasons show that the relative height of the mixed-layer depth with respect to the valley depth, along with the valley width-to-depth ratio, determine whether the diurnal valley circulation is a “confined” system or an “open” system. The open system has a smaller pressure amplitude and an earlier pressure phase.

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Yanping Li, Kit Szeto, Ronald E. Stewart, Julie M. Thériault, Liang Chen, Bohdan Kochtubajda, Anthony Liu, Sudesh Boodoo, Ron Goodson, Curtis Mooney, and Sopan Kurkute

Abstract

A devastating, flood-producing rainstorm occurred over southern Alberta, Canada, from 19 to 22 June 2013. The long-lived, heavy rainfall event was a result of complex interplays between topographic, synoptic, and convective processes that rendered an accurate simulation of this event a challenging task. In this study, the Weather Research and Forecasting (WRF) Model was used to simulate this event and was validated against several observation datasets. Both the timing and location of the model precipitation agree closely with the observations, indicating that the WRF Model is capable of reproducing this type of severe event. Sensitivity tests with different microphysics schemes were conducted and evaluated using equitable threat and bias frequency scores. The WRF double-moment 6-class microphysics scheme (WDM6) generally performed better when compared with other schemes. The application of a conventional convective/stratiform separation algorithm shows that convective activity was dominant during the early stages, then evolved into predominantly stratiform precipitation later in the event. The HYSPLIT back-trajectory analysis and regional water budget assessments using WRF simulation output suggest that the moisture for the precipitation was mainly from recycling antecedent soil moisture through evaporation and evapotranspiration over the Canadian Prairies and the U.S. Great Plains. This analysis also shows that a small fraction of the moisture can be traced back to the northeastern Pacific, and direct uptake from the Gulf of Mexico was not a significant source in this event.

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