User:Cocoabon/sandbox

= Narrow cold-frontal rainbands = A narrow cold-frontal rainband (NCFR) is an organization of kilometers-wide narrow and heavy precipitations which occur with and move along with surface cold fronts. Such rainbands can have a vertical extent of approximately 2 to 6 km.

Commonly, the heaviest precipitation coming from the NCFR takes place in various elliptically shaped “precipitation cores” (PCs), coincidentally with areas of strong convergence on the surface. Main mechanisms for the formation of NCFRs and PCs are convective instabilities, forced lifting and gravity currents.

Surface convergence, deep wind shear and low-level jet can be observed alongside NCFRs. During the passage of PCs, a sequence of pressure changes (pressure checks), wind shift, peak rain rate and temperature drops can occur.

The mechanisms of NCFR are better understood than its hazards and predictability. Mesoscale numerical weather prediction models and mesoscale models are deemed as confident models for forecasting NCFR events. Knowledge about synoptic scale forcing and large scale dynamically active events can provide forecasters with confidence at predicting NCFR events with multiday lead time. Improvements in terms of resolving the finescale structure within NCFRs and assessing rain intensities are still required for a better predictability in local scales and prevention of life-threatening hazards.

Formation of NCFR and precipitation
NCFRs are formed by converging air at the leading edge of a cold front which leads to updrafts. It takes place above the wind shift zone of the cold front. A cloud band formed by the updraft contains a large amount of liquid water. The source of moisture in the updrafts are originated from a low-level jet ahead of and parallel to the cold front. Hails may be formed from the cloud band. The updraft is coupled with a system of downdraft. Heaviest precipitation within the NCFR of approximately 100 mm per hour can occur within this system of downdraft. (HERE REFERENCE TO IMAGE). In radar images, NCFRs can be identified as elongated bands of reflectivity larger than 40 to 50 dBZ.

Precipitation pattern and characteristics
Areas of heaviest precipitation are organized into ellipsoidal shapes, which are the PCs. These are regularly shaped and oriented at angles of 29 to 35 degrees to the surface cold front. Areas with reduced convergence between the PCs have weaker precipitation. Such areas are called the “gap regions” (GRs).

Studies have shown that PCs are located in areas of prominent updraft and strong surface convergence behind the wind shift zone. Convergence, which decreases with increasing height, and updraft occur inside the wind shift zone. Similar to observed strong cold fronts, vertical airflow within PCs can show an alternating pattern of updraft and downdraft.

In terms of the horizontal wind near the PCs, they can move perpendicularly to the synoptic scale front with the same speed as the front. Along the synoptic-scale front, PCs can move with the mean wind on both sides of the front. The wind shift zone shows a similar meso-scale pattern as the cold front. A low-level jet in the parallel wind component to the PCs can be observed ahead of the wind shift zone at higher altitudes (approximately 1.8 km). At lower altitudes, the parallel component decreases, with decreasing altitude due to surface friction.

Dynamical differences between PCs and GRs can be implied by the difference in horizontal shears. While strong cyclonic shears are centered within the wind shift zone ahead of the PCs, the horizontal shears parallel to GRs are weaker than those of the PCs. The overall low-level flow associated with the PCs is dominated by air coming from the warm sector ahead of the wind shift zone into the PCs. For GRs, the relative flow consists of a southerly component in the warm sector and a northerly component in the cold air. Both components are approximately parallel to the wind shift zone.

Meteorological impact
The sequence of changes in meteorological variables during the passage of PCs confirms the dynamical structure and precipitation pattern associated with the passage of a NCFR. Observations have shown that pressure checks and wind shift occur before the peak in rain rate. The wind shift and pressure check are signs of disturbance in higher altitudes, as cold air moves forwards. Surface observations before the passage of PCs are similar to the outflow of cold air from a squall line. Furthermore, there are also similarities between mesoscale lows associated with PCs, and bow echoes associated with downbursts and tornado development.

The sequence of changes coincident with the passage of GRs depends on whether the measurement station was in the precipitation area. Pressure check and wind shift can occur before or simultaneously with the temperature drop, and there is no peak in rain rate, if a precipitation area did not pass through the measurement station. If a precipitation area passes through the station, the peak in rain rate occurs before the pressure check, wind shift and temperature drop.

Post-wildfire debris flows, shallow landslides and flash flooding due to short and intense rainfall events such as the NCFRs often are the main dangers to life, property, and infrastructure in southern California.

Mechanisms for formations of NCFRs and PCs
Studies and authors have shown that the organization of convection in NCFR is due to instability caused by wind shear, as cold front reaches the surface. PCs can be the result of instability due to strong horizontal shears of the wind component parallel to the front. However, instability is insufficient for shaping the convective organization of NCFR. In addition, forced convection, triggered by strong convergence at the leading edge of the cold front, is another reason for producing updrafts at the cold front.

Initial convection at the cold front, which is assumed to be a uniform line of convection, is one of the possible drivers for the formation of PCs. A wave-shaped perturbation along the front can then enhance convergence, which then leads to updrafts and precipitations, then forming PCs. Regions on the front, where the flow is weakly convergent or even divergent, are the GRs.

On small meso-scale, the shape of the surface cold front is similar to observed gravity currents in tank experiments. Case study shows that forced convection and convection due to convective instability can be present near the cold front. The former is due to gravity-current of cold air lifting the warm air, the latter due to overhang of cold air in the cleft regions along the cold front.

Predictability and forecasting of NCFRs
A configuration of the Weather Research and Forecasting (WRF) Model successfully resolved the presence of a NCFR event in southern California, which occurred in San Diego, California on the 2nd of February 2019. The WRF model consists of an outer grid space of 9 km and an inner grid space of 3 km. Outputs from the WRF Model ensemble profiles and the data from radiosondes deployed at 30 minutes intervals during the NCFR passage over San Diego showed similarity in the relative magnitude of winds and frontal structure. The frontal structure included the horizontal wind shear, reduction of wind speed and water vapor after the front passage. Simulated profiles remained representative 2 days after the model's initialization. NCFR's association with the presence of a low-level jet, large drop in potential temperature field by >2 K, convergence and release of instability at the frontal boundary, and its depth could already be well captured by the model.

Model ensemble's ability to capture the primary components of NCFR's development encourages the usage of NWP in forecasting NCFR. However, the structure of PCs and GRs, which is relevant for publishing hazard warnings, were not well resolved. Despite an adequate simulation of the frontal structure, the 3 km resolution is insufficient for resolving the detailed structure of NCFRs. The variation in NCFR's propagation, structure, and intensity was large across ensemble members. The region of simulated largest reflectivity also varied across different ensemble members. Ensemble analyses also showed high sensitivity in timing of the NCFRs. The NCFR was also only observed until it was approximately 100 to 200 km away from the coastal radars, but conditions for the development of the NCFR were present well offshore, such as strong frontogenesis.

Hence, improvements of representing variability of NCFRs at small scales will improve forecasting of hazard precipitation events.