A modelling case-study of soil moisture atmosphere coupling

Transcription

1 QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Q. J. R. Meteorol. Soc. 136(s1): (2010) Published online 8 December 2009 in Wiley InterScience ( DOI: /qj.541 A modelling case-study of soil moisture atmosphere coupling M. A. Gaertner, a *M.Domínguez b and M. Garvert c a Environmental Sciences Faculty, University of Castilla-La Mancha, Toledo, Spain b Environmental Sciences Institute, University of Castilla-La Mancha, Toledo, Spain c 3TIER North America, Seattle, Washington, USA ABSTRACT: A sensitivity analysis of a moist convection case-study of the 2005 West African monsoon has been performed with the limited-area model PROMES. In the control simulation, soil moisture was initialized based on European Centre for Medium-Range Weather Forecasts (ECMWF) soil moisture. A relatively wet patch appears over an area that was affected later by the passage of a squall line. The control simulation is able to reproduce a squall line that crosses over this wet surface, which makes it possible to analyse the sensitivity of the modelled convective system to a soil moisture increase up to saturation in the cited patch. The wetter land surface in the sensitivity run generates a cooler and moister area in the near-surface atmospheric fields compared to the control run, and the surface wind differences between both runs show a divergent pattern. The humidity and temperature differences follow the diurnal cycle of the monsoon flow, as the cool and moist atmospheric perturbation remains over the wet patch during the afternoon, and moves towards the northeast during the night. There is a clear interaction between the soil moisture perturbation and an approximating African Easterly Wave. This interaction generates larger differences between the two runs on the second simulation day than on the first. The precipitation is reduced over the wet patch in the sensitivity run, but over a larger area a precipitation increase is obtained, reflecting a complex soil moisture-precipitation feedback. Copyright c 2009 Royal Meteorological Society KEY WORDS West African monsoon; squall line; soil moisture atmosphere interaction; mesoscale circulation; mesoscale numerical model; African Monsoon Multidisciplinary Analysis (AMMA) Received 16 December 2008; Revised 8 October 2009; Accepted 12 October Introduction Soil moisture plays an important role in the hydrological cycle in certain semi-arid regions like the West African Sahel (Koster et al., 2004). Over this region, the convective nature of precipitation leads to a high spatial variability in soil moisture. This generates in turn spatially variable surface fluxes that can affect atmospheric fields and even precipitation, through soil moisture-precipitation feedbacks. The soil moisture-precipitation feedback can be positive, when the additional atmospheric humidity from a positive soil moisture perturbation tends to generate higher precipitation over the wetter surface area. Persistent rainfall patterns were found in the Sahel by Taylor et al. (1997), with strong precipitation gradients over distances of about 10 km. These authors suggested the possibility that the associated planetary boundary layer (PBL) anomalies could locally enhance individual convective cells within larger-scale convective systems passing over the soil moisture anomalies. This feedback mechanism was analysed under idealized conditions with a numerical mesoscale model by Clark et al. (2003). Correspondence to: M. A. Gaertner, Environmental Sciences Faculty, University of Castilla-La Mancha, Toledo, Spain. miguel.gaertner@uclm.es Alternatively, the soil moisture precipitation feedback can be negative, as the cool anomaly developing over wet soils generates a sea-breeze-like circulation with subsidence and therefore reduced moist convection over the soil moisture anomaly. An observational example of such mesoscale atmospheric circulations induced by soil moisture over the Sahel is discussed by Taylor et al. (2007). Suppression of afternoon convection initiation above wet soils has been also observed over this region (Taylor and Ellis, 2006), in a consistent way with a negative soil moisture precipitation feedback mechanism. The sign of the soil moisture precipitation feedback can depend also on the development phase of a convective system. This is shown by Gantner and Kalthoff (2009), who obtain in their simulations a negative feedback for triggering of convection and a positive feedback for a mature system. An important feature of the West African monsoon is the diurnal cycle of the monsoon winds (Parker et al., 2005). In the afternoon, the turbulent mixing within the PBL causes weak winds, whereas during the night winds intensify, leading to a northward extension of the southwesterly monsoon flow and to the formation of a low-level jet. This diurnal cycle can also interact with surface-induced circulations, as proposed by Parker et al. (2005). Another feature of the West African monsoon that can interact with the land surface are the Copyright c 2009 Royal Meteorological Society

2 484 M. A. GAERTNER ET AL. African Easterly Waves, as discussed by Taylor and Clark (2001). Mesoscale convective systems generate a large fraction of total precipitation during the West African monsoon (Mathon et al., 2002). In this paper, we analyse the interaction of such a system (a squall line) with a soil moisture perturbation through a sensitivity study with a mesoscale numerical model. The method and data are described in section 2. The response to the soil moisture perturbation is analysed in section 3, including the interaction of the surface fluxes with an approaching African Easterly Wave and with the diurnal cycle of the monsoon circulation. The main conclusions are presented in section Method and data 2.1. Model description The two simulations of the sensitivity study have been performed with the limited-area model PROMES. This is a mesoscale atmospheric model developed by MOMAC (MOdelización para el Medio Ambiente y el Clima) research group at the Complutense University of Madrid (UCM) and the University of Castilla-La Mancha (UCLM) (Castro et al., 1993). It is a hydrostatic atmospheric model which uses pressure-based sigma coordinates in the vertical and a Lambert conformal projection for the Cartesian horizontal coordinates. An Arakawa C grid is used for the spatial arrangement of variables. For the lateral boundary conditions a Davies-type relaxation scheme (Davies, 1976) is used. Large-scale values are updated every 6 h. The vertical interpolation of the driving fields to model levels follows the procedure described in Gaertner and Castro (1996). The PROMES model uses a split-explicit integration scheme, based on Gadd (1978). The different terms of the equations are integrated with time steps depending on their typical time-scale. The absorption and scattering of short-wave radiation by clouds is based on the method proposed by Anthes et al. (1987), and long-wave radiation processes are parameterized according to Stephens (1978) and Garand (1983). Explicit cloud formation and precipitation processes are modelled with Hong et al. (2004) parameterization, which includes ice processes. Subgrid-scale convective clouds and their precipitation are parameterized using the Kain and Fritsch (1993) scheme, with some of the modifications described in Kain (2004). Turbulence is parameterized with a 1.5-order closure scheme, and a non-local and physically based mixing length is used, which is computed from the buoyancy and turbulent kinetic energy fields over the whole PBL (Cuxart et al., 2000). The exchanges between soil, vegetation and atmosphere are parameterized using the land-surface scheme SECHIBA (Schématisation des EChanges Hydriques à l Interface entre la Biosphère et l Atmosphère: Ducoudré et al., 1993; de Rosnay and Polcher, 1998). The version of SECHIBA used in the present study can include up to eight land surface types in one grid box: bare soil, tundra, grass, steppe, savannah, conifers, deciduous forest and tropical forest. The soil water content is calculated in two layers: a superficial layer and a subsurface layer. The upper reservoir has variable depth to allow rapid response of the evaporation to a shower, and it is created as soon as precipitation is larger than evaporation. The evaporation of soil moisture is controlled by transpiration (which combines root properties and soil moisture) and bare-soil evaporation. Even though several vegetation types can coexist in one grid box, they share the same atmospheric forcing and soil hydrology. Transpiration and interception losses are computed for each of the seven types of vegetation. Soil temperature is calculated in seven layers. Biosphere parameters such as albedo, roughness length or leaf area index evolve with a 15-day time step through the year Set-up of simulations For the present study, a domain of horizontal grid points covering the area approximately limited by 12 W 36 Eand2 S 27 N with 15 km horizontal grid size has been used. This domain has been selected so that it fully includes the area of formation of the mesoscale convective system which is a central element of this study. Thirty-six vertical levels up to 10 hpa with more resolution near the surface (8 levels in the lowest km), and a 10 points wide lateral relaxation zone, have been used. A 3-day period has been simulated, from 0000 UTC 27 August to 0000 UTC 30 August This period includes a mesoscale convective system which formed on 28 August over Nigeria, crossing the central part of the domain. A more detailed description of this mesoscale convective system can be found in Guichard et al. (2010). European Centre for Medium-Range Weather Forecasts (ECMWF) analysis data have been used for assigning initial and lateral boundary conditions. PROMES initial soil moisture and soil temperature data have been obtained from the corresponding ECMWF fields. Two simulations have been performed: a control simulation (CTRL hereafter), and a sensitivity simulation (WET hereafter) with a perturbed soil moisture field. In the control simulation, the initial soil moisture has been assigned through the model standard interpolation from ECMWF soil moisture. As can be seen in Figure 1 (which shows only the central part of the computational domain which is the focus of the analysis), the initial soil moisture has a typical north south gradient structure, but in the Sahel there is an increased soil moisture patch about 300 km long and 100 km wide in the area approximately limited by N and 2 5 E, probably as a result of previous convective precipitation. This patch is crossed by a squall line on 28 August, both in the observations and in the simulations. We have increased the initial soil moisture over this patch up to saturation (300 mm in SECHIBA land surface scheme) in the sensitivity run (WET simulation). This increase is an idealized one, probably larger than what is observed in reality after precipitation events, but it has been selected in order to detect

3 MODELLING SOIL MOISTURE ATMOSPHERE COUPLING 485 Figure 1. Initial values (0000 UTC 27 August 2005) of total soil moisture in mm (shaded) for CTRL simulation. The area enclosed by the thick black contour is the wet patch where the initial soil moisture is increased to saturation in the WET simulation. Country borders are superimposed. Figure h precipitation accumulation (upper frame) for the CTRL simulation, and 24 h precipitation difference (WET-CTRL, lower frame), in mm, for the period 0600 UTC 28 August 0600 UTC 29 August. The position of the wet patch is indicated by the thick black contour.

4 486 M. A. GAERTNER ET AL. Figure 3. Hourly precipitation (mm/h) averaged over the whole domain shown in Figure 2 (upper frame) and averaged only over the wet patch (lower frame) for the period 0600 UTC 28 August 0600 UTC 29 August. The continuous lines correspond to the CTRL simulation and the dashed lines to the WET simulation. as clearly as possible the impact of such a soil moisture perturbation on the moving squall line. The modelling of mesoscale convective systems over the Sahel is a difficult issue, and the availability of a realistic simulation of a squall line makes this an interesting case to analyse the interaction between the squall line and soil moisture Control simulation analysis The control simulation has been compared with observations in a different study (Guichard et al., 2010), and therefore we only summarise here the main aspects of that comparison. That study is a model intercomparison focused on precipitation and evaporation for the mesoscale convective system of August During 28 August, the PROMES CTRL simulation generates a propagating squall line with a timing that agrees well with the observed westward propagation of rain. The amount of precipitation is underestimated, as the simulation does not extend the precipitation enough to the north and fails in generating high precipitation near the Guinean coast. Regarding evapotranspiration, it is generally underestimated in the CTRL simulation, when compared to the offline simulations performed in the AMMA Land surface Model Intercomparison Project (ALMIP: Boone et al., 2009). This underestimation may be one reason for the too-low simulated precipitation. However, the latitudinal extension of the evapotranspiration (as measured by the 1 mm/day northern limit) is represented quite well by the model. This is a desirable feature when trying to analyse the impact of a wet patch located in the Sahel, as the evapotranspiration gradients are large under these conditions. In the following description, the term difference refers to changes (WET-CTRL) as a response to the imposed soil moisture perturbation. The analysed area

5 MODELLING SOIL MOISTURE ATMOSPHERE COUPLING 487 Figure 4. The 2 m specific humidity difference in g/kg (WET-CTRL) averaged over the period UTC 28 August. The corresponding wind difference at the surface in m/s (WET-CTRL) is superimposed. The wind scale is indicated by the arrow below the frame. is the part of the whole simulation domain indicated in Figure Response to the soil moisture perturbation 3.1. Impact on near-surface atmospheric fields and precipitation In the first day of the simulation (27 August), widespread evening convection with weak rainfall amounts was observed, as explained by Guichard et al. (2010). The lack of organized convection is reflected in PROMES simulations (not shown). The impact of the soil moisture perturbation on atmospheric fields is simulated mainly over the wet patch. As this impact is qualitatively similar to (but smaller than) the differences simulated during the second day, no figures are shown for the first day. The interaction between the simulated squall line and the wet patch occurs during the second day of the simulation, and the following analysis is focused on this day. Total CTRL precipitation for the period 0600 UTC 28 August to 0600 UTC 29 August is shown in Figure 2. During this period a squall line is generated in CTRL, passing over the wet patch from east to west during the night. Precipitation differences for the same period between the two simulations (WET CTRL) show a basically dipolar structure, with a reduction of precipitation over the wet patch and near its southern border, while a precipitation increase is found farther south. As shown in Figure 3 (upper frame), the hourly evolution of precipitation averaged over the whole domain of Figure 2 shows more increases than decreases. In contrast, if the same average is calculated only over the wet patch (lower frame of Figure 3), a clear precipitation decrease is obtained in the WET simulation. The soil moisture precipitation feedback is therefore complex: locally negative over the wet patch, but globally positive if we look at a larger area. The evolution in time of the differences (WET-CTRL) of near-surface fields and precipitation offers some insight into how the interaction between the wet patch and the squall line occurs. We show the results at two times: when the modelled squall line is approaching the wet patch ( UTC 28 August) and when the squall line is passing over the wet patch ( UTC 28 August). Figure 4 shows the 2 m specific humidity difference, averaged over the period UTC. The 2 m temperature difference (not shown) has a very similar spatial distribution and indicates a decrease of 2 3 K, which is larger than during the first day. The humidity difference is also larger than during the first day and reaches values of 2 3 g/kg over the wet patch. The surface wind differences are superimposed and show a divergent pattern. It is noteworthy that these differences are not symmetric with respect to the wet patch, but are larger at its eastern border and in the southwesterly direction. At this time, the humidity and temperature differences are still basically centred over the soil moisture perturbation. Figure 5 shows the accumulated precipitation and average wind field for the CTRL run for the same 2-hour period ( UTC 28 August), together with the precipitation difference between the two runs. The main area of precipitation is over central Nigeria, with an additional rainfall band north of it at about 13 N. The precipitation differences are small at this time, with a small increase of precipitation near the eastern limit of the patch, probably due to the increased convergence there, and a noisy pattern coinciding with the main precipitation area. If we look at the same fields 6 hours later, when the squall line is passing over the wet patch, we find that the cooler and wetter area has diminished strongly in magnitude (Figure 6), as a result of the reduced soil atmosphere interaction during the night but also due to the effect of the wet patch on the squall line.

6 488 M. A. GAERTNER ET AL. Figure 5. Upper frame: accumulated precipitation (mm) and mean wind field at the surface (m/s) for the period UTC 28 August in the CTRL simulation. Lower frame: differences of precipitation (mm) and wind at the surface (m/s, WET-CTRL) for the same period. Wind scales are different in both frames, and are indicated under the respective frames. Figure 6. As Figure 4, but for the period UTC 28 August.

7 MODELLING SOIL MOISTURE ATMOSPHERE COUPLING 489 Figure 7. As Figure 5, but for the period UTC 28 August. The squall line appears also in the sensitivity run, but it is delayed and generates less rain over the wet patch (Figure 7). This delay and the precipitation reduction limit the cooling and moistening effect of the rain to the southwest of the wet patch, where drier air is simulated in the sensitivity run as shown in Figure 6. The precipitation differences seem to be associated with the differences in the low-level convergence due to the effect of the near-surface divergent circulation induced over the wet patch by the soil moisture perturbation. To analyse this, Figure 8 shows the accumulated precipitation for two consecutive 2-hour periods, superimposed on the near-surface divergence field at the middle of the respective 2-hour period. In the first period ( UTC 28 August), increased convergence is simulated in the WET run to the east of the wet patch. This increased convergence strengthens the approaching precipitation band, as seen through the larger extension of the 1 mm precipitation contour and also through the higher precipitation values in Figure 5. But when the squall line begins to cross the wet patch ( UTC 28 August), the divergent winds over the wet patch weaken the precipitation band and begin to delay it. This process continues while the squall line crosses the wet patch. The impact of the wet patch is not limited to the wet patch and nearby areas. More to the south (between 9 and 10 N) there is an increase in precipitation in the WET run. It is also interesting to see how the northward displacement of the southwesterly monsoon flux at night pushes the cooler and wetter air towards the northeast (Figure 6). The differences during the third simulation day (29 August) are rather disorganized, with the largest changes in surface fluxes and 2 m temperature and humidity placed not over the wet patch, but over the areas with the highest precipitation differences during the previous hours.

8 490 M. A. GAERTNER ET AL. Figure 8. Accumulated 2-hour precipitation (fine continuous contours, corresponding to 1 mm) superimposed on the near-surface (10 m) divergence field (indicated by the pattern filling: the horizontal-line filling corresponds to divergence values lower than s 1, i.e. convergent areas, and the dotted filling corresponds to divergence values larger than s 1, i.e. divergent areas). (a) CTRL simulation ( UTC 28 August) (b) WET simulation for the same 2-hour period. (c) CTRL simulation ( UTC 28 August) (d) WET simulation for the same 2-hour period. Figure 9. Evolution of the difference of latent (dashed line) and sensible (continuous line) heat fluxes (WET CTRL, W/m 2 ), averaged over the wet patch, during the three days of simulation. Fluxes are taken as positive upwards Impact on surface fluxes: Interaction with an African Easterly Wave The effects of the soil moisture perturbation are transmitted to the atmosphere through changes in surface fluxes. Figure 9 shows the time evolution of the anomalies of latent and sensible heat fluxes, averaged over the wet patch. The wet patch generates a latent heat increase and a simultaneous sensible heat decrease. During the first day, both differences grow approximately until midday and decrease to zero at nightfall. The latent heat flux difference is larger in absolute value than the sensible heat

9 MODELLING SOIL MOISTURE ATMOSPHERE COUPLING 491 Figure 10. Latitude height cross-sections of differences between 28 August and 27 August ( UTC averages) for temperature (upper frame, shaded, K) and humidity (lower frame, shaded, g/kg). The corresponding differences (28 27 August) for meridional wind component (thin contour lines, m/s) are superimposed. Negative meridional wind differences are indicated with dashed contours. The position of the wet patch is indicated through the thick black line at the bottom. The variables are averaged over the longitudinal extension of the wet patch. The thick contour line corresponds to zero difference for temperature and humidity, respectively. Pressure in the vertical axis is indicated in hpa. flux difference, by up to 20 W/m 2. The same qualitative evolution is simulated the second day until the evening, but the absolute value of the changes is much larger. This may seem contradictory with the fact that the soil moisture perturbation is smaller on the second day, due to the stronger evaporation during the first day in the WET run. However, the surface heat fluxes depend not only on the soil, but also on the atmospheric state, and there are clear changes in the atmospheric fields from the first to the second day. As indicated in Guichard et al. (2010), an African Easterly Wave begins to cross the analysed area on 28 August, with a change towards northerly meridional wind. This wind change causes warmer and drier air from the north to be advected over the wet patch, as seen in Figure 10, which shows the temperature and humidity differences between the second and the first simulation day, around midday, for the CTRL run. Near the surface, the air over the wet patch is drier on the second day by about 1 g/kg, and warmer by about 3 K. This combination will likely favour a stronger impact of the soil moisture perturbation during the second day in the WET run, due to higher evaporation. Changes in sensible and latent heat fluxes are linked to changes in the radiation budget at the surface. Figure 11 shows the time evolution of the differences in several components of this budget, together with the net radiation difference. The differences in solar radiation terms are rather small during the first two days, reflecting a similar cloud cover in both simulations. It is the long-wave radiation which changes most. Downward long-wave radiation decreases in the WET run, likely due to the cooler PBL over the wet patch. The stronger impact is seen in the upward long-wave radiation, as the cooler soil surface emits less radiation. The overall effect of these

10 492 M. A. GAERTNER ET AL. Figure 11. Evolution of the difference (WET-CTRL) of the components of radiation balance at the surface: downward solar radiation (thin continuous line), reflected solar radiation (dotted line), downward long-wave radiation (dashed line), upward long-wave radiation (dot-dashed line) and net radiation (thick black line), in W/m 2. All quantities are averages over the wet patch. All fluxes are taken as positive downward. changes is an increase in net radiation, in correspondence with the increased total heat flux from the soil. The influence of the approaching African Easterly Wave is also seen on the PBL height differences. During the first day, the daily average PBL height over the wet patch reduces from 928 m (CTRL run) to 776 m (WET run). During the second day, the daily average PBL height is much larger in the CTRL run (2219 m) and the impact of the soil moisture perturbation is also much larger than during the first day, as the PBL height reduces to 1386 m in the WET run Interaction of the response with the diurnal cycle of the monsoon flow An interesting aspect of the response to the soil moisture perturbation is its interaction with the diurnal changes of the monsoon flow. The diurnal cycle of the monsoon circulation, as explained by Parker et al. (2005), produces weak winds in the afternoon over the Sahel, due to turbulent mixing of momentum in the vertical. After sunset, the low-level monsoon winds intensify and advance to the north. This variation is reflected clearly in the simulated differences. Figure 12 shows latitude height cross-sections of humidity and meridional wind differences at several times. The first cross-section is shown for 1800 UTC of the first day. The humidity differences are located mainly over the wet patch, and extend vertically up to approximately 800 hpa due to the vertical mixing in the PBL. During the previous hours, the humidity differences have extended vertically, but almost have not moved horizontally. The difference of the meridional wind component shows a weak divergent circulation pattern. After sunset, the large-scale southwesterly monsoon flux at low levels intensifies and progresses northward. This causes the moist air pool to expand and move towards the north, as can be seen in the vertical crosssection for 0300 UTC of the second day. The flow differences weaken and move northward with the moist air. This simulated behaviour indicates that wet patches in the Sahel due to previous storms may increase the northward transport of humidity during the night. When the PBL begins to build up during the second day, new and more intense humidity differences develop over the wet patch. As the 1800 UTC cross-section for this day shows, the humidity differences extend also higher. The flow differences near the surface are clearly more intense than on the first day, reaching values of more than 1 m/s at the northern border of the humidity differences. The wind differences are stronger at the northern border of the humidity differences, where they have a southerly component, than at the southern border. The direction of the wind differences is in accordance with the local changes of monsoon circulations found by Parker et al. (2005) in observed fields as a response to cool anomalies associated to the previous passage of convective systems. To the north of a cool anomaly the large-scale gradients are enhanced, whereas to the south of it these gradients are weakened. 4. Conclusions The interaction of a squall line with a wet patch has been analysed through a sensitivity run with the PROMES model for a moist convection case-study of the 2005 West African monsoon. The control simulation is able to reproduce a squall line crossing over a relatively wet patch in the Sahel, with a realistic timing compared to the observed convective system. In the sensitivity run, the soil

11 MODELLING SOIL MOISTURE ATMOSPHERE COUPLING 493 Figure 12. Latitude height cross-sections of humidity differences (shaded, WET-CTRL, g/kg) and meridional wind differences (contours, WET- CTRL, m/s) at 1800 UTC 27 August (upper frame), 0300 UTC 28 August (middle frame) and 1800 UTC 28 August (lower frame). The position of the wet patch is indicated through the thick black line at the bottom. The variables are averaged over the longitudinal extension of the wet patch. Pressure in the vertical axis is indicated in hpa.

12 494 M. A. GAERTNER ET AL. moisture is increased up to saturation in this mesoscalesized wet patch. The wetter land surface in the sensitivity run generates a cooler and moister area compared to the control run, and the surface wind differences between the runs show a divergent pattern in the sensitivity simulation. The main impact of the wet patch on precipitation is a reduction of precipitation over the wet patch and near its southern border, together with a precipitation increase farther south, reflecting a complex soil moisture precipitation feedback which is locally negative over the soil moisture perturbation. The sign of the local feedback can be different under other circumstances, as shown e.g. by Gantner and Kalthoff (2009) who obtain a positive local feedback for a mature system passing over a dry soil-moisture anomaly, in sensitivity simulations in a different modelling case-study. Many aspects can influence the feedback, from model parametrizations to specific features of the atmospheric conditions. In the present study we have shown such an atmospheric feature affecting the feedback (the approximation of an African Easterly Wave). Regarding the model parameterizations, as the effect of the soil moisture perturbation on the squall line depends on the modification of the low-level divergence field and the associated vertical winds, an example of a parametrization feature that could affect the simulated response is the formulation of the impact of vertical wind on deep convection. The time evolution of the differences of the surface fluxes over the wet patch shows an interesting change from the first to the second day, as they are clearly larger on the second day. This occurs despite the fact that the soil moisture perturbation is smaller on the second day. A likely reason for this behaviour is an African Easterly Wave that begins to cross the analysed area on the second day. This wave generates northerly advection of drier and warmer air over the wet patch, which should increase the latent and sensible heat flux differences simulated during the second day through increased evapotranspiration in comparison with the first day. Finally, it is shown how the atmospheric differences linked to the wet patch interact with the diurnal cycle of the monsoon flow. In the afternoon, the horizontal flow is relatively weak, and the humidity differences remain over the wet patch. These humidity differences extend vertically due to turbulent mixing in the PBL. After sunset, the southwesterly monsoon flux at low levels intensifies and progresses northward. This causes the moist air pool to expand and move towards the north during the night. 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